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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.
Billy
ampman1961
czarivey
5,941 posts
 
i think it has nothing to do with coupling caps.
it's all in DC chain not in signal...
ampman1961 OP
4 posts
 
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.
Thanks.
ampman1961 OP
4 posts
 
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!
Thanks
bifwynne
2,206 posts
 
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.
bifwynne
2,206 posts
 
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.
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