I'm working on a silverface Bassman head. It has been blackfaced, changed to a level bias setup, but I noticed it had a lot of hiss and just seemed unstable, so I left the caps to ground on the power tubes. Initially I thought the hiss was from plate resistors, so I changed those out and it lowered it a bit, also did the power supply resistors (caps are sprague atoms for the 1st pair, grey illionois for the rest).
Played the amp, all seemed fine, gave it back to my buddy. He played it a while, no problems, then jumpered the channels and kept playing. After a half hour or so, he said the volume started dropping and he heard a buzzing sound that got louder and louder until he shut it off.
I was thinking it was from the grids of the preamp tubes having unshielded wire, so I replaced them with shielded. Played it, and it still seems unstable, or on the verge of oscillation. Even with the volume controls down, there is a good amount of hiss, and it doesn't stop entirely until I pull the 2nd tube (gain stage for both channels).
I have a scope but don't know how to use it to find oscillations. How do I proceed?
Played the amp, all seemed fine, gave it back to my buddy. He played it a while, no problems, then jumpered the channels and kept playing. After a half hour or so, he said the volume started dropping and he heard a buzzing sound that got louder and louder until he shut it off.
I was thinking it was from the grids of the preamp tubes having unshielded wire, so I replaced them with shielded. Played it, and it still seems unstable, or on the verge of oscillation. Even with the volume controls down, there is a good amount of hiss, and it doesn't stop entirely until I pull the 2nd tube (gain stage for both channels).
I have a scope but don't know how to use it to find oscillations. How do I proceed?
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