Marshall JCM 2000 DSL bias help

[antieatingactivist]

So my friend has a DSL that he's been playing for about 2 years on the same set of tubes. Other than sounding a little crappy, it has been fine. I put in a set of JJ el34s and biased it to 90mv on each side. It sounded fine until one of the tubes red plated. I rearranged tubes and tried again. After it warms up, the bias drops on one side and remains that way even after turning it to standby. I had some 47mfd caps, so I changed the bias caps. That didn't help. All of the resistors seem to be fine.

I suspect it is the bias pots. Does this ever happen?

It is either this, or a PCB problem as far as I can tell, and I don't think im prepared for that. I've heard that these things can be nightmares.

[Enzo]

How about one of the new tubes is bad? If one tube on one side shorts its grid to something it kills the bias to both tubes on that side. So swap both left ones for both right ones. If teh red plating tube now moves to the other side, then the tubes are bad. If the same sockets red plate regardless then the amp has a problem.

The old worn out tubes at least worked, put them back in, does the amp work OK with them?

If the amp is at fault, pull all the power tubes. Fire up the amp, and probe down each tube socket. Verify you have B+ on every pin 3 and every pin 4. And you want to find proper bias voltage on each pin 5. Let it run a while and verify those grid voltages remain where they belong. I forget, about -45v or so?

And keep this possibility in mind. Your 90ma reading is for two tubes. Normally that means 45ma per tube in the pair. But if one tube is not running in a pair, you might be able to bias up to 90ma, but then it would all be flowing through one tube, which most certainly would be red plating.

[antieatingactivist]

How about one of the new tubes is bad? If one tube on one side shorts its grid to something it kills the bias to both tubes on that side. So swap both left ones for both right ones. If teh red plating tube now moves to the other side, then the tubes are bad. If the same sockets red plate regardless then the amp has a problem.

I changed the tubes around, and it stays in the same socket. After the thing gets to the point that it runs away, The negative voltage reads about -45 on one side, -25 on the tube that is red plating, and about -35 on the other tube on that side.

I put a bias probe on the problem socket. It reads 45mv until the problem starts. It then goes above 100. shortly after it red plates.

I will try putting the old tubes back in to see if I can recreate the problem.

[Enzo]

The bias collapses to 25v, OK, but does it do that without a tube? That was the point of the tubeless test, to see if the bias voltage collapses on its own. WHile that COULD be a bad trim pot, I tend to suspect coupling caps from the phase inverter stage, in those cases.

[Mars Amp Repair]

hello,
as for the bias issue, you might wiggle the red-plating tube around to be certain you don't just have a loose tube socket pin on pin 5 in the tube socket. That would certainly cause the tube to red-plate, too.

As an aside when all is said & done & you've resolved your bias issue, don't forget to replace that little Disc cap that is installed next to the power tube that is right next to the power transformer. If it hasn't already, it will get leaky & cause you hum issues.

Replace it with a 22pf @ 2KVolt version. Most of us know about this issue.

glen

[antieatingactivist]

Turns out it was a bad tube. Once they bias got all wacky, it would stay that way even when you turned the amp on standby. When you pulled the red plating tube, the bias goes back to normal on the socket and the socket on the same side. Pulling the other tube on that side did not correct the bias. I was unable to recreate the situation with another tube set. I guess this is a lesson learned.

Anyone that works on amps every day, how often do you get a brand new tube that is bad, and what brands are they usually?

[Amp Kat]

Any tube can be bad out of box and I've seen even the best NOS go bad out of box but they are usually very good because they are just that much better. I like the JJ and the Svetlana winged C for reliability but like I said they can go bad too. I do run them thru my tube tester before firing them up.

[antieatingactivist]

Any tube can be bad out of box and I've seen even the best NOS go bad out of box but they are usually very good because they are just that much better. I like the JJ and the Svetlana winged C for reliability but like I said they can go bad too. I do run them thru my tube tester before firing them up.

I guess they don't run them very hard when they are matching them at Tube Depot. Or something could have gotten shaken loose during shipping maybe. Ill be more careful next time because getting those bias supply caps back in that PCB was no fun at all.

[Enzo]

Or they ran them for 10 hours and the tube was destined to fail at 11.

As KB said, never assume a tube is good just because it is new. ANy brand can fail. The finest tubes made can be damaged when the semi truck hauling them to you takes a railroad track at speed, and bounces them in the air.

Buy your tubes from a good seller who stands behind his tubes.