Ampeg V4B instability

[Jonathan Krogh]

I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.

What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

[Jonathan Krogh]

Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

[[e-mail]]

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

I'm pretty sure I've seen this technique in some Carvin and maybe Mesa
Boogie amps. The late Ken Fisher has a few wise things to say
regarding this "protection" technique in that "guru" book. Basically
they might offer some protection, but a well-designed and maintained
amp that is not abused doesn't really need them. I believe Fisher
referred to them as "amulets".

-dave M.

[m***@yahoo.com]

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

[Mike Schway]

Post by m***@yahoo.com

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.

What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.

I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.

To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[m***@yahoo.com]

Post by Mike Schway

Post by m***@yahoo.com

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.
What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.
I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.
To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

it definitely does prevent oscillation, ultra high frequency,
low frequency and "motor boating". that's why the manufacturer
installed the diodes to start with.
if you want to modify the amp by removing them, you must
accept the consequence.
" I took out the diodes and now my amp oscillates"
just where is the mystery in this?
if you are brave enough to re install them you would soon
find out that I am 100% correct.
you can resolder all you want, check the capacitors all
you want, etc, etc, etc... the oscillation will not stop
until you install the diodes.
(finding a bad solder connection in a magnavox ampeg
would really be something, cause there aren't any, these amps
are not PVS!)
for many years i have shared knowledge and many laugh at it,
but sooner or later what i have been saying all this time
will be revealed as truth. i sincerely doubt that anyone reading
AGA has repaired more tube amps than i have, and you could
possibly learn something from a person with more experience
than yourself- if you are willing to accept new ideas.
many amps including Marshall contain the same circuit.
see old Marshall schematics, learn something new.
you will also find the same circuit in some versions of
(real) SVT's.
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.

[Jonathan Krogh]

Post by m***@yahoo.com

Post by Mike Schway

Post by m***@yahoo.com

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.
What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.
I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.
To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

it definitely does prevent oscillation, ultra high frequency,
low frequency and "motor boating". that's why the manufacturer
installed the diodes to start with.
if you want to modify the amp by removing them, you must
accept the consequence.
" I took out the diodes and now my amp oscillates"
just where is the mystery in this?
if you are brave enough to re install them you would soon
find out that I am 100% correct.
you can resolder all you want, check the capacitors all
you want, etc, etc, etc... the oscillation will not stop
until you install the diodes.
(finding a bad solder connection in a magnavox ampeg
would really be something, cause there aren't any, these amps
are not PVS!)
for many years i have shared knowledge and many laugh at it,
but sooner or later what i have been saying all this time
will be revealed as truth. i sincerely doubt that anyone reading
AGA has repaired more tube amps than i have, and you could
possibly learn something from a person with more experience
than yourself- if you are willing to accept new ideas.
many amps including Marshall contain the same circuit.
see old Marshall schematics, learn something new.
you will also find the same circuit in some versions of
(real) SVT's.
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.- Hide quoted text -
- Show quoted text -

Well I did reinstall them, so we shall soon find out, apparently
theyre originally a pair of 1400piv in each string, so ive put in
three IN4007 per string, I will leave out the coupling cap replacement
and get it going. I was at this time installing some spring retainers
and a set of KT88s, but their matching is way off, so back to the
6L6GEs ive had in.

[Jonathan Krogh]

Post by Jonathan Krogh

Post by m***@yahoo.com

Post by Mike Schway

Post by m***@yahoo.com

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.
What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.
I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.
To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

it definitely does prevent oscillation, ultra high frequency,
low frequency and "motor boating". that's why the manufacturer
installed the diodes to start with.
if you want to modify the amp by removing them, you must
accept the consequence.
" I took out the diodes and now my amp oscillates"
just where is the mystery in this?
if you are brave enough to re install them you would soon
find out that I am 100% correct.
you can resolder all you want, check the capacitors all
you want, etc, etc, etc... the oscillation will not stop
until you install the diodes.
(finding a bad solder connection in a magnavox ampeg
would really be something, cause there aren't any, these amps
are not PVS!)
for many years i have shared knowledge and many laugh at it,
but sooner or later what i have been saying all this time
will be revealed as truth. i sincerely doubt that anyone reading
AGA has repaired more tube amps than i have, and you could
possibly learn something from a person with more experience
than yourself- if you are willing to accept new ideas.
many amps including Marshall contain the same circuit.
see old Marshall schematics, learn something new.
you will also find the same circuit in some versions of
(real) SVT's.
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.- Hide quoted text -
- Show quoted text -

Well I did reinstall them, so we shall soon find out, apparently
theyre originally a pair of 1400piv in each string, so ive put in
three IN4007 per string, I will leave out the coupling cap replacement
and get it going. I was at this time installing some spring retainers
and a set of KT88s, but their matching is way off, so back to the
6L6GEs ive had in.- Hide quoted text -
- Show quoted text -

Ok, back to the diode strings installed, no effect on the low/sub
sonic oscillation/motorboating, im also seeing that it can be
triggered by noise on the vol pot of one of the channels, im not
getting any DC on that pot either, so I need to take that control
board out and start shotgunning caps, I cant work on that board in
situ. I did shotgun out caps on the main board, didnt help any. GE
6L6's reinstalled all biased up around 35ma per tube.

[Mike Schway]

Post by Jonathan Krogh
Ok, back to the diode strings installed, no effect on the low/sub
sonic oscillation/motorboating, im also seeing that it can be
triggered by noise on the vol pot of one of the channels, im not
getting any DC on that pot either, so I need to take that control
board out and start shotgunning caps, I cant work on that board in
situ. I did shotgun out caps on the main board, didnt help any. GE
6L6's reinstalled all biased up around 35ma per tube.

Hi, JK

More shots in the dark:

Did you do anything with the filter caps on this beast? I know it's a
royal PITA on a classic Ampeg (it ain't like recapping a Fender!), but
it may be something to look at. At the very least, double-check the
ground connections on the filters.

However, before doing the most difficult and most expensive fix,
eliminate the easy stuff first. There's that diode (D5?) in the power
supply section which couples the first two of stages of DC supply to
true ground. You'll see it on the schematic as being next to the pilot
lamp, and standby switch. Check the diode, and/or its solder connection.

There's also that standby switch. Ampeg's standby involves lifting the
ground from the power supply, not directly switching B+.Sounds weird,
but if the switch is high impedance, it'll also create too high a
resistance path to ground and motorboating could result.

Good luck. This will be one gloriously beefy sounding amp once you get
it running.

