Fender Twin Amp tone controls

[Gareth Magennis]

Hi,

I have a Twin Amp (modern build) whose owner says has lost all its bass. He
descibes it as sounding like the speakers are out of phase.

I've changed all the valves to no effect, and all the 12AT7's are where they
ought to be. Bias and output power good.
Trouble is I have nothing to compare it with, and nothing is obviously
broken. The Bass pots don't seem to be as effective as they should though,
but otherwise there is no major sonic problem as far as I can tell..

I took some measurements of the tone circuitry and would like to know if
this is typical of what they are supposed to be doing, or is indicative of
the problem:

Bass Mid and Treble controls at 0:
60Hz -3dB
250Hz +6dB
1Khz 0dB
1.4Khz -3dB


Bass Mid and Treble controls at 5:
55Hz -3dB
160Hz +4.2dB
1Khz 0dB
3.8KHz +4.4dB


Bass Mid and Treble controls at 10:
625Hz -3dB
1Khz 0dB
4Khz +5dB
(Note very poor bass response/much greater Mid and Treble response)


In addition, the treble control range at 10KHz is 16dB
Mid pot control range at 1Khz is 15dB
Bass pot control range at 100Hz is only 8dB.

I am measuring at the output of an 8 ohm dummy load at low power, but there
is not a significant difference to the measurements at the preamp output,
and frequency sweeping into the power amp input shows a pretty flat
response.
I am measuring the clean channel, as this is the only one used, but a quick
check of the dirty channel reveals a similar 8dB range of the bass pot at
100Hz, and similar 16dB ranges of Mid and Treble.



http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf

This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.


Any insight appreciated.


Cheers,


Gareth.

[Gareth Magennis]

Should have done this earlier but:

http://users.sa.chariot.net.au/~gmarts/ampbasic.htm



this shows approximately 16dB range on all three controls. Both channels do
the same 8dB bass range, which is odd. Not sure how this could happen, even
if the bass response is poor on the input circuit.



Gareth.

[jh]

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
this shows approximately 16dB range on all three controls. Both channels do
the same 8dB bass range, which is odd. Not sure how this could happen, even
if the bass response is poor on the input circuit.
Gareth.

Hi Gareth,

you might model your particular tonestack and the freq response with TSC
from duncanamps.com (BTW AFAIK MiUK !)

it's a terrific tool; highly recommended.


regards

Jochen

[Gareth Magennis]

Post by jh

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
this shows approximately 16dB range on all three controls. Both channels
do the same 8dB bass range, which is odd. Not sure how this could
happen, even if the bass response is poor on the input circuit.
Gareth.

Hi Gareth,
you might model your particular tonestack and the freq response with TSC
from duncanamps.com (BTW AFAIK MiUK !)
it's a terrific tool; highly recommended.
regards
Jochen

Unfortunately I only had the amp for the afternoon, as the guy is gigging
with it this weekend, and I didn't take note of the component values, ran
out of time. The schematics I posted doesn't seem to be exactly the same
circuit. I'll see it again monday.




Cheers,


Gareth.

[Gareth Magennis]

Post by jh

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
this shows approximately 16dB range on all three controls. Both
channels do the same 8dB bass range, which is odd. Not sure how this
could happen, even if the bass response is poor on the input circuit.
Gareth.

Hi Gareth,
you might model your particular tonestack and the freq response with TSC
from duncanamps.com (BTW AFAIK MiUK !)
it's a terrific tool; highly recommended.
regards
Jochen

Been playing with this terrific tool, and so it is. Looks like my
measurements are a long way off what they should be.

I suspect something down the line from the 2 input channels, will get
scoping again monday.



Cheers,



Gareth.

[RS]

On Sat, 16 Aug 2008 09:42:26 GMT, "Gareth Magennis"

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass. He
descibes it as sounding like the speakers are out of phase.
I've changed all the valves to no effect, and all the 12AT7's are where they
ought to be. Bias and output power good.
Trouble is I have nothing to compare it with, and nothing is obviously
broken. The Bass pots don't seem to be as effective as they should though,
but otherwise there is no major sonic problem as far as I can tell..
I took some measurements of the tone circuitry and would like to know if
this is typical of what they are supposed to be doing, or is indicative of
60Hz -3dB
250Hz +6dB
1Khz 0dB
1.4Khz -3dB
55Hz -3dB
160Hz +4.2dB
1Khz 0dB
3.8KHz +4.4dB
625Hz -3dB
1Khz 0dB
4Khz +5dB
(Note very poor bass response/much greater Mid and Treble response)
In addition, the treble control range at 10KHz is 16dB
Mid pot control range at 1Khz is 15dB
Bass pot control range at 100Hz is only 8dB.

