JCM 2000 Dual Super Lead BIAS damage

[mike1950r]

Hello,

this is my first thread:

We have a JCM 2000 Dual Super Lead with BIAS problem.
We get BIAS values between min 80mA and max 120mA
measured with TAD BIAS Master.

TAD confirmes, that they cannot offer any tube, which accepts such BIAS values.
They say, that there must be a damage in the BIAS circuit.

Thanks for any ideas, how to fix this.

Cheers mike

 

[mike1950r]

forgot the tube values:

G1 -30V
G2 406V
G3 150mV
Anode 429V
Kathode 245mV

Cheers mike

 

[Central Scrutinizer]

Welcome to the forum

Some years and models JCM 2000 DSLs are infamous for bias drift issues.
I have a 2004 JCM 2000 DSL 50 that has had no problems.

Someone more knowledgeable than I will be able to tell you more information.

Good luck.

 

[Leonard Neemoil]

Yes, welcome to the forum.

Old link, but maybe still good. This should fix it AFAIK. I think other board issues can cause problems too, so just beware.

Stable Bias Mod Kit - Dr.Tube

The Dr.Tube JCM2000 Stable Bias Mod Kit fixes the problem of the drifting bias in the Marshall JCM2000 DSL50, DSL100, TSL100 & TSL122 amplifiers.

www.drtube.com

 

[nortiks]

Hello,

this is my first thread:

We have a JCM 2000 Dual Super Lead with BIAS problem.
We get BIAS values between min 80mA and max 120mA
measured with TAD BIAS Master.

TAD confirmes, that they cannot offer any tube, which accepts such BIAS values.
They say, that there must be a damage in the BIAS circuit.

Thanks for any ideas, how to fix this.

Cheers mike

Are those measurements when the amp is hot or cold?

The infamous bias drift issue symptoms are you set it cold at 35ma or whatever and an hour later after it heats up its red plating.

So if that is the range when cold, that's not the drift issue symptom.

 

[mike1950r]

Hello to everybody,
thanks for the welcome.

Well, indeed the values are measured cold.
With such values I did not have the courage to keep on going.

Sometimes we had the case in other amps, that the BIAS pot does not deliver the range of values needed in the amp. So we modified the range a bit by changing the pre resistor value.

We tried this also with the JCM 2000 by modifying the value of R77 (10k).
We could lower this resistor and came into the acceptable range between 20 and 50 mA.
And this value rested stable with no shift.

But this cannot be the solution, cause we had to lower the value to 1k. (really a lot)

This only shows, that there is a problem in the BIAS ciruit.
I cannot imagine, that the value of R77 has been faulty designed by Marshall.

May be I should look for R68 (33k).
If this resistor gets "high ohmed" say looses it's value completely, this could perhaps cause a similar problem.

This is the only idea I have for the moment.

I'll keep you informed.

Cheers mike

 

[paul-e-mann]

Hello,

this is my first thread:

We have a JCM 2000 Dual Super Lead with BIAS problem.
We get BIAS values between min 80mA and max 120mA
measured with TAD BIAS Master.

TAD confirmes, that they cannot offer any tube, which accepts such BIAS values.
They say, that there must be a damage in the BIAS circuit.

Thanks for any ideas, how to fix this.

Cheers mike

Did you buy the amp 2nd hand? Someone may have done a resistor change to bias a certain set of tubes. You may need to change those resistors to a lower value so you can get your bias in range with your current tubes. I'm no tech but it's supposed to be an easy fix. You may want to move this thread to the workbench section so you get some tech guys to chime in.

 

[nortiks]

Hello to everybody,
thanks for the welcome.

Well, indeed the values are measured cold.
With such values I did not have the courage to keep on going.

Sometimes we had the case in other amps, that the BIAS pot does not deliver the range of values needed in the amp. So we modified the range a bit by changing the pre resistor value.

We tried this also with the JCM 2000 by modifying the value of R77 (10k).
We could lower this resistor and came into the acceptable range between 20 and 50 mA.
And this value rested stable with no shift.