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[m***@yahoo.com]

you still have not returned the amp to stock circuit.
you are using 6L6 instead of 7027
as i said these are NOT interchangeable.
you are trying to troubleshoot an amp that has been
modified, and the problem is the modifications,
not the amp itself.
you are changing parts randomly, hoping to
accidentally solve the problem, but it will never be solved
with this approach.
if you return all the proper parts to the stock circuit
then it will work right.
but you can change and replace all you want, it will
never be solved until it's un- modified.
you are so quick to criticize the voice of experience
and sound reason- but those who criticize are the ones
who will prove to have the least experience.
who knows how many things have been changed,
and if the changed parts were installed correctly to
start with?
you started by assuming that you could replace
7027 with 6L6, mistake one.
then you assumed you could remove the feedback diodes,
mistake two.
then you went on and on randomly replacing parts,
mistake three.
now you assume the person with the experience is
an idiot because he does not make all the assumptions
you have made, mistake four.
a good tech never assumes anything- he VERIFIES.
this is the difference between LV and a good tech.
"look at me, i work for the derrick trucks band, that means
all other techs are inferior, i MUST be the greatest".
good techs don't need to brag about who they work for
either; they get plenty of recognition by being humble.
the more they brag, the more the lack of experience shows.

[Mike Schway]

Post by m***@yahoo.com
a good tech never assumes anything- he VERIFIES.

One thing I **HAVE** verified is that these amps run just fine (with
minor differences in tube longevity and tone) running 7027, 6L6GC, 5881
and plastic-base 6550s. Bias has to be adjusted a bit, but oscillation
has NEVER been a problem in my EXPERIENCE. Not just on a scope, but
through a speaker, playing guitar and bass LOUD. (sorry for shouting,
but I'm getting a bit testy about all of this).

To JK: Another wild thought came to me. You mentioned the LF
oscillation has been a problem over a period of time. Did it **ever**
run OK for you?

The reason I'm asking is just *maybe* a previous "tech" reversed the
polarity on the OPT primary. This means that the NFB loop would
actually give POSITIVE feedback and instability across the entire
spectrum would be expected. You'd have, in essence, a 60lb oscillator.
To test this, put the amp on a scope with a moderate 1KHz signal and
monitoring output across an appropriate load. Now, temporarily lift one
end of R34 and C14 Did the output level increase? Fine; that would be
expected and you can ignore this comment.

If, however, output decreased with removal of R34/C14, you have a
reversed OPT primary. Put everything back to where it belongs and see
if the oscillation goes away.

Are we having fun yet?
--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[Mike Schway]

Post by Mike Schway
To JK: Another wild thought came to me. You mentioned the LF
oscillation has been a problem over a period of time. Did it **ever**
run OK for you?
The reason I'm asking is just *maybe* a previous "tech" reversed the
polarity on the OPT primary. This means that the NFB loop would
actually give POSITIVE feedback and instability across the entire
spectrum would be expected. You'd have, in essence, a 60lb oscillator.
To test this, put the amp on a scope with a moderate 1KHz signal and
monitoring output across an appropriate load. Now, temporarily lift one
end of R34 and C14 Did the output level increase? Fine; that would be
expected and you can ignore this comment.
If, however, output decreased with removal of R34/C14, you have a
reversed OPT primary. Put everything back to where it belongs and see
if the oscillation goes away.

OK, on second thought, this may not be an issue with your amp. On an
Ampeg, the NFB reduces gain of the V3b voltage amp prior to the p/i and
OPT polarity might not matter here.

OTOH, With Fenders, Marshalls and most of the rest of the known world,,
NFB adds back to the non-inverting input of the p/i (a long tail P/I is
a differential amp, after all), and reversing polarity will reverse the
effect of NFB.

Sorry about the temporary confusion.

--Mike

--------------------------------------------------------------------
Mike Schway | "Oh, that's different. Never mind."
***@nas.com | --Gilda Radner as Emily Litella
--------------------------------------------------------------------

[Jonathan Krogh]

Post by Mike Schway

Post by m***@yahoo.com
a good tech never assumes anything- he VERIFIES.

One thing I **HAVE** verified is that these amps run just fine (with
minor differences in tube longevity and tone) running 7027, 6L6GC, 5881
and plastic-base 6550s. Bias has to be adjusted a bit, but oscillation
has NEVER been a problem in my EXPERIENCE. Not just on a scope, but
through a speaker, playing guitar and bass LOUD. (sorry for shouting,
but I'm getting a bit testy about all of this).
To JK: Another wild thought came to me. You mentioned the LF
oscillation has been a problem over a period of time. Did it **ever**
run OK for you?
The reason I'm asking is just *maybe* a previous "tech" reversed the
polarity on the OPT primary. This means that the NFB loop would
actually give POSITIVE feedback and instability across the entire
spectrum would be expected. You'd have, in essence, a 60lb oscillator.
To test this, put the amp on a scope with a moderate 1KHz signal and
monitoring output across an appropriate load. Now, temporarily lift one
end of R34 and C14 Did the output level increase? Fine; that would be
expected and you can ignore this comment.
If, however, output decreased with removal of R34/C14, you have a
reversed OPT primary. Put everything back to where it belongs and see
if the oscillation goes away.
Are we having fun yet?
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

I have done full replacement of diodes and filter capacitors in this
amplifier over the years, also screen resistors and plate resistors.
Currently the plate filtering is two 680uf 350v cans in series, the
screen filtering is two 220uf 300v axials up inside on a terminal
strip, and the remaning two filter sections in this amp are a 50/50uf
500v LCR can, the schematic for these shows a redunant extra filter
for the 'reverb', but this I dont have in place, altho i do have the
two 4.7k resistors there that break down the voltage to the PI and
preamp.
I have the OPT primary wires correct, I myself have taken this OT out
to take off some minor rust and paint it, im very familiar with the
positive feedback mistake.
I have converted the powertube sockets to be able to use EL34/6L6/6550
etc by joining the pins 1 and 9 together. I have installed a bias pot
in line with one of the existing bias resistors, normal correct setup.
I have had this amp with and without the reverse OPT lead diodes.

The oscillation is triggered intermittently by low signals, some from
playing, right now as i say some noise on a vol pot is trigering it
also.

The amp 'works' fine otherwise, huge volume, unreal bass, all controls
work perfectly, VERY low hiss and hum. I have tried every tube listed
above in this amp, and all behaved the same. (EL34 seem to struggle
tho, the screens cant take it).

I am appreciating all of the responses and suggestions, but definitely
not found any of the usual suspect causes yet.

[Mike Schway]

Post by Jonathan Krogh
I have done full replacement of diodes and filter capacitors in this
amplifier over the years, also screen resistors and plate resistors.
Currently the plate filtering is two 680uf 350v cans in series, the
screen filtering is two 220uf 300v axials up inside on a terminal
strip, and the remaning two filter sections in this amp are a 50/50uf
500v LCR can, the schematic for these shows a redunant extra filter
for the 'reverb', but this I dont have in place, altho i do have the
two 4.7k resistors there that break down the voltage to the PI and
preamp.

Hi, Jonathan

When you recapped did you use the old ground wiring scheme or did you
establish a new star ground. I'm only asking since if you check the
schemos, you'll see that the main filters, screen filter and bridge
connect to a floating ground which ties to earth ground (and the other
filter stages) via that D5 diode. I wouldn't know for sure, but just
possibly circumventing the original system may lead to instability here.

