Bias voltage range on JCM-800?

[Mark]

I’m working on a JCM-800 and I find that the bias voltage ranges from -58VDc to -42VDC.

I have tried the EL-34’s that the amp came in with and a new set. I find with a new Marshall branded set if EL-34’s the maximum they will pull is 27mA.

The bias circuit is incredibly simple. A diode, two resistors, two caps and a pot. The caps are original, but should be fine as I measured them with my Peak Electronics ESR-70, the resistors look good too though the 56K is difficult to measure as the meter tries to charge one of the bias caps.

Have any of you guys encountered this?
Advice appreciated.

https://www.drtube.com/schematics/marshall/jcm800pw.gif

[Roe]

I’m working on a JCM-800 and I find that the bias voltage ranges from -58VDc to -42VDC.

I have tried the EL-34’s that the amp came in with and a new set. I find with a new Marshall branded set if EL-34’s the maximum they will pull is 27mA.

The bias circuit is incredibly simple. A diode, two resistors, two caps and a pot. The caps are original, but should be fine as I measured them with my Peak Electronics ESR-70, the resistors look good too though the 56K is difficult to measure as the meter tries to charge one of the bias caps.

Have any of you guys encountered this?
Advice appreciated.

https://www.drtube.com/schematics/marshall/jcm800pw.gif

to get the right bias range you often need to replace a resistor

[statorvane]

Maybe the amplifier was originally set up for 6550s. Check out the bias circuit here:

https://mhuss.com/MyJCM/JCM800_2204.gif

[Stevem]

With a -58 available I would also say that the amp was born with 6550s.
With 34s in the amp you want to have the low end of the bias range go down to like-36.

Start off with adding in series to the input side of the bias adjust pot a 10k to and then see what that gives you .

Of course do this bias voltage test without having the output tubes in the amp.

[danman]

I have often needed to drop the value of the resistor that is inline with the bias pot. If it is a 47k or 56k, lower it down to the next value and try again.

[Mark]

Thanks for your advice I appreciate that.