JCM 900 DR bias catastrophe

[j***@gmail.com]

Hello all. I'm fairly new to the world of amp modding, and all my
attempts have been successful....until now.

I recently converted my Dual Reverb 4100 to EL34 specs, as described on
this page: http://web.aoct.org/marshall/jcm900-tube-conversion.txt

I have attempted to set the bias according to Lord Valve's directions
(read Pin 5 without tubes, set max neg voltage, read Pin 3 with tubes,
read 1 ohm resistors, multiply readings, and adjust bias accordingly).

Now here's the problem:

I get a max negative voltage reading of -89 on Pin 5 with the tubes
OUT. But when I put the EL34's in, I get no negative voltage reading on
Pin 3. As well, I get no mA reading across the 1ohm resistors on the
cathodes.

Any ideas as to what could be happening?

[j***@gmail.com]

New development....

When I adjusted the bias pot all the way to the other side (minimum
negative voltage), I got a reading of 17mA across the 1ohm resistors.
Still no reading from Pin 3 on the power tubes though.

[Dave Curtis]

Post by j***@gmail.com
New development....
When I adjusted the bias pot all the way to the other side (minimum
negative voltage), I got a reading of 17mA across the 1ohm resistors.
Still no reading from Pin 3 on the power tubes though.

You mean 17mV, right?
-DC

[Stephen Cowell]

Post by j***@gmail.com
New development....
When I adjusted the bias pot all the way to the other side (minimum
negative voltage), I got a reading of 17mA across the 1ohm resistors.
Still no reading from Pin 3 on the power tubes though.

What kind of meter do you have? I'm worried that you're
not reading HV... this is dangerous, since it's most likely
there, but you're either not connecting the meter properly,
or your meter has blown a fuse (it's not set for amps, right?).
Verify all functions of your meter before continuing.
__
Steve
.

[Jim]

Post by Stephen Cowell

Post by j***@gmail.com
New development....
When I adjusted the bias pot all the way to the other side (minimum
negative voltage), I got a reading of 17mA across the 1ohm resistors.
Still no reading from Pin 3 on the power tubes though.

What kind of meter do you have? I'm worried that you're
not reading HV... this is dangerous, since it's most likely
there, but you're either not connecting the meter properly,
or your meter has blown a fuse (it's not set for amps, right?).
Verify all functions of your meter before continuing.

Check to see leads are properly seated whenever you are expecting to
read a high voltage. I *almost* made painful mistake when working on my
old 105W Univox. I did a quick voltage check for B+, got nuttin. I
figgered, "cool, bleed resistors." Decided to do the screwdriver to
ground just in case, got a BIG spark. The probe wasn't seated in the
meter. DOH!

[j***@gmail.com]

Sorry....yes, mV is what I meant.

I've checked and double checked my DMM. It works fine. I even tried
reading Pin 3 with my analog meter, and I still didn't get any reading.

However, I found that if I took a reading from Pin 4, it read around
470 V. It seems to me that this should be the reading for Pin 3, no? I
know for sure I'm not reading the socket numbers wrong, because I start
counting from Pin 1 (which is tied to Pin 8) clockwise to Pin 3.

[Stephen Cowell]

Post by j***@gmail.com
Sorry....yes, mV is what I meant.
I've checked and double checked my DMM. It works fine. I even tried
reading Pin 3 with my analog meter, and I still didn't get any reading.
However, I found that if I took a reading from Pin 4, it read around
470 V. It seems to me that this should be the reading for Pin 3, no? I
know for sure I'm not reading the socket numbers wrong, because I start
counting from Pin 1 (which is tied to Pin 8) clockwise to Pin 3.

Find the plate transformer wires.. what pin are they on?
There should be grid stoppers on the grid, and screen resistors
on the screen... verify all this, and pin numbers, before proceeding.
Pin 1 is NC... it's conceivable that pins 7 and 8 could be tied
together... pin 7 is Heater, which could be grounded. Only
trust the locator pin... you can trust the tiny numbers as well.
__
Steve
.