Grasping at straws....

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[Jonathan Krogh]

Post by Mike Schway

Post by Jonathan Krogh
I have done full replacement of diodes and filter capacitors in this
amplifier over the years, also screen resistors and plate resistors.
Currently the plate filtering is two 680uf 350v cans in series, the
screen filtering is two 220uf 300v axials up inside on a terminal
strip, and the remaning two filter sections in this amp are a 50/50uf
500v LCR can, the schematic for these shows a redunant extra filter
for the 'reverb', but this I dont have in place, altho i do have the
two 4.7k resistors there that break down the voltage to the PI and
preamp.

Hi, Jonathan
When you recapped did you use the old ground wiring scheme or did you
establish a new star ground. I'm only asking since if you check the
schemos, you'll see that the main filters, screen filter and bridge
connect to a floating ground which ties to earth ground (and the other
filter stages) via that D5 diode. I wouldn't know for sure, but just
possibly circumventing the original system may lead to instability here.
Grasping at straws....
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

I've got the original grounding scheme in it, I seem to remember once
doing the filters directly to ground, and something went weird, that
it effectively bypassed the standby and the amp stayed in run mode or
something, depends on how exactly I goofed.
What exactly is the purpose of the diode in that ground/standby
scheme?

I think I do want to replace both the power and standby plastic
switches, the power one seems to arc internally and have an odd feel
to it, altho it 'works fine'. If i cant another plastic rocker in
close size i may use toggles i guess

[Mike Schway]

Post by Jonathan Krogh
I've got the original grounding scheme in it, I seem to remember once
doing the filters directly to ground, and something went weird, that
it effectively bypassed the standby and the amp stayed in run mode or
something, depends on how exactly I goofed.
What exactly is the purpose of the diode in that ground/standby
scheme?

I'm not surprised that your amp stayed in the "on" condition. The
standby switch works by disconnecting ground to the main filters and
bridge.

I'm not sure of the primary intended function of D5, but one result is
that by isolating the main filters (which have the highest ripple
current to ground) from the preamp filter grounds, you'll reduce hum.
I've seen several amps from other mfrs where there's a 4.7 ohm resistor
between the main PS ground and the "local" preamp ground.

Post by Jonathan Krogh
I think I do want to replace both the power and standby plastic
switches, the power one seems to arc internally and have an odd feel
to it, altho it 'works fine'. If i cant another plastic rocker in
close size i may use toggles i guess

Not a bad idea to replace it, IMO. Does your standby lamp behave
normally without flicker?

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[Stephen Cowell]

"Jonathan Krogh" <***@gmail.com> wrote in message news:***@e65g2000hsc.googlegroups.com...

...

Post by Jonathan Krogh
The oscillation is triggered intermittently by low signals, some from
playing, right now as i say some noise on a vol pot is trigering it
also.

Check your feedback resistor and capacitor... you can raise the
value of the NFB cap (or lower the R) to decrease the frequency
corner, your final amp is lacking LF stability. This NFB is injected
at the cathode stage of the 12DW7... if this has been converted from
the normal valve, this NFB value may have changed (say, if a
different cathode resistor has been installed). I don't think you will
hurt the amp by giving it *more* NFB.

Post by Jonathan Krogh
The amp 'works' fine otherwise, huge volume, unreal bass, all controls
work perfectly, VERY low hiss and hum. I have tried every tube listed
above in this amp, and all behaved the same. (EL34 seem to struggle
tho, the screens cant take it).

I've found that bigger screen resistors help with this.

Post by Jonathan Krogh
I am appreciating all of the responses and suggestions, but definitely
not found any of the usual suspect causes yet.

You can also try paralleling in extra filter capacitance
at certain circuit spots to see if you get more LF stability.

Does the 'Ultra Low' switch change the effect?
__
Steve
.

[Stephen Cowell]

<***@yahoo.com> wrote

...

Post by m***@yahoo.com
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.

Two of those diodes, in series, reverse biased, gives
about 8pF capacitance... connected to ground, not B+.
That's not a lotta pF... I'm more inclined to the flyback
explanation, one that Pittman endorses, oddly enough.


Amazing how this schematic won't work, as drawn...
missing ground.

http://www.schematicheaven.com/ampegamps/v4bpowramp.pdf
__
Steve
.

[[e-mail]]

Post by m***@yahoo.com

Post by Mike Schway

Post by m***@yahoo.com

Post by Jonathan Krogh
Oh BTW I did take out those 'flyback' diode strings off the power
plates, surely this wouldnt be the cause....? I mean.. no other amps
have them

you need to keep the diodes on the power plates,
this is what prevents the oscillation, and yes
a bunch of amps DO have them including Marshalls.
we have cured many a mesa boogie design screw up
using the same method.
prefer to use 1000V diodes (two) in series, it
works better than one; on each plate.
this will also cure ultra high frequency oscillation
problems in many amps.
reinstall the diodes and the problem will be cured.
is this a real V4B or a fake SLM (crate) V4B?
from what you describe, sounds like a real one.
used to run an ampeg repair center (real magnavox
ampeg) that's how i know about this problem.
and as far as SLM (fake) ampegs, they do make excellent
boat anchors; or in case you don't have a boat, Reynold's
pays 15 cents a pound and it's all CASH!

How could these diodes POSSIBLY prevent oscillation? In order for them
to conduct, the plate would have go lower than NEGATIVE 2-3 V (assuming
two 1000V diodes). That just ain't gonna happen unless you have
grid-to-plate short, and if you do, you'll have bigger problems than
oscillation!!!.
What they do accomplish is to act as a sort of sacrificial inverse-fuse
which will short plate-to-ground if the plate voltage ever exceeds the
PIV of the diode string. This CAN happen under flyback conditions (high
signal with no load on secondary. If your fuse is of the correct value,
the diode string will protect sockets and your trannies.
I do agree, however, about the difference between East Coast Ampeg and
Midwest Ampeg. Different beasts entirely, though I'm not sure SLM ever
made a V4.
To the OP: you may want to reflow all solder joints in the power amp
section. These things notoriously overheat and/or crack in a V4,
especially if there's been a previous tube failure.
--Mike
--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
--------------------------------------------------------------------

it definitely does prevent oscillation, ultra high frequency,
low frequency and "motor boating". that's why the manufacturer
installed the diodes to start with.
if you want to modify the amp by removing them, you must
accept the consequence.
" I took out the diodes and now my amp oscillates"
just where is the mystery in this?
if you are brave enough to re install them you would soon
find out that I am 100% correct.
you can resolder all you want, check the capacitors all
you want, etc, etc, etc... the oscillation will not stop
until you install the diodes.
(finding a bad solder connection in a magnavox ampeg
would really be something, cause there aren't any, these amps
are not PVS!)
for many years i have shared knowledge and many laugh at it,
but sooner or later what i have been saying all this time
will be revealed as truth. i sincerely doubt that anyone reading
AGA has repaired more tube amps than i have, and you could
possibly learn something from a person with more experience
than yourself- if you are willing to accept new ideas.
many amps including Marshall contain the same circuit.
see old Marshall schematics, learn something new.
you will also find the same circuit in some versions of
(real) SVT's.
the diode is used as an audio feedback path to stabilize
the power supply. its not a DC path. the fact that two out of phase
audio signals are converging at the B+ is creating a "zero point"
reference balancing the plates to that reference.
its not my fault it works.- Hide quoted text -
- Show quoted text -

