I finished debugging and restoring a pair of Mk IIIs and am quite happy with them. Now I am rebuilding a ST-70 for the first time. Two of the four EL 34 outputs (one from of each pair) show dull red glowing plates. This is barely visible with normal lighting, but quite clearly visible in the dark. I lowered the bias to 1.4V and see (maybe) a small change but 2 of the four still show some red glow. The amp is stock and has the stock bias circuit in place. I am assuming that any plate glow indicates a problem. The tubes are marked Dynaco and are presumably the original matched pairs. I don't have a curve tracer to use. Any ideas?
I also have unacceptable hum from one channel that seems to follow one of the 7199s. Is there a source of (quiet) 7199 anywhere these days? I am not considering a board upgrade for this amp at this time--even though that usually allows separate biasing of all four outputs. I plan to keep this particular amp very stock.
...j
I also have unacceptable hum from one channel that seems to follow one of the 7199s. Is there a source of (quiet) 7199 anywhere these days? I am not considering a board upgrade for this amp at this time--even though that usually allows separate biasing of all four outputs. I plan to keep this particular amp very stock.
...j