[Stephen Cowell]

Post by Stephen Cowell

Post by j***@gmail.com
Sorry....yes, mV is what I meant.
I've checked and double checked my DMM. It works fine. I even tried
reading Pin 3 with my analog meter, and I still didn't get any reading.
However, I found that if I took a reading from Pin 4, it read around
470 V. It seems to me that this should be the reading for Pin 3, no? I
know for sure I'm not reading the socket numbers wrong, because I start
counting from Pin 1 (which is tied to Pin 8) clockwise to Pin 3.

Find the plate transformer wires.. what pin are they on?
There should be grid stoppers on the grid, and screen resistors
on the screen... verify all this, and pin numbers, before proceeding.
Pin 1 is NC... it's conceivable that pins 7 and 8 could be tied
together... pin 7 is Heater, which could be grounded. Only
trust the locator pin... you can trust the tiny numbers as well.

Oops... nope, EL34 is different, has the G3 pin on pin1...
still... be careful!
__
Steve
.

[Tony Hwang]

Post by j***@gmail.com
Sorry....yes, mV is what I meant.
I've checked and double checked my DMM. It works fine. I even tried
reading Pin 3 with my analog meter, and I still didn't get any reading.
However, I found that if I took a reading from Pin 4, it read around
470 V. It seems to me that this should be the reading for Pin 3, no? I
know for sure I'm not reading the socket numbers wrong, because I start
counting from Pin 1 (which is tied to Pin 8) clockwise to Pin 3.

Hi,
Pin 3 is plate. Maybe you are counting pin no. wrong.
And 1mV is same as 1mA in plate+Sg current combined per Ohm's law
where Sg draws very small amount of current.
Tony

[Dave Curtis]

Post by Tony Hwang
Hi,
Pin 3 is plate. Maybe you are counting pin no. wrong.
And 1mV is same as 1mA in plate+Sg current combined per Ohm's law
where Sg draws very small amount of current.
Tony

Hi Tony!

(I know you know this; it's for the OP):

You can measure the V across the screen R to calculate the screen
current and deduct that from the total current measured across the 1R
cathode R to get a more exact plate current.

(if you want)

-DC

[Stephen Cowell]

Post by Dave Curtis

Post by Tony Hwang
Hi,
Pin 3 is plate. Maybe you are counting pin no. wrong.
And 1mV is same as 1mA in plate+Sg current combined per Ohm's law
where Sg draws very small amount of current.
Tony

Hi Tony!
You can measure the V across the screen R to calculate the screen
current and deduct that from the total current measured across the 1R
cathode R to get a more exact plate current.
(if you want)

Or, you can do like I did... build a bias probe with
the 1ohm resistor in the plate side... and then grab
ahold of the banana plugs for the meter and shock
the sh!t out of yourself.

Seriously, put the resistor in the cathode side... not
worth avoiding the extra math.
__
Steve
.

[Dave Curtis]

Post by Stephen Cowell

Post by Dave Curtis

Post by Tony Hwang
Hi,
Pin 3 is plate. Maybe you are counting pin no. wrong.
And 1mV is same as 1mA in plate+Sg current combined per Ohm's law
where Sg draws very small amount of current.
Tony

Hi Tony!
You can measure the V across the screen R to calculate the screen
current and deduct that from the total current measured across the 1R
cathode R to get a more exact plate current.
(if you want)

Or, you can do like I did... build a bias probe with
the 1ohm resistor in the plate side... and then grab
ahold of the banana plugs for the meter and shock
the sh!t out of yourself.

Ouch (puts it mildly)! Proceed at your own risk! ;o)

Nothing commands more respect than the possibility of a few million
electrons rapidly coursing through your body tissue.

Post by Stephen Cowell
Seriously, put the resistor in the cathode side... not
worth avoiding the extra math.

Well, yeah.

I didn't mean add a 1R current sense resistor to the screen. I meant
measure the V drop across *the* screen resistor and calculate the
current through it (I thought I said that).


-DC

[Stephen Cowell]

...

Post by Dave Curtis

Post by Stephen Cowell
Seriously, put the resistor in the cathode side... not
worth avoiding the extra math.

Well, yeah.
I didn't mean add a 1R current sense resistor to the screen. I meant
measure the V drop across *the* screen resistor and calculate the
current through it (I thought I said that).

Of course... I was just pointing out that you can get
an exact reading, but have a dangerous situation, by
doing as I did. If sheathed banana plugs were available
(like the ones Fluke uses) then putting the resistor in
the plate circuit might be not such a stupid thing.
__
Steve
.