I have had to use some tricks on a lot of V4's to prevent actual
melting of solder joints on the screen-grid supply resistor, which can
result in that resistor losing an electrical connection. This
effectively removes the screen-grid voltage, and the output stage gain
drops to almost nothing. I've solved this problem by replacing the
resistor, along with the individual S-G feed resistors with new
wirewounds, keeping the lead long, bending them along the traces and
each other to create a larger area to dissipate heat, then soldering
the whole nest of leads. It has worked great in every case, and I
haven't seen a single one come back for that problem again. I mention
this because it is one example of solder joint related problems I've
found in many amps, and Ampegs are definitely *not* immune to this
problem.

I don't see how those diodes could possibly have any effect on low
frequency oscillation- not saying you're wrong, as I've seen weird
stuff that didn't make sense but just worked. I have removed these
diodes in a few V4's and never had any motorboating occur, and many
high power amps work just fine without these diodes, so...

I have worked on a few dozen Ampegs in my time, as well as some
Fenders, Marshalls, Traynors, Carvins, Mesas, Sovteks, Gibsons,
Epiphones, Peaveys (ugh), Crates, etc. etc. etc.

-dave M.

[m***@yahoo.com]

Post by ***@choiceonemail.com
I don't see how those diodes could possibly have any effect on low
frequency oscillation- not saying you're wrong, as I've seen weird
stuff that didn't make sense but just worked. I have removed these
diodes in a few V4's and never had any motorboating occur, and many
high power amps work just fine without these diodes, so...

the ultra high / low oscillation / motorboating occurs only when the
amp is plugged into a speaker, and only when the instrument is
played,
and will not occur under test bench conditions. in many cases it only
occurs with the controls set in certain positions.
you have removed the diodes without understanding the purpose of
the diodes and you have really made a big mistake. down the road these
amps you decided to "modify" by removing the factory designed parts
could cause major problems and tube failures.
you have concluded that these diodes have no purpose, but you are 100%
wrong.
when the ultra high frequency oscillation occurs, for example it can
cause one or more of the output tubes to turn red and the tubes
overheat
and short. the diodes will prevent this.
mesa boogie bass amps have this problem, and the factory is too
stupid to install the diodes. in fact the factory is too stupid to
realize it only oscillates under actual playing conditions, and these
oscillations do not occur when the amp is loaded with a resistor and
driven with a sine wave on the test bench.
so let that be a lesson. besides bench testing an amp, the only REAL
test is to play it and play it LOUD. this is the ONLY way to find out
what an amp will REALLY do when it's played. unfortunately, this is
the test that most repair shops SKIP.
and furthermore (the wind is really starting to bellow), if you are
changing from 7027 original tubes to KT88's (or other) you should be
changing several values in the circuit to compensate for a very
different tube. 7027, 6L6, KT88 ARE NOT interchangeable. yes they
have the same pin out, but VERY different characteristics.
OK so I have changed several SVTs over to 88's because we lack
quality 7027s. but i have also changed the circuit values so that
the tubes will be biased correctly and will work correctly. and try
to keep that philosophy in mind when you are modifying an amp,
and play it very loud, (eviction level) fucking loud when testing it.

mykey may be the oldest living tube amp technician, he has worked
with so many famous people that he doesn't need to brag about it.
(quote): "if we don't complain, things will NEVER get better".
also, he doesn't use the caps key unless he REALLY FUCKING
MEANS IT.

[Mike Schway]

Post by m***@yahoo.com
the ultra high / low oscillation / motorboating occurs only when the
amp is plugged into a speaker, and only when the instrument is
played,
and will not occur under test bench conditions. in many cases it only
occurs with the controls set in certain positions.
you have removed the diodes without understanding the purpose of
the diodes and you have really made a big mistake. down the road these
amps you decided to "modify" by removing the factory designed parts
could cause major problems and tube failures.
you have concluded that these diodes have no purpose, but you are 100%
wrong.
when the ultra high frequency oscillation occurs, for example it can
cause one or more of the output tubes to turn red and the tubes
overheat
and short. the diodes will prevent this.

At the risk of redundantly repeating myself in a redundant fashion:

I still am at a loss to see HOW these diodes could possibly have any
function other than a crowbar. The way they're biased (plate to
*GROUND*), they'll only conduct if the tube anode goes negative by a
couple of volts. DC, AC or whatever, inductive load or resistive load,
it doesn't matter, they'll ONLY conduct if the tube's anode goes
negative with respect to ground. That'll NEVER (repeat, NEVER) happen
in real space-time.

On the other hand, if Va exceeds the PIV rating of the diode string by a
couple fold, the diodes will short and of course conduct very well.
This unfortunately does happen under conditions where you've removed the
secondary load and have maximally high signal reflection.

You can get a moderately high signal reflection under normal conditions
at a speaker's free air resonance where impedance is at a maximum, but
we're only talking around 70-80 ohms or so, not thousands of ohms which
would be necessary for the reflected primary V to exceed 1KV or so.
Besides, if you are talking about using a pair of 1000V diodes, the
string's PIV is then 2KV. It would take a reflected V of about 2.5-3KV
to get these diodes to zener. You won't see this under any condition
where a speaker is attached, even at its resonant freq.

(and as an aside, re: NJ Ampeg reliability: Most of the V4s and VT22s
I've worked on have had failed (overheated, cracked, crystallized)
solder joints on the PCB where the screen resistors reside. Maybe they
were just fine when Mykey worked on them 20 years ago, but the NJ Ampegs
I'm seeing in my shop are now 25 years old and have withstood a couple
cycles of failed tubes. Screen resistors belong on tube sockets, NOT
printed circuit boards!)

--Mike

--------------------------------------------------------------------
Mike Schway | [Picture your favorite quote here]
***@nas.com |
--------------------------------------------------------------------

[RonSonic]

Post by Mike Schway

Post by m***@yahoo.com
the ultra high / low oscillation / motorboating occurs only when the
amp is plugged into a speaker, and only when the instrument is
played,
and will not occur under test bench conditions. in many cases it only
occurs with the controls set in certain positions.
you have removed the diodes without understanding the purpose of
the diodes and you have really made a big mistake. down the road these
amps you decided to "modify" by removing the factory designed parts
could cause major problems and tube failures.
you have concluded that these diodes have no purpose, but you are 100%
wrong.
when the ultra high frequency oscillation occurs, for example it can
cause one or more of the output tubes to turn red and the tubes
overheat
and short. the diodes will prevent this.

