2. amps-preamps
  3. 1405363835

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
 
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.
ampman1961 OP
4 posts
 
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.
Billy
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.
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