[Dave Curtis]

Post by Stephen Cowell
...

Post by Dave Curtis

Post by Stephen Cowell
Seriously, put the resistor in the cathode side... not
worth avoiding the extra math.

Well, yeah.
I didn't mean add a 1R current sense resistor to the screen. I meant
measure the V drop across *the* screen resistor and calculate the
current through it (I thought I said that).

Of course... I was just pointing out that you can get
an exact reading, but have a dangerous situation, by
doing as I did.

Oh. Minor misunderstanding. Carry on.

Post by Stephen Cowell
If sheathed banana plugs were available
(like the ones Fluke uses) then putting the resistor in
the plate circuit might be not such a stupid thing.

Not stupid at all, as long as one remembers what's hot and what's not.
Pain has an uncanny way of reinforcing one's memory, doesn't it? If
the sense resistor is soldered in-circuit, there'd be even less of a
chance of "memory reinforcement".

;o)

-DC

[j***@gmail.com]

Alright everyone, I seem to have solved part of the problem....

I went over the amp in it's entirety, and found that I had not fitted
the link across the HT capacitor, as specified in the conversion
directions. Oops.

After fixing that, the amp now gives me a reading on Pin 3!! However, I
still have to max out the bias trimmer to get around 33-35mA on the
cathodes.

Also, Pin 3 reads only 480V when the bias is maxed out. So, when I do
the math, the tubes are only dissapating 16 or 17 watts. Isn't that
rather low for an EL34?

Is the bias circuit still not letting enough juice through?

[Walter Campbell]

Post by j***@gmail.com
Hello all. I'm fairly new to the world of amp modding, and all my
attempts have been successful....until now.
I recently converted my Dual Reverb 4100 to EL34 specs, as described on
this page: http://web.aoct.org/marshall/jcm900-tube-conversion.txt
I have attempted to set the bias according to Lord Valve's directions
(read Pin 5 without tubes, set max neg voltage, read Pin 3 with tubes,
read 1 ohm resistors, multiply readings, and adjust bias accordingly).
I get a max negative voltage reading of -89 on Pin 5 with the tubes
OUT. But when I put the EL34's in, I get no negative voltage reading on
Pin 3. As well, I get no mA reading across the 1ohm resistors on the
cathodes.
Any ideas as to what could be happening?

Let's get this straight...

"
But when I put the EL34's in, I get no negative voltage reading on

Post by j***@gmail.com
Pin 3."

You shouldn't get a *negative voltage* reading on pin 3. that's the
plate and you should get a very high positive reading. The negative
voltage is still being applied (we hope) on pin 5. You're looking for
the plate (positive) voltage to use with the below info to get max plate
dissipation.

second:
"
As well, I get no mA reading across the 1ohm resistors on the

Post by j***@gmail.com
cathodes. "

You should be looking for a voltage (mV) reading and converting to amps
by dividing by 1 (that's why you're using the 1 ohm resistors in series
between the cathode and ground). Plug the numbers into the formula and
see what you get and adjust the bias accordingly.

In my experience it is sometimes necessary to swap in a different value
fixed resistor into the bias circuit when converting from 5881s to
EL-34s because as shipped the bias circuit sometimes isn't capable of
producing sufficient negative voltage to adequately bias an EL-34.
Fairly easy to do, but can be tedious on a printed circuit board amp
like the JCM-900.

Walt Campbell
Campbell Sound
http://www.campbellsound.com/

[j***@gmail.com]

Walter....replying to your last point, that's exactly what I did. I
modified the bias circuit as per these directions:
http://web.aoct.org/marshall/jcm900-tube-conversion.txt

Putting aside the fact that I'm not getting any reading on Pin 3, I
find it odd that with the bias trimmer maxed out, I'm only getting
about 13 or 14 mA on the cathodes. Shouldn't EL34's be between 30-50 mA
when properly biased?

It seems to me that the bias circuit isn't getting enough power through
to the tubes?