I still am at a loss to see HOW these diodes could possibly have any
function other than a crowbar. The way they're biased (plate to
*GROUND*), they'll only conduct if the tube anode goes negative by a
couple of volts. DC, AC or whatever, inductive load or resistive load,
it doesn't matter, they'll ONLY conduct if the tube's anode goes
negative with respect to ground. That'll NEVER (repeat, NEVER) happen
in real space-time.
On the other hand, if Va exceeds the PIV rating of the diode string by a
couple fold, the diodes will short and of course conduct very well.
This unfortunately does happen under conditions where you've removed the
secondary load and have maximally high signal reflection.

At which time those diodes short taking the B+ to ground and the fuse blows and
hopefully all this happens before anything expensive arcs.

I don't see those diodes having any other effect either.

Post by Mike Schway
You can get a moderately high signal reflection under normal conditions
at a speaker's free air resonance where impedance is at a maximum, but
we're only talking around 70-80 ohms or so, not thousands of ohms which
would be necessary for the reflected primary V to exceed 1KV or so.
Besides, if you are talking about using a pair of 1000V diodes, the
string's PIV is then 2KV. It would take a reflected V of about 2.5-3KV
to get these diodes to zener. You won't see this under any condition
where a speaker is attached, even at its resonant freq.
(and as an aside, re: NJ Ampeg reliability: Most of the V4s and VT22s
I've worked on have had failed (overheated, cracked, crystallized)
solder joints on the PCB where the screen resistors reside. Maybe they
were just fine when Mykey worked on them 20 years ago, but the NJ Ampegs
I'm seeing in my shop are now 25 years old and have withstood a couple
cycles of failed tubes. Screen resistors belong on tube sockets, NOT
printed circuit boards!)

Yep.



Ron

Effect pedal demo's up at http://www.soundclick.com/ronsonicpedalry

[Lord Valve]

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.

LV

[Jonathan Krogh]

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

[Lord Valve]

Post by Jonathan Krogh

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

Use tubes AT prodigy DOT net .

LV

[[e-mail]]

Post by Jonathan Krogh

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

Use tubes AT prodigy DOT net .
LV

Doing a sentimental trip thru AGA, sad state.
Anyways, a few years ago I finally cured what could best be described as that subsonic oscillation. I had a brainwave regarding the powertube grid stoppers, in this Vseries, they're over on the pcb lined up along with the screen grid resistors. I did some surgery moving them from the pcb over to be directly on the socket pins. Instant cure. That weird behaviour was all from that control grid line. I acquired another V4 since then and added that mod to the routine maintenance on it, hasnt given a moments problems.
Grid stoppers! Put em on the socket!

[Fat Willie]

Post by ***@gmail.com

Post by Jonathan Krogh

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

Use tubes AT prodigy DOT net .
LV

Doing a sentimental trip thru AGA, sad state.
Anyways, a few years ago I finally cured what could best be described as that subsonic oscillation. I had a brainwave regarding the powertube grid stoppers, in this Vseries, they're over on the pcb lined up along with the screen grid resistors. I did some surgery moving them from the pcb over to be directly on the socket pins. Instant cure. That weird behaviour was all from that control grid line. I acquired another V4 since then and added that mod to the routine maintenance on it, hasnt given a moments problems.
Grid stoppers! Put em on the socket!

Duh.

The youngsters companies like Fender, Ampeg, etc. hired to realize the
old /vintage circuits as PCBs for use in the shitty "re-issue" amps don't
know shit from apple butter when it comes to vacuum tube circuits -
that's why you can find positively HILARIOUS shit like a 220K bias feed
resistor connected directly to a 1.5K grid stopper a quarter of an inch
away on the board...after all, that's what it shows on the schematic right?
No-one looked at the PCB layout and said, "Hey, wait a minute...why do
we have two resistors in series that add up to 221.5K when we could just
use a single precision 221K resistor instead? what does this tell us, class?
It tells us that younger "engineers" have jack shit for tube knowledge,
because regardless of how the circuit may be represented on the schematic,
the ACTUAL PHYSICAL PLACEMENT of the grid swamp resistors matters,
and they need to be right on the tube pin to swamp oscillation which may
occur on the grid feed wire. First time I ever saw one of those PCBs it
took me about five minutes to quit laughing. Another thing I noticed is
that the screen resistors are also mounted on the main PCB; this doesn't
have much effect on how they operate, but if a screen shorts and takes
out the resistor the PCB is going to get burned, making the board conductive
in that region. You can replace it, but you're going to have leakage unless
you get your Dremel (actually, I'm using a Milwaukee precision rotary tool
nowadays - VASTLY superior...) and carve away all the burned stuff...it'll
look like shit, and you'll wind up with parts hanging in the air and whatnot.
Here's what you do if you encounter one of these boards in a re-issue BF
amp - and you can do this without pulling up the main board, which is
a royal pain in the ass: Locate the 1K5 grid swamp resistors and solder
a short piece of 22-24G bare wire across them. Do the same thing for
the 470R screen resistors - short 'em out. Go to the tube sockets and
mount the 470R parts between pins 4 and 6, connecting the existing
screen grid leads to pin 6 on each socket. (Pin 6 will be unused when
you start this procedure.) Likewise, remove the signal grid feed wire
from the socket (pin 5) and solder a 1K5 - 1/2W part to the pin, tying
the wire you removed to the free end of the resistor. Depending on
whether they used pin 1 for anything, you may be able to use the
old-school setup of mounting the swampers between pin 1 and pin
5. If you're not familiar with the old shit, here's an AB763 schemo
and wiring layout for a Super Reverb:

https://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf

Truth be told, there are a whole metric shitload of people working
for the legendary names (Fender, Ampeg, Vox, etc.) who just don't know
their ass from a hole in the ground when it comes to vacuum tube
circuitry - hell, they're so fuckin' dumb they can't even correctly copy
the old shit, when all they needed to do was get one, open it up,
and LOOK.

Don't get me started.

Lord Valve
Expert (fuck you)

[[e-mail]]

Post by ***@gmail.com

Post by Jonathan Krogh

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

Use tubes AT prodigy DOT net .
LV

Doing a sentimental trip thru AGA, sad state.
Anyways, a few years ago I finally cured what could best be described as that subsonic oscillation. I had a brainwave regarding the powertube grid stoppers, in this Vseries, they're over on the pcb lined up along with the screen grid resistors. I did some surgery moving them from the pcb over to be directly on the socket pins. Instant cure. That weird behaviour was all from that control grid line. I acquired another V4 since then and added that mod to the routine maintenance on it, hasnt given a moments problems.
Grid stoppers! Put em on the socket!