Actually the thing that seems abnormal is the wide range of the mid
pot. Double check that.

The bass control doesn't normally have a very wide swing.

Also odd is that all of your numbers above show 0db variation at 1khz.
Looks like you 'rezero'd or re-referenced with the assumption that 1k
was center? (It's not, and you shouldn't rezero anyway, especially in
an amp with a mid control). No way will 1khz stay the same at all
control settings.

You also said that your measurements at the dummy load were similar to
those post-preamp. There's usually a rolloff induced by a coupling cap
(.001 ?) between the preamp and the power amp. You should have
noticed some bass rolloff from that unless your 'post-pre'
measurements included it. BTW, it's intended to tighten up the bottom
end, so expect other problems if you use a higher value (like output
stage saturation/bias shift when the amp is cranked).

I presume that you tested with other speakers? Have you compared the
amp with other Fenders? The response curve is not linear--it's
tailored for guitar.

And what jh said: See Duncan's simulator to get a better idea of what
to expect.

[Gareth Magennis]

Post by RS
On Sat, 16 Aug 2008 09:42:26 GMT, "Gareth Magennis"

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass. He
descibes it as sounding like the speakers are out of phase.
I've changed all the valves to no effect, and all the 12AT7's are where they
ought to be. Bias and output power good.
Trouble is I have nothing to compare it with, and nothing is obviously
broken. The Bass pots don't seem to be as effective as they should though,
but otherwise there is no major sonic problem as far as I can tell..
I took some measurements of the tone circuitry and would like to know if
this is typical of what they are supposed to be doing, or is indicative of
60Hz -3dB
250Hz +6dB
1Khz 0dB
1.4Khz -3dB
55Hz -3dB
160Hz +4.2dB
1Khz 0dB
3.8KHz +4.4dB
625Hz -3dB
1Khz 0dB
4Khz +5dB
(Note very poor bass response/much greater Mid and Treble response)
In addition, the treble control range at 10KHz is 16dB
Mid pot control range at 1Khz is 15dB
Bass pot control range at 100Hz is only 8dB.

Actually the thing that seems abnormal is the wide range of the mid
pot. Double check that.
The bass control doesn't normally have a very wide swing.
Also odd is that all of your numbers above show 0db variation at 1khz.
Looks like you 'rezero'd or re-referenced with the assumption that 1k
was center? (It's not, and you shouldn't rezero anyway, especially in
an amp with a mid control). No way will 1khz stay the same at all
control settings.

I used 1KHz as a reference 0dB for convenience. It is much easier to see how
the three frequency bands vary with each other that way. I am concerned with
the ratio of Bass to Mid and Treble as the pots are altered. The Bass seems
way behind the others, and this is the reported fault, which suddenly
happened.

Post by RS
You also said that your measurements at the dummy load were similar to
those post-preamp. There's usually a rolloff induced by a coupling cap
(.001 ?) between the preamp and the power amp. You should have
noticed some bass rolloff from that unless your 'post-pre'
measurements included it. BTW, it's intended to tighten up the bottom
end, so expect other problems if you use a higher value (like output
stage saturation/bias shift when the amp is cranked).

Well again I'm not trying to be that accurate, just confirming there isn't
anything wrong with the power amp. There was pretty much the same 8dB range
of bass control at preamp output as at power amp output.

Post by RS
I presume that you tested with other speakers? Have you compared the
amp with other Fenders? The response curve is not linear--it's
tailored for guitar.

Yes, my test speaker sounds similar. I know the response is far from linear
by design, and that the controls do not often behave in the manner expected
with other audio equipment.

Post by RS
And what jh said: See Duncan's simulator to get a better idea of what
to expect.

Cheers,



Gareth.

[Stephen Cowell]

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

My guess... C1, the first stage cathode bypass. You can shunt around it
with another 47uF and see if bass picks up... you'd be down on gain too.
Also, there are two relays and a 'de-thumper' circuit involved in channel
switching... make sure that you don't have both channels on at once, or
that one's staying on all the time, by verifying the operation of the tone
stacks in each channel. Good luck!
__
Steve
.

[Burnham Treezdown]

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?

Has the owner been futzing around inside without mentioning it?

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

[Gareth Magennis]

Post by Burnham Treezdown

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?
Has the owner been futzing around inside without mentioning it?

No, both move the same way with a battery. 2 x 8 ohm in series.

Post by Burnham Treezdown

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

No. C6 is 0.022 and connects to the Mid Wiper, not Mid top. The Mid Wiper
goes nowhere else. The top of the Mid pot goes to the bottom of the Bass
pot, nothing else is connected to this point. Bottom of Mid pot goes to
ground.
All other values, including the pots are the same, except R7 is 1Meg,
reading 500K because of R6 in parallel. Ch2 seems to be wired the same.