[Mike Schway]

Post by j***@gmail.com
Walter....replying to your last point, that's exactly what I did. I
http://web.aoct.org/marshall/jcm900-tube-conversion.txt
Putting aside the fact that I'm not getting any reading on Pin 3, I
find it odd that with the bias trimmer maxed out, I'm only getting
about 13 or 14 mA on the cathodes. Shouldn't EL34's be between 30-50 mA
when properly biased?
It seems to me that the bias circuit isn't getting enough power through
to the tubes?

What's your voltage range on pin 5 when you rotate the bias pot? You
should be able to get -35V with the pot somewhere in the midrange. If
you can only get "up" to -40V with the pot fully turned, the bias supply
is still set up for 5881s.

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

[Walter Campbell]

Post by j***@gmail.com
Walter....replying to your last point, that's exactly what I did. I
http://web.aoct.org/marshall/jcm900-tube-conversion.txt
Putting aside the fact that I'm not getting any reading on Pin 3, I
find it odd that with the bias trimmer maxed out, I'm only getting
about 13 or 14 mA on the cathodes. Shouldn't EL34's be between 30-50 mA
when properly biased?
It seems to me that the bias circuit isn't getting enough power through
to the tubes?

What negative voltage reading are you getting on pin 5? that's the grid
and that's where the bias voltage should be. If you've got too high a
bias voltage you can easily have "turned off" the tube (valve <g>) and
that would give you a very small reading on the cathode.

And again.... when you say you're not getting any reading on pin 3, what
do you mean? You should be getting a very high (around 400-500 volt DC)
plate current reading there and *no* negative DC reading. If you're not
getting "any" reading there (no plate current?) then that's not good and
could also explain the low cathode current flow.

Check the bias current (negative DC) at the bias pot, at the junction of
the bias resistors (150K?, should be around -50 for 5881s and -40 or so
for EL-34s according to the schematics I looked at), at the other side
of the bias resistors, at the grid stoppers, and at pin 5. What do you
get?

Without that info we're shooting blind here...

Walt Campbell
Campbell Sound
http://www.campbellsound.com/

[n***@real.com]

On Sun, 23 Jul 2006 11:05:02 -0700, Walter Campbell

... You should be getting a very high (around 400-500 volt DC)
plate current reading there and *no* negative DC reading. If you're not
getting "any" reading there (no plate current?) then that's not good and
could also explain the low cathode current flow.
Check the bias current (negative DC) at the bias pot, at the junction of
the bias resistors (150K?, should be around -50 for 5881s and -40 or so
for EL-34s according to the schematics I looked at), at the other side
of the bias resistors, at the grid stoppers, and at pin 5. What do you
get?

Your advise is excellent but some might get confused when you refer to
voltage readings as current. Sorry to be pedantic...

regards,
(!)

[Dave Curtis]

Post by n***@real.com
On Sun, 23 Jul 2006 11:05:02 -0700, Walter Campbell

... You should be getting a very high (around 400-500 volt DC)
plate current reading there and *no* negative DC reading. If you're not
getting "any" reading there (no plate current?) then that's not good and
could also explain the low cathode current flow.
Check the bias current (negative DC) at the bias pot, at the junction of
the bias resistors (150K?, should be around -50 for 5881s and -40 or so
for EL-34s according to the schematics I looked at), at the other side
of the bias resistors, at the grid stoppers, and at pin 5. What do you
get?

Your advise is excellent but some might get confused when you refer to
voltage readings as current. Sorry to be pedantic...
regards,
(!)

Not quite excellent then, huh?

-DC

[Walter Campbell]

Post by n***@real.com
On Sun, 23 Jul 2006 11:05:02 -0700, Walter Campbell

... You should be getting a very high (around 400-500 volt DC)
plate current reading there and *no* negative DC reading. If you're not
getting "any" reading there (no plate current?) then that's not good and
could also explain the low cathode current flow.
Check the bias current (negative DC) at the bias pot, at the junction of
the bias resistors (150K?, should be around -50 for 5881s and -40 or so
for EL-34s according to the schematics I looked at), at the other side
of the bias resistors, at the grid stoppers, and at pin 5. What do you
get?

Your advise is excellent but some might get confused when you refer to
voltage readings as current. Sorry to be pedantic...
regards,
(!)

not pedantic, accurate. my fingers got ahead of my brain (again <G>).

Walt Campbell
Campbell Sound
http://www.campbellsound.com/