Duh.
The youngsters companies like Fender, Ampeg, etc. hired to realize the
old /vintage circuits as PCBs for use in the shitty "re-issue" amps don't
know shit from apple butter when it comes to vacuum tube circuits -
that's why you can find positively HILARIOUS shit like a 220K bias feed
resistor connected directly to a 1.5K grid stopper a quarter of an inch
away on the board...after all, that's what it shows on the schematic right?
No-one looked at the PCB layout and said, "Hey, wait a minute...why do
we have two resistors in series that add up to 221.5K when we could just
use a single precision 221K resistor instead? what does this tell us, class?
It tells us that younger "engineers" have jack shit for tube knowledge,
because regardless of how the circuit may be represented on the schematic,
the ACTUAL PHYSICAL PLACEMENT of the grid swamp resistors matters,
and they need to be right on the tube pin to swamp oscillation which may
occur on the grid feed wire. First time I ever saw one of those PCBs it
took me about five minutes to quit laughing. Another thing I noticed is
that the screen resistors are also mounted on the main PCB; this doesn't
have much effect on how they operate, but if a screen shorts and takes
out the resistor the PCB is going to get burned, making the board conductive
in that region. You can replace it, but you're going to have leakage unless
you get your Dremel (actually, I'm using a Milwaukee precision rotary tool
nowadays - VASTLY superior...) and carve away all the burned stuff...it'll
look like shit, and you'll wind up with parts hanging in the air and whatnot.
Here's what you do if you encounter one of these boards in a re-issue BF
amp - and you can do this without pulling up the main board, which is
a royal pain in the ass: Locate the 1K5 grid swamp resistors and solder
a short piece of 22-24G bare wire across them. Do the same thing for
the 470R screen resistors - short 'em out. Go to the tube sockets and
mount the 470R parts between pins 4 and 6, connecting the existing
screen grid leads to pin 6 on each socket. (Pin 6 will be unused when
you start this procedure.) Likewise, remove the signal grid feed wire
from the socket (pin 5) and solder a 1K5 - 1/2W part to the pin, tying
the wire you removed to the free end of the resistor. Depending on
whether they used pin 1 for anything, you may be able to use the
old-school setup of mounting the swampers between pin 1 and pin
5. If you're not familiar with the old shit, here's an AB763 schemo
https://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf
Truth be told, there are a whole metric shitload of people working
for the legendary names (Fender, Ampeg, Vox, etc.) who just don't know
their ass from a hole in the ground when it comes to vacuum tube
circuitry - hell, they're so fuckin' dumb they can't even correctly copy
the old shit, when all they needed to do was get one, open it up,
and LOOK.
Don't get me started.
Lord Valve
Expert (fuck you)

Well the problem here Willie is that the amp where I'm critiquing the placement of the grid stoppers *IS* the vintage amp. The 70's V series ampegs have all the screen and grid resistors on the pcb quite a distance away from the tubes, and nothing on the sockets themselves. I trusted 70s ampeg enough that I overlooked that detail for about a decade of wondering why that amp didnt work right. Subsonic oscillation? I'd never seen it happen any other time.

Jonathan Krogh

[Fat Willie]

Post by ***@gmail.com

Post by ***@gmail.com

Post by Jonathan Krogh

Post by Lord Valve

Post by m***@yahoo.com
OK so I have changed several SVTs over to 88's because we lack
quality 7027s.

Stuff like this is the way people who are actually good at fixing MI
equipment know you're full of shit.
LV

Elvie, where do I email you at, I got an SVT here needs some new glass.

Use tubes AT prodigy DOT net .
LV

Doing a sentimental trip thru AGA, sad state.
Anyways, a few years ago I finally cured what could best be described as that subsonic oscillation. I had a brainwave regarding the powertube grid stoppers, in this Vseries, they're over on the pcb lined up along with the screen grid resistors. I did some surgery moving them from the pcb over to be directly on the socket pins. Instant cure. That weird behaviour was all from that control grid line. I acquired another V4 since then and added that mod to the routine maintenance on it, hasnt given a moments problems.
Grid stoppers! Put em on the socket!

Duh.
The youngsters companies like Fender, Ampeg, etc. hired to realize the
old /vintage circuits as PCBs for use in the shitty "re-issue" amps don't
know shit from apple butter when it comes to vacuum tube circuits -
that's why you can find positively HILARIOUS shit like a 220K bias feed
resistor connected directly to a 1.5K grid stopper a quarter of an inch
away on the board...after all, that's what it shows on the schematic right?
No-one looked at the PCB layout and said, "Hey, wait a minute...why do
we have two resistors in series that add up to 221.5K when we could just
use a single precision 221K resistor instead? what does this tell us, class?
It tells us that younger "engineers" have jack shit for tube knowledge,
because regardless of how the circuit may be represented on the schematic,
the ACTUAL PHYSICAL PLACEMENT of the grid swamp resistors matters,
and they need to be right on the tube pin to swamp oscillation which may
occur on the grid feed wire. First time I ever saw one of those PCBs it
took me about five minutes to quit laughing. Another thing I noticed is
that the screen resistors are also mounted on the main PCB; this doesn't
have much effect on how they operate, but if a screen shorts and takes
out the resistor the PCB is going to get burned, making the board conductive
in that region. You can replace it, but you're going to have leakage unless
you get your Dremel (actually, I'm using a Milwaukee precision rotary tool
nowadays - VASTLY superior...) and carve away all the burned stuff...it'll
look like shit, and you'll wind up with parts hanging in the air and whatnot.
Here's what you do if you encounter one of these boards in a re-issue BF
amp - and you can do this without pulling up the main board, which is
a royal pain in the ass: Locate the 1K5 grid swamp resistors and solder
a short piece of 22-24G bare wire across them. Do the same thing for
the 470R screen resistors - short 'em out. Go to the tube sockets and
mount the 470R parts between pins 4 and 6, connecting the existing
screen grid leads to pin 6 on each socket. (Pin 6 will be unused when
you start this procedure.) Likewise, remove the signal grid feed wire
from the socket (pin 5) and solder a 1K5 - 1/2W part to the pin, tying
the wire you removed to the free end of the resistor. Depending on
whether they used pin 1 for anything, you may be able to use the
old-school setup of mounting the swampers between pin 1 and pin
5. If you're not familiar with the old shit, here's an AB763 schemo
https://schematicheaven.net/fenderamps/super_reverb_ab763_schem.pdf
Truth be told, there are a whole metric shitload of people working
for the legendary names (Fender, Ampeg, Vox, etc.) who just don't know
their ass from a hole in the ground when it comes to vacuum tube
circuitry - hell, they're so fuckin' dumb they can't even correctly copy
the old shit, when all they needed to do was get one, open it up,
and LOOK.
Don't get me started.
Lord Valve
Expert (fuck you)

Well the problem here Willie is that the amp where I'm critiquing the placement of the grid stoppers *IS* the vintage amp. The 70's V series ampegs have all the screen and grid resistors on the pcb quite a distance away from the tubes, and nothing on the sockets themselves. I trusted 70s ampeg enough that I overlooked that detail for about a decade of wondering why that amp didnt work right. Subsonic oscillation? I'd never seen it happen any other time.
Jonathan Krogh

As I said - placement of the screen resistors has little or no effect on their
function in the circuit. It's roasting the board during failure that is the much
bigger problem, and that is the main reason to put them on the socket pins.
In your case, there was obviously some sort of lead-dress mojo going on;
I don't know what it is/was, but if it's fixed, it's fixed. Can't argue with success.
What you report is rare in those amps, though. Re-routing signal grid wiring
would probably have done the trick - you can tweak for shit like that with the
amp up and running - get it oscillating and shove the wiring around with a
spudger. When it stops oscillating, fix the wiring in position.

