Inside the amp was a handwritten note that said:
"Dear Repairman: Amp is noisy on clean channel only. Volume control makes little difference in the loudness of the noise (it is noisy when volume is at 0). Noise sounds like wind blowing across a live microphone. It is not always noisy. Sometimes it happens as soon as you turn it on, other times it may be 30 minutes. Noise can last 10 seconds or 1 hour. Nothing has to be plugged into it for the noise to happen. I have checked tubes and they appear to be ok, none are microphonic. If however you find something wrong with a power tube, please replace all with a matched set and set bias. Hope this helps!"
I replaced a bad preamp tube and some noisy plate resistors.
"Dear Repairman: Amp is noisy on clean channel only. Volume control makes little difference in the loudness of the noise (it is noisy when volume is at 0). Noise sounds like wind blowing across a live microphone. It is not always noisy. Sometimes it happens as soon as you turn it on, other times it may be 30 minutes. Noise can last 10 seconds or 1 hour. Nothing has to be plugged into it for the noise to happen. I have checked tubes and they appear to be ok, none are microphonic. If however you find something wrong with a power tube, please replace all with a matched set and set bias. Hope this helps!"
I replaced a bad preamp tube and some noisy plate resistors.
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