Marshall jcm2000 dsl 100 Bias low

[pbdog]

I'm getting only -34 volts on Pin 5 of the EL34s sockets.I haven't had time to dig any deep yet.Maybe Sunday.Is there anything I should look for first.The old el34s were cooked. Thanks pb

[Enzo]

Flip your meter to AC and measure that bias - looking to see how much ripple is there - if there is a ton, then the filter caps in the bias suply are not doing their job. Alternatively a very leaky cap could be dragging the bias down. And is this at both sockets? If only one, a leaky coupling cap could be the problem. After all that explore the voltaeg divider resistors in the bias supply.

[Amp Kat]

And the negative grid voltage doesn't always meand that is a relationship to current flow. -35ma's always seems to come up when you do the calculation on an El-34 at around 450 to 500 volts at around 70% dissipation. There are much hotter EL34's around that many are using that draw more screen current and overall current. Although you do want your bias voltage rock solid you do need to calculate how much current is flowing through the tube with either a bias rite meter,shunt test or current sense test. Current production tubes played a bunch will simply burn out faster than most NOS even if the bias is right on and solid which it may be but do the test and find out for sure.

[pbdog]

Low bias on one pair of sockets.

You think it could be one of the 22@630v caps in the PI leaking ? pb

[pbdog]

Thats .022@630volts.

The Caps C7 on the schematic.pb

[Enzo]

Did you do the test I suggested? How much ripple voltage is on the bias supply?

If you think an interstage (coupling) cap is leaky, disconnect one end of it. If the bias returns to normal, then it was in fact leaky. If the bias remains unchanged it was not.

Was it both tubes the same or just one? If both, that would mean BOTH coupling caps were leaky - not very likely. Just one - pretty likely.

Make SURE the polarity is right on those filters.

You have low voltage at the grids, but what is it at the various steps in the bias supply circuit?

[pbdog]

no AC ripple voltage.

It's got low voltage up one side of the circuit to the coupling cap.I'll try lifting one end of the cap.Polaritys good on the filters. Thanks Enzo.pb

[pbdog]

New coupling caps on PI end no change.

I replaced the .022@630v at the PI.No change.Is there any other problems with the bias in this model? Thanks pb

[Enzo]

With power off and circuits discharged, have you verified the resistors are close to value? A 220k resistor that wants to be more like 2.2Meg will mess with you.

So if I read you right, bias is OK at one tube but not at the other? If bias is off at only one side, then there is something about that side. If you eliminated the coupling cap, then we are down to bad resistors or conductive crap on the circuit board, or something.

[Amp Kat]

Yeah Enzo is right on that there isn't very much left but a few resistors and lead dress. You know this sounds crazy but once again that rectifier in these Marshalls that heats up is tied to gound and all of those grounds are tied together so it is possible that it is effecting the bias through ground so you may want to shoot at changing it and with a higher rated rectifier. Some of these are a bitch to change as you have to take the whole power tube board off to get to it. That's the only other thing I can think of.

[pbdog]

Bias circuit board.

Could it be that little bias board with the 22k pots on it? Where can I buy one? Everything else checks OK.Thanks pb

[Enzo]

Unplug it and check the pots with your ohm meter. It is just a couple little trim pots, they can be replaced. FIND OUT if it is the problem.

[drewl]

I just had one last week that had those bias pots intermittant/flaky.
I replaced them with higher wattage pots.
they seem awful cheesy/flimsy to be handling such an important job...what say you Enzo?

[Enzo]

In my view, they are not working very hard, I am not so concerned about power handling, but if a heftier power rating includes more mechanical strength, I'm all for it.

[tim]

I had this same problem on one of the DSL's I had in my shop about a year ago...I pulled my hair out for weeks before I found it. I don't have the info here and it's not coming to me where the part is on the schematic. I know it's one of the little white pf type caps on the end of the one of the preamp boards...as I remember, with the amp upside down and the front towards you, it's on the right hand end. there are like three or four of those caps in one area and it's one of those...it starts to short out and causes the bias voltage to get dragged low. If I can't remember where it was this weekend, I'll go through my notes when I get to the shop on Monday and find out exactly where it is.