If I sweep an input tone between 100 Hz and 1K, the input to the tone stack
at C4/R5 is flat down to less than 30 Hz or so, so there doesn't appear to
be anything up with the input ciruitry. The output, measured at the Treble
wiper exhibits the same readings the amp is giving overall, notably the Bass
pot has around 8.5db range only at 100 Hz. From this it looks like there is
no fault.


Thanks,


Gareth.

[TD Madden]

Post by Gareth Magennis

Post by Burnham Treezdown

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?
Has the owner been futzing around inside without mentioning it?

No, both move the same way with a battery. 2 x 8 ohm in series.

Post by Burnham Treezdown

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

No. C6 is 0.022 and connects to the Mid Wiper, not Mid top. The Mid Wiper
goes nowhere else. The top of the Mid pot goes to the bottom of the Bass
pot, nothing else is connected to this point. Bottom of Mid pot goes to
ground.
All other values, including the pots are the same, except R7 is 1Meg,
reading 500K because of R6 in parallel. Ch2 seems to be wired the same.
If I sweep an input tone between 100 Hz and 1K, the input to the tone stack
at C4/R5 is flat down to less than 30 Hz or so, so there doesn't appear to
be anything up with the input ciruitry. The output, measured at the Treble
wiper exhibits the same readings the amp is giving overall, notably the Bass
pot has around 8.5db range only at 100 Hz. From this it looks like there is
no fault.
Thanks,
Gareth.

plugged into a 16 ohm jack?

[Gareth Magennis]

Post by Burnham Treezdown

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?
Has the owner been futzing around inside without mentioning it?

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

Just to confirm, with reference to the circuits here:
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm

This amp has the Fender circuit for the Treble and Bass pots, but uses the
Marshall circuit at the middle pot, so is some kind of hybrid.



Cheers,


Gareth.

[Burnham Treezdown]

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
This amp has the Fender circuit for the Treble and Bass pots, but uses the
Marshall circuit at the middle pot, so is some kind of hybrid.

And you're sure the owner or one of his buddies hasn't been messing in
there, maybe after reading some amp mods webpage?

Does the owner say it once had more bass and it suddenly disappeared, or
that it always sounded this way?

The missing connection from the mids pot wiper to the top would seem to
account for this at least partially.

[jh]

Post by Burnham Treezdown

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
This amp has the Fender circuit for the Treble and Bass pots, but uses the
Marshall circuit at the middle pot, so is some kind of hybrid.

And you're sure the owner or one of his buddies hasn't been messing in
there, maybe after reading some amp mods webpage?
Does the owner say it once had more bass and it suddenly disappeared, or
that it always sounded this way?
The missing connection from the mids pot wiper to the top would seem to
account for this at least partially.

If it's the '94Twin, then the C8 to wiper would be correct.

T: 250pF 250K
B: .1F 250K
M: .047F 25k

slope R: 100K
shunt to ground from treble cap input: 1M

regards

Jochen

[Gareth Magennis]

Post by Burnham Treezdown

Post by Gareth Magennis
http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
This amp has the Fender circuit for the Treble and Bass pots, but uses
the Marshall circuit at the middle pot, so is some kind of hybrid.

And you're sure the owner or one of his buddies hasn't been messing in
there, maybe after reading some amp mods webpage?
Does the owner say it once had more bass and it suddenly disappeared, or
that it always sounded this way?
The missing connection from the mids pot wiper to the top would seem to
account for this at least partially.

I'm beginning to seriously distrust the owners' reports. It could be too
much Mid, or there may even not be any fault at all.
Anyway, his friend has an identical amp and I have persuaded him to try and
get a loan of it to do a proper A/B.


For those interested, I have posted part of the Schematic that Jochen kindly
sent me, on:

alt.binaries.schematics.electronic

Tone stack really is partly Fender and partly Marshall, so Duncanamps'
rather spiffing simulator won't work AFAICT.

Does this mean the Bass pot has less range than Mid and Treble?


Cheers,


Gareth.

[RS]

On Mon, 18 Aug 2008 21:53:13 GMT, "Gareth Magennis"

Post by Gareth Magennis
Does this mean the Bass pot has less range than Mid and Treble?

That's about what I was trying to tell you. Except the normal Fender
mid control has relatively little swing.

To retrace: The center frequency used by most guitar amp designers is
-not- 1Khz. It is usually closer to 300 hz. This means that the low E
on guitar is less than 2 octaves down. If the RC slope is 6db per
octave, that's 12db abs max, and you won't get that.

Again, the stock Fender mid doesn't do a whole lot with the main
slopes, but it does cancel much of the large phase-based notch (180
shifts in opposite directions around the treble vs bass RC nets).

