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Volume drop when playing and hiss noise at turning off from one channel
Volume drop when playing and hiss noise at turning off from one channel
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Old 2nd March 2015, 07:11 PM   #1
giulio82 is offline giulio82
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Default Volume drop when playing and hiss noise at turning off from one channel

Hi, my name is Giulio and I'm new to this forum, so hello everyone!

I'm starting this thread because I have one of those weird issues which don't happen all the times I use the amplifier, therefore it is difficult to make an accurate diagnosis.

My brother's Copland CTA405 (an integrated tube amp), which is about 7-8 years old but with very little listening hours on its back, has an intermittent problem: quite often, but not always, I hear a very noticeable and persistent volume drop from the left channel after a few minutes of operating (with no additional noises); if I turn the amp off (which is what I normally do when this happens), I hear a hiss with some little cracklings from that same channel, lasting about 3-4 seconds. If I turn the amp on again, the problem often disappears (with no noises when I turn the amp off, this time), or it appears again after some more minutes of listening.

I already excluded a speaker problem (I recently moved to another house with new cables and speakers, with no variations in the problem) or a tube problem (if I invert the left and right preamp or power tubes, the problem is always on the left channel, when present). Bias test point voltages are steady.
As the problem only appears after the amp has warmed up, and it seems to appear less often if I bias a little colder than suggested (41mV vs 45 mV), I suppose it is something related to temperature/overheating. In your opinion, what do I have to check?

Thank you in advance,
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Old 2nd March 2015, 11:11 PM   #2
Miles Prower is offline Miles Prower  United States
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It's difficult to diagnose remotely, but it could be a couple of things. Cold solder joints could cause problems like this. It could also be a case of marginal stability. The left channel could be breaking out in oscillation at super sonic frequencies. That would account for the loss in volume. You'd need to o'scope to see the oscillation.

Marginally stable amps can frequently go unstable at power off. As the cathodes cool, and the voltage drops, so does the open loop gain. This could shift the operating point past the -1.0 + j0 Nyquist instability point. Yes, not having enough open loop gain can have detrimental consequences under gNFB. You might want to check out components in the feedback loop (phase compensating capacitor gone bad) or the Zobels across front end voltage amp plate loads, or compensating capacitors/Zobels paralleling the OPT primary. Duff parts there can compromise stability.
There are no foxes in atheistholes
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Old 3rd March 2015, 12:05 AM   #3
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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I would check for dry joints and poor connections.
Support for Fender, Marshall and all Valve Equipment; Audio Innovations, Audiorama FU29, Quad and Leak. www.jonsnell.co.uk
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Old 3rd March 2015, 05:46 PM   #4
giulio82 is offline giulio82
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Thank you very much for your help
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Old 3rd March 2015, 09:30 PM   #5
KenTajalli is offline KenTajalli  United Kingdom
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Does the amp have manual bias trimmers?
If yes, try turning the same side trimmer a little back and forth a few times, putting it back where it was.
It could be that the wiper is corroded and this could clean it up.
It could be the bias drifting due to this.
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Old 11th March 2015, 11:07 PM   #6
giulio82 is offline giulio82
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Thank you for your smart suggestion: I tried it even though there is no measurable bias drifting, and in fact I had no results.

One thing I forgot to mention is that the noise coming from the left channel at switching off, when present, is independent from the volume level, and happens even if I switch on the amp with the volume closed, I wait, and I switch off without opening the volume at all.
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