But this cannot be the solution, cause we had to lower the value to 1k. (really a lot)

This only shows, that there is a problem in the BIAS ciruit.
I cannot imagine, that the value of R77 has been faulty designed by Marshall.

May be I should look for R68 (33k).
If this resistor gets "high ohmed" say looses it's value completely, this could perhaps cause a similar problem.

This is the only idea I have for the moment.

I'll keep you informed.

Cheers mike

View attachment 146817

View attachment 146816

You want to be 100% sure the revision of schematic matches the components on the board.

Unfortunately I only have multiple years of schematics for the DSL50 since that's what I own and had to repair, but looking at ISS3 (1997) for it, R77 is 10K and the grid blocking resistors are 220K. The ISS7 (2003) revision shows R77 as 3K9 and the grid blockers at 5K6. So imo, make sure everything in the bias circuit including grid blockers match the schematic you have. FWIW, consensus is that the early version with 220K grid blockers sound better after being converted to the newer 5.6K topology.

Imo, also verify the cathode resistors to make 100% sure they measure 1 ohm since the bias measurement is 100% dependent on those values being right.

Also imo, measure the bias AC output of the transformer to verify its in spec.

This information should give you a good list of what can be conclusively ruled out if they all check out.

 

[mike1950r]

Did you buy the amp 2nd hand? Someone may have done a resistor change to bias a certain set of tubes. You may need to change those resistors to a lower value so you can get your bias in range with your current tubes. I'm no tech but it's supposed to be an easy fix. You may want to move this thread to the workbench section so you get some tech guys to chime in.

Hi Paul,
it's not my amp.
I got it in the workshop for checking out.
Sorry if I did not open this thread in the right section.
Cheers mike

 

[mike1950r]

You want to be 100% sure the revision of schematic matches the components on the board.

Unfortunately I only have multiple years of schematics for the DSL50 since that's what I own and had to repair, but looking at ISS3 (1997) for it, R77 is 10K and the grid blocking resistors are 220K. The ISS7 (2003) revision shows R77 as 3K9 and the grid blockers at 5K6. So imo, make sure everything in the bias circuit including grid blockers match the schematic you have. FWIW, consensus is that the early version with 220K grid blockers sound better after being converted to the newer 5.6K topology.

Imo, also verify the cathode resistors to make 100% sure they measure 1 ohm since the bias measurement is 100% dependent on those values being right.

Also imo, measure the bias AC output of the transformer to verify its in spec.

This information should give you a good list of what can be conclusively ruled out if they all check out.

Hi Nortiks,
thanks for the information.
The values from the older versions are interesting and come nearer to the value I tried.
BTW My idea of R68 did not bring me further. The value is alright.
Transformer output and Kathode Resistor are OK.
Anyway it should be a good idea, if the user buys best fitting tubes from his distributor.
The tubes that were in this one (Svetlana) were still working even with these terrible values.
Cheers mike

 

[nortiks]

IMO, I'd replace the 3 47uF filter caps. If any of those get somewhat conductive, it will pull the bias voltage towards the ground rail, and cause the tubes to run hot. Especially C42 since it would affect both tubes the same, which is the symptom you have. Of course jmo, based on the information exchanged.

 

[mike1950r]

IMO, I'd replace the 3 47uF filter caps. If any of those get somewhat conductive, it will pull the bias voltage towards the ground rail, and cause the tubes to run hot. Especially C42 since it would affect both tubes the same, which is the symptom you have. Of course jmo, based on the information exchanged.

thanks Nortiks,
for the details.
I will check this.
Cheers mike

 

[mike1950r]

Hi Nortiks,
the filter caps are all OK.
Best results I have by lowering R77.
TAD proposes BIAS values for Marshall 100 Watt EL34 between 25 mA and 40 mA.
I'm in this values now and have tested the amp.
Sounds excellent and do not get any red plating.
So your (2003) revision values are a good solution.
Thanks all for your help.
Cheers mike