LV

[[e-mail]]

Post by m***@yahoo.com

Post by ***@choiceonemail.com
I don't see how those diodes could possibly have any effect on low
frequency oscillation- not saying you're wrong, as I've seen weird
stuff that didn't make sense but just worked. I have removed these
diodes in a few V4's and never had any motorboating occur, and many
high power amps work just fine without these diodes, so...

the ultra high / low oscillation / motorboating occurs only when the
amp is plugged into a speaker, and only when the instrument is
played,
and will not occur under test bench conditions. in many cases it only
occurs with the controls set in certain positions.
you have removed the diodes without understanding the purpose of
the diodes and you have really made a big mistake. down the road these
amps you decided to "modify" by removing the factory designed parts
could cause major problems and tube failures.
you have concluded that these diodes have no purpose, but you are 100%
wrong.
when the ultra high frequency oscillation occurs, for example it can
cause one or more of the output tubes to turn red and the tubes
overheat
and short. the diodes will prevent this.
mesa boogie bass amps have this problem, and the factory is too
stupid to install the diodes. in fact the factory is too stupid to
realize it only oscillates under actual playing conditions, and these
oscillations do not occur when the amp is loaded with a resistor and
driven with a sine wave on the test bench.
so let that be a lesson. besides bench testing an amp, the only REAL
test is to play it and play it LOUD. this is the ONLY way to find out
what an amp will REALLY do when it's played. unfortunately, this is
the test that most repair shops SKIP.
and furthermore (the wind is really starting to bellow), if you are
changing from 7027 original tubes to KT88's (or other) you should be
changing several values in the circuit to compensate for a very
different tube. 7027, 6L6, KT88 ARE NOT interchangeable. yes they
have the same pin out, but VERY different characteristics.
OK so I have changed several SVTs over to 88's because we lack
quality 7027s. but i have also changed the circuit values so that
the tubes will be biased correctly and will work correctly. and try
to keep that philosophy in mind when you are modifying an amp,
and play it very loud, (eviction level) fucking loud when testing it.
mykey may be the oldest living tube amp technician, he has worked
with so many famous people that he doesn't need to brag about it.
(quote): "if we don't complain, things will NEVER get better".
also, he doesn't use the caps key unless he REALLY FUCKING
MEANS IT.

OK, first off, my primary method for testing guitar amps is actually
plugging a *guitar* into the amp, stuffing my ears with plugs, and
running the bastard full tilt into 2x12 test cabinet, which no "bench"
is complete without. I usually don't break out the scope unless
there's something I can't diagnose easily by other means. I also don't
go around pulling parts out of amps just for the hell of it- in the
case of the V4, the diodes had shorted due to a failed output tube
(IIRC) and blew it's fuse. I decided to see how the amp would do
without them, and all was well- at high volumes as well as low, and
with the tone controls in various positions.

I understand your frustration with half-assed repair shops that don't
know shit from sandwiches, but I don't get mad at 'em. I've gotten a
lot of cutomers from them- people who have experienced bad service can
better appreciate the competent service that I provide.

Why *are* you typing in caps?

We should all keep in mind that none of us is born with a soldering
iron in his hand...

-dave M.

[RG]

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

Trouble-shooting over the AGA is tough, but here is something that you can
try; move the speakers at an angle to the opposing wall, you could be
having 'standing-waves' that are causing the issue.

Best regards,

Rich
www.guytronix.com

[[e-mail]]

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

I would check the coupling caps, as you have suggested. When I first
started modifying amplifiers, I kind of went for the jugular and
bumped up the coupling/ tone stack caps like crazy, thinking all the
while about how thick and meaty the tone would be. All I accomplished
was causing the amp to exhibit exactly the symptoms you describe.

Measure the DC voltage on both sides of every coupling cap. It'll take
you five minutes, and it'll turn up any failing caps. 99% of the time,
coupling caps are there to block DC, so you should see some (positive)
DC on one side of the cap, and the other should probably be 0 VDC.

Is there anything you haven't replaced in the amp? That's the first
place to look. Cathode bypass caps maybe?

If you're near Rhode Island, I can look at it for you. If I *can't*
figure it out I won't even charge you :)

-dave M.

[Jonathan Krogh]

Post by ***@choiceonemail.com

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

I would check the coupling caps, as you have suggested. When I first
started modifying amplifiers, I kind of went for the jugular and
bumped up the coupling/ tone stack caps like crazy, thinking all the
while about how thick and meaty the tone would be. All I accomplished
was causing the amp to exhibit exactly the symptoms you describe.
Measure the DC voltage on both sides of every coupling cap. It'll take
you five minutes, and it'll turn up any failing caps. 99% of the time,
coupling caps are there to block DC, so you should see some (positive)
DC on one side of the cap, and the other should probably be 0 VDC.
Is there anything you haven't replaced in the amp? That's the first
place to look. Cathode bypass caps maybe?
If you're near Rhode Island, I can look at it for you. If I *can't*
figure it out I won't even charge you :)
-dave M.- Hide quoted text -
- Show quoted text -

Haha, well thanks very much for the offer, but i'm in the caribbean,
the... only tube amp tech around I guess.
I tossed it up on the bench tonight and did a few overdue mods, new
spring powertube retainers, took out a MV I had done (on these, why
bother), and I was looking thru the coupling caps to see if i have the
values for a full swap out. I will go ahead and check all for leaks,
but as i say this is an intermittent thing, I might shotgun out and
replace all the caps, measuring the ones on the control pot board
might be a hassle.

