Audio Research CL-30 Bias Drift

A client brought a CL-30 in for a re-tube because the amp blew a fuse when a tube shorted. While doing my usual voltage checks without tubes installed, I noticed that each half of the amplifier has a tube socket that has a tremendous bias drift that slowly drops from -50 volts to -19 volts. The other two sockets remain at around -49.
Not having studied the amplifier schematic, does this sound like a common problem associated with these amps? This would certainly explain why it cooked a 6550. Does anyone have any suggestions in sourcing the problem? I build and repair vacuum tube guitar amps, so this sort of hybrid circuitry isn't what I see very often.
Thank you in advance.
the cathode dc supply capacitor may be blown. do a visual and then remove cap(s) and test them with cap tester.
input resistor to the grid may also get blown as well. check it with continuity tester.
it's common problem with manymany tube amps.
Here's the test condition:
No tubes installed, power on and warmed up for a minute.
The bias rail before the grid circuits measures -53 volts and maintains that value. However, from left to right as viewed from the top rear, of the 4 6550 tube sockets, the two inboard sockets slowly go down in bias voltage. This is after I have replaced all of the resistors in the circuit for good measure. This makes absolutely no sense at all, and lifting the cold side of the 1uf coupling caps to the grids confirms there is no DC leakage causing any problems. The only thing I can think is the opto couplers but it still doesn't make any sense. There should be no voltage drop without a load, and even with the tubes installed, there should be minimal if any drop with a bias supply so well regulated. I have slept on on it and I woke up without any answers. A real head scratcher.
Thanks for your response.
i think it has nothing to do with coupling caps.
it's all in DC chain not in signal...
I agree. I just stated that as a measure of test procedure. In a common PP amplifier, the bias voltage can be affected by leaky coupling caps. I see this fairly often in guitar amplifiers. I will check the DC cathode circuit as you suggested.
Replacement of the optical isolators (LDR) made a difference. The bias has stabilized but is uneven at each pair. The difference averages out to be around 1.4 volts. I can live with that after I pair up the tubes where the current is best divided amongst the pairs.
Audio Research is a prime example of pushing the envelope at the expense of simplicity. A biasing scheme should never have a problem like this. I mean, if the design goal was to "track" the bias and adjust according to impedance swing at the output, I can see why. But, troubleshooting is another matter entirely. It's like an automatic variable voltage regulator, or power scaling but to what purpose I can only theorize. Has anyone read about this type of output stage control as it relates to the CL-30 design?
I will be glad to see this go back home with the owner never to be heard from again. I'll take a pair of class A monoblock's any day!
Sorry if this heretical, but is there any wisdom is sending the amp to an authorized ARC service tech. Perhaps even back to the factory.

The CL-30 is a bit long in the tooth. The factory or an experienced ARC tech could replace worn parts and get the amp back into original or maybe even better condition.

This is no slight to Ampman. Just that ARC and its authorized service techs know their ARC patients.
Btw, I obviously don't know where Ampman lives. If he is located in the Philly/New York/Delaware area, I have a great ARC authorized tech who makes house calls and is very reasonable. He works closely with Chris Ossana back at the factory.

If Ampman is located on the West Coast, Gary (Hifigeek1) is an ARC authorized tech.

If it's the heartland, there's always good old ARC.

Just trying to help.
My name is Billy Yates and I live in Venice Ca.
My client found me through several registries regarding amplifier repair. I fixed the amplifier, and is now back in service. Apparently, there are very few resources in Los Angeles to repair this amp according to ARC standards.
The amp sat for 2 years before it came to me. I can only assume he didn't have very much luck. Fortunately, I was able to get it running again.
Thank you for the comments.
Billy Yates
That's because the bias on 2 of the 4 tubes use a servo. The best way to check that is to wait a half hour while monitoring the tubes that are part of the servo. It has a slow time constant so you will have to wait. If the servo tubes don't pull in it will require service. BTW I am an authorized factory repair facility in L.A.