• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Marshall JCM900 rebiasing

Carlosraj

Member
2010-01-17 11:12 am
Hi


I have this JCM900 Model 4100 which is originally fitted with 5881 tubes. A friend bought 4 new 6l6s and asked me to replace them. After fitting them, when I turn on the amp, there is some low hum from the amp. The bias was at -50V, when I set the bias to minimum (-63V), the hum has reduced very much but it is still there. How do I set the bias. I heard that some resistors value need to be change. If so, which are the resistors and what are the values. Please assist.


Thanks
 
Is this the right one?
 

Attachments

  • Marshall_jcm900_dual_reverb_jmp50a_poweramp_psu_4100_60_02_2_iss11.gif
    Marshall_jcm900_dual_reverb_jmp50a_poweramp_psu_4100_60_02_2_iss11.gif
    74.7 KB · Views: 83

6A3sUMMER

Member
2016-06-07 6:50 am
How much hum was there Before you took the 5881 tubes out, and put in the 6L6s?
Always check an amp before you modify it, including tube rolling.
If you did not do that, then put the 5881 tubes back in and check the hum.

Tube rolling is probably not the cause of the hum, but here are some tube differences:

5881 plate dissipation is 23 Watts; Screen 3 Watts dissipation, 270V.

6L6, 6L6G, 6L6GA, 6L6GB plate dissipation is only 19 Watts; Screen is only 2.5 Watts dissipation, 270V.

6L6GC plate dissipation is 30 Watts; screen is 5 Watts dissipation, 450V.

All tube substitutes are created equal, some tube substitutes are more equal than others (stolen from "Animal Farm" by George Orwell).
 
Last edited:

Zung

Member
2005-04-30 7:30 pm
Geneva
Some substitutes are simply other tubes, with model numbers stamped at random: i have a bunch of Sovtek 6L6WXT's that are way different, requiring 17V instead of 14V to bias near the nominal 72mA as per the databooks.

In your case, you might want to increase the bias by perhaps decreasing R29 (8K2 to 4K7, or even lower). Be sure to properly measure the cathode current, either by inserting a 10 ohms resistor in serie with the cathode, or using a bias probe; 25-35 mA is about right, fine tune according to the desired tone.
 
Some substitutes are simply other tubes, with model numbers stamped at random: i have a bunch of Sovtek 6L6WXT's that are way different, requiring 17V instead of 14V to bias near the nominal 72mA as per the databooks.

In your case, you might want to increase the bias by perhaps decreasing R29 (8K2 to 4K7, or even lower). Be sure to properly measure the cathode current, either by inserting a 10 ohms resistor in serie with the cathode, or using a bias probe; 25-35 mA is about right, fine tune according to the desired tone.

Why not parallel another resistor across R29 and use a switch to put it in circuit if needed?
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
...when I set the bias to minimum (-63V), the hum has reduced very much but it is still there. How do I set the bias....

You need to be able to *measure* the bias current in the output tubes. Can you?

Turning down to near zero current is shutting-off the tubes. Yes, hum goes down but so do small signals. Bias for appropriate current. Treating hum flaws is a different step.
 
I have this JCM900 Model 4100 which is originally fitted with 5881 tubes. A friend bought 4 new 6l6s and asked me to replace them. After fitting them, when I turn on the amp, there is some low hum from the amp. The bias was at -50V, when I set the bias to minimum (-63V), the hum has reduced very much but it is still there. How do I set the bias. I heard that some resistors value need to be change. If so, which are the resistors and what are the values.
Repeat with me: BIAS IS NOT A HUM CONTROL DEVICE

Bias is set to a given idle current so you avoid crossover distortion.

Setting tubes to cutoff (-63V :eek: ) is a TERRIBLE idea.

Set bias as specified by Marshall or by feeding a sinewave and adjusting for minimum visible crossover or by having tubes dissipate at idle soe 60% of full plate dissipation or simply set it back to original -50V, at least better than MESSING bias without clue.

Hum self cancels in the OT if it appears with same intensity on both halves of a push pull power stage so your hearing it means both halves are NOT matched.

measure individual tube cathode current, I think each has an individual 1 or 10 ohm resistor from cathode to ground, and if not, add it.

Then mix the tubes you have until both halves match.

Absolute worst case, live with a little hum which is only heard in a bedroom setting, while this amp is designed for ear bleeding sound level.
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> sinewave and adjusting for minimum visible crossover or by having tubes dissipate at idle soe 60% of full plate dissipation

Many guitar amps, and quite a few hi-fi amps, trimming for minimum crossover leads to over-dissipation.

I had a Fisher. I trimmed it for best low-level sound. It sounded best when the 12W tubes were idling 18W-20W.

The 60% rule has no solid justification but is almost always near where you want a "fix"-biased guitar amp to be.

I do like the factory-spec system except modern tubes may vary a lot from what the factory was getting when the amp was new. Agree about 1r resistors in any amp you care about.
 
I don't play a guitar but I have a few thoughts I'd like to share. First of all its just a guitar amp not a 10K tube amp meant for home enjoyment. Its going to have some noise in my opinion every guitar amp I've ever seen/heard had some hum. Some have commented on a "Matched set" of output tubes but then again how long does a matched set actually stay matched? If it were mine I would modify the bias circuit with an additional resistor that could be put in circuit via a small switch. I would also install individual bias controls and bias resistors as a means of checking the bias on each tube. I would drill the chassis and install a test probe thing that your meter probe could be inserted in and the other meter lead grounded.

If there is a little hum...live with it.
 

Zung

Member
2005-04-30 7:30 pm
Geneva
Like I said, most guitar amps are designed for "tone", often with a non-negligible amount of chemicals in the body, think THC or worse, and built to a price, the lower the better. In a bed room situation, a bit of hum is quite the norm, and it doesn't matter at all when gigging; it can be objectionable when recording.

If the hum is an issue, it can be fixed: I've successfully tamed my Fender Champ by using DC for the heater of the preamp. However, my "big" amps, a Pignose GR60, and before that a Selmer T&B 50 and a Fender Deluxe Reverb clone never had that problem to start with.

With that said, "designing" an amp that demands matched output tubes to eliminate the hum requires a serious dose of chemicals in the body.

My advice to the OP: try to be a little quantitative:
  • Measure and set the bias as outlined previously
  • Measure the hum at the output: it'll change a bit depending on the knobs, but anything below 30mVac should be OK (-60dB re [email protected] ohms)