Will update on this issue, also going thru a 78 SVT, full restoration

BTW anyone ever really checked out the pcb holding the 6K11? its got
unused circuit traces and two unused 9pin spots, I always wondered if
it was alternate circuitry to 'replace' the 6K11, or if it was unused
reverb circuitry that was used in the V4

[d***@yahoo.com]

Post by Jonathan Krogh

Post by ***@choiceonemail.com

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

I would check the coupling caps, as you have suggested. When I first
started modifying amplifiers, I kind of went for the jugular and
bumped up the coupling/ tone stack caps like crazy, thinking all the
while about how thick and meaty the tone would be. All I accomplished
was causing the amp to exhibit exactly the symptoms you describe.
Measure the DC voltage on both sides of every coupling cap. It'll take
you five minutes, and it'll turn up any failing caps. 99% of the time,
coupling caps are there to block DC, so you should see some (positive)
DC on one side of the cap, and the other should probably be 0 VDC.
Is there anything you haven't replaced in the amp? That's the first
place to look. Cathode bypass caps maybe?
If you're near Rhode Island, I can look at it for you. If I *can't*
figure it out I won't even charge you :)
-dave M.- Hide quoted text -
- Show quoted text -

Haha, well thanks very much for the offer, but i'm in the caribbean,
the... only tube amp tech around I guess.
I tossed it up on the bench tonight and did a few overdue mods, new
spring powertube retainers, took out a MV I had done (on these, why
bother), and I was looking thru the coupling caps to see if i have the
values for a full swap out. I will go ahead and check all for leaks,
but as i say this is an intermittent thing, I might shotgun out and
replace all the caps, measuring the ones on the control pot board
might be a hassle.
Will update on this issue, also going thru a 78 SVT, full restoration
BTW anyone ever really checked out the pcb holding the 6K11? its got
unused circuit traces and two unused 9pin spots, I always wondered if
it was alternate circuitry to 'replace' the 6K11, or if it was unused
reverb circuitry that was used in the V4- Hide quoted text -

I have a pair of V4s on the bench; it's for the reverb.

Post by Jonathan Krogh
- Show quoted text -

[Jonathan Krogh]

Post by d***@yahoo.com

Post by Jonathan Krogh

Post by ***@choiceonemail.com

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

I would check the coupling caps, as you have suggested. When I first
started modifying amplifiers, I kind of went for the jugular and
bumped up the coupling/ tone stack caps like crazy, thinking all the
while about how thick and meaty the tone would be. All I accomplished
was causing the amp to exhibit exactly the symptoms you describe.
Measure the DC voltage on both sides of every coupling cap. It'll take
you five minutes, and it'll turn up any failing caps. 99% of the time,
coupling caps are there to block DC, so you should see some (positive)
DC on one side of the cap, and the other should probably be 0 VDC.
Is there anything you haven't replaced in the amp? That's the first
place to look. Cathode bypass caps maybe?
If you're near Rhode Island, I can look at it for you. If I *can't*
figure it out I won't even charge you :)
-dave M.- Hide quoted text -
- Show quoted text -

Haha, well thanks very much for the offer, but i'm in the caribbean,
the... only tube amp tech around I guess.
I tossed it up on the bench tonight and did a few overdue mods, new
spring powertube retainers, took out a MV I had done (on these, why
bother), and I was looking thru the coupling caps to see if i have the
values for a full swap out. I will go ahead and check all for leaks,
but as i say this is an intermittent thing, I might shotgun out and
replace all the caps, measuring the ones on the control pot board
might be a hassle.
Will update on this issue, also going thru a 78 SVT, full restoration
BTW anyone ever really checked out the pcb holding the 6K11? its got
unused circuit traces and two unused 9pin spots, I always wondered if
it was alternate circuitry to 'replace' the 6K11, or if it was unused
reverb circuitry that was used in the V4- Hide quoted text -

I have a pair of V4s on the bench; it's for the reverb.

Post by Jonathan Krogh
- Show quoted text -- Hide quoted text -

- Show quoted text -- Hide quoted text -
- Show quoted text -

Well im going to figure out if this means i can add reverb to the V4b,
I have the reverb tank from a VT22 laying around, well everything
except the pc mount sockets...

[[e-mail]]

Post by Jonathan Krogh

Post by ***@choiceonemail.com

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

I would check the coupling caps, as you have suggested. When I first
started modifying amplifiers, I kind of went for the jugular and
bumped up the coupling/ tone stack caps like crazy, thinking all the
while about how thick and meaty the tone would be. All I accomplished
was causing the amp to exhibit exactly the symptoms you describe.
Measure the DC voltage on both sides of every coupling cap. It'll take
you five minutes, and it'll turn up any failing caps. 99% of the time,
coupling caps are there to block DC, so you should see some (positive)
DC on one side of the cap, and the other should probably be 0 VDC.
Is there anything you haven't replaced in the amp? That's the first
place to look. Cathode bypass caps maybe?
If you're near Rhode Island, I can look at it for you. If I *can't*
figure it out I won't even charge you :)
-dave M.- Hide quoted text -
- Show quoted text -

Haha, well thanks very much for the offer, but i'm in the caribbean,
the... only tube amp tech around I guess.
I tossed it up on the bench tonight and did a few overdue mods, new
spring powertube retainers, took out a MV I had done (on these, why
bother), and I was looking thru the coupling caps to see if i have the
values for a full swap out. I will go ahead and check all for leaks,
but as i say this is an intermittent thing, I might shotgun out and
replace all the caps, measuring the ones on the control pot board
might be a hassle.
Will update on this issue, also going thru a 78 SVT, full restoration
BTW anyone ever really checked out the pcb holding the 6K11? its got
unused circuit traces and two unused 9pin spots, I always wondered if
it was alternate circuitry to 'replace' the 6K11, or if it was unused
reverb circuitry that was used in the V4- Hide quoted text -
- Show quoted text -

It's possible that the MV modifiacation is where the LFO problem is
happening. Is the MV capacitively coupled? I've seen a few done this
way- If the cap is too large, then that creates an opportunity for LFO
to happen. I've used .022 uF here with no problems, any higher and you
have to start worrying about motorboating. The layout for wiring the
MV is important, too, especially if you use unshielded cable.

The only differences I know of between the V4 and V4B are the value of
two series resistors, and whether or not it's got the reverb circuit
stuffed.

An amusing thing I've seen in those amps is the screen printing around
the phase inverter tube- the socket terminals are labelled in reverse!
(ie 9-1 instead of 1-9)

-dave M.

[Jim]

Post by Jonathan Krogh
I've had a problem in my V4B for years that i've never properly cured,
mostly because ive never used it much and I have been using it for
guitar instead of bass.
Heres the behaviour, when I have the bass knob up high, any thumping
on the low strings/muting/ or tapping on the strings when used with
bass, the speakers seem very 'out of control', and have a huge back
and forth excursion that seems.. more than the notes and sounds being
produced, almost like the appearance of a subsonic oscillation.
Sometimes, from this it would break into a motorboating that cannot be
stopped without turning the standby off or smacking the strings really
hard, it seems feedback could also start this motorboating.
Now, this has been happening for a long time, and i have been thru
several different sets and types of power tubes, swapped out all its
preamp tubes, screen resistors, filter capacitors, diodes over the
course of this time,. and nothing ever eliminated it, it would always
happen in the bass was up enough.
What could this be? Is it possibly a leaking coupling cap? I just..
never really chased down the issue, the amp worked so fine otherwise,
doesnt seem to be tube microphonics. Ive never had either coupling
caps or microphonic problems so steadily related to these symptoms and
triggers, the high bass setting string thumping.
Any ideas?

Research "motor boating."