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Well, there's your problem

Hi-fi tube amp came to me for a hum on the left channel; after some poking around it was pretty apparent that one of the cathode bypass caps let the smoke out.

Either a tube failed and shorted, or it was roasted to death from the resistor it’s piggybacking
Either a tube failed and shorted, or it was roasted to death from the resistor it’s piggybacking
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Humorously, I had the exact same part on-hand.

Illustration for article titled Well, theres your problem
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Less humorously, this amp is set up for old KT77s rated at 30+ watts and is cathode-bias class A (push-pull) so it’s trying to push 35W/tube on current wall voltage. This isn’t jiving with the new-production JJ KT77s in there (rated at 25W) at all, so I’ve just ordered some new cathode resistors to bias it back to 25 or so watts so it doesn’t eat tubes weekly.

I’m also going to move the bypass caps further from those cathode resistors when I have it apart so they don’t get cooked again.

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This is reminding me how I’ve been meaning to build myself a hi-fi set for a long time but have never gotten around to it. I have a design I did years ago... somewhere.

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