But you should have seen both of these effects with Duncan's
simulator, no? It's also easy to find a Fender amp at a music store
and just bang on strings while you turn pots.

[Gareth Magennis]

Post by RS
On Mon, 18 Aug 2008 21:53:13 GMT, "Gareth Magennis"

Post by Gareth Magennis
Does this mean the Bass pot has less range than Mid and Treble?

That's about what I was trying to tell you. Except the normal Fender
mid control has relatively little swing.
To retrace: The center frequency used by most guitar amp designers is
-not- 1Khz. It is usually closer to 300 hz. This means that the low E
on guitar is less than 2 octaves down. If the RC slope is 6db per
octave, that's 12db abs max, and you won't get that.
Again, the stock Fender mid doesn't do a whole lot with the main
slopes, but it does cancel much of the large phase-based notch (180
shifts in opposite directions around the treble vs bass RC nets).
But you should have seen both of these effects with Duncan's
simulator, no? It's also easy to find a Fender amp at a music store
and just bang on strings while you turn pots.

Well well, well, I got the other amp in today for an A/B. This one behaves
as a stock Fender with the Mid centred around 300 Hz just as you say.
(Duncanamps says more like 500Hz but there you go). The "faulty" one has the
94 Twin tone stack with the Mid centred around 1.1kHz, a little higher than
the Duncanamps Marshall simulation says.

It didn't suddenly lose bass, the owner suddenly realised it didn't sound
like the other amp, which has a very different tone stack, and sounds a lot
better IMHO. And his.


Blimey O'Reilly.

Thanks to all, at least I've learnt something from all those hours finding a
fault that never was.



Gareth.

[JP]

On Fri, 22 Aug 2008 17:52:22 GMT, "Gareth Magennis"

Post by Gareth Magennis

Post by RS
On Mon, 18 Aug 2008 21:53:13 GMT, "Gareth Magennis"

Post by Gareth Magennis
Does this mean the Bass pot has less range than Mid and Treble?

That's about what I was trying to tell you. Except the normal Fender
mid control has relatively little swing.
To retrace: The center frequency used by most guitar amp designers is
-not- 1Khz. It is usually closer to 300 hz. This means that the low E
on guitar is less than 2 octaves down. If the RC slope is 6db per
octave, that's 12db abs max, and you won't get that.
Again, the stock Fender mid doesn't do a whole lot with the main
slopes, but it does cancel much of the large phase-based notch (180
shifts in opposite directions around the treble vs bass RC nets).
But you should have seen both of these effects with Duncan's
simulator, no? It's also easy to find a Fender amp at a music store
and just bang on strings while you turn pots.

Well well, well, I got the other amp in today for an A/B. This one behaves
as a stock Fender with the Mid centred around 300 Hz just as you say.
(Duncanamps says more like 500Hz but there you go). The "faulty" one has the
94 Twin tone stack with the Mid centred around 1.1kHz, a little higher than
the Duncanamps Marshall simulation says.
It didn't suddenly lose bass, the owner suddenly realised it didn't sound
like the other amp, which has a very different tone stack, and sounds a lot
better IMHO. And his.
Blimey O'Reilly.
Thanks to all, at least I've learnt something from all those hours finding a
fault that never was.
Gareth.

Sell him a "Fix" ...$$$$

[jh]

Post by Gareth Magennis

Post by Burnham Treezdown

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its bass.
He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?
Has the owner been futzing around inside without mentioning it?

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid pot
wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
This amp has the Fender circuit for the Treble and Bass pots, but uses the
Marshall circuit at the middle pot, so is some kind of hybrid.
Cheers,
Gareth.

Gareth,

you've got mail

regards

Jochen

[Gareth Magennis]

Post by jh

Post by Gareth Magennis

Post by Burnham Treezdown

Post by Gareth Magennis
Hi,
I have a Twin Amp (modern build) whose owner says has lost all its
bass. He descibes it as sounding like the speakers are out of phase.

So the first question might be....are they?
Has the owner been futzing around inside without mentioning it?

Post by Gareth Magennis
http://www.fender.com/support/amp_schematics/pdfs/Twin_Pro_Tube_Amp_SchE45.pdf
This is a similar circuit, except the cap C6 actually goes to the Mid
pot wiper, and the wiper and top of the pot are not connected together.

Does C6 and the wiper connect to the bass pot as shown?

http://users.sa.chariot.net.au/~gmarts/ampbasic.htm
This amp has the Fender circuit for the Treble and Bass pots, but uses
the Marshall circuit at the middle pot, so is some kind of hybrid.
Cheers,
Gareth.

Gareth,
you've got mail
regards
Jochen

Excellent, thanks Jochen. Looks like it is indeed the '94 Twin.


Cheers,


Gareth.