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After some weeks of successful use, I had a strange noise coming from one channel (sometimes sort of a whistle, other times an intermittent crackling sound) - traded out the driver tube for a new one, no change - traded out the output tube and it was silent again. Left the amp on a few minutes with some test speakers to be sure everything was OK before putting it back in the system and suddenly it started smoking. The cathode resistors in the channel that was noisy (R16 a, b, c) burned big time - board delaminated, adjacent coupling cap sort of melted on exterior.

Greatly appreciate anybody's idea as to what might have happened? Or where to look? Thanks very much!
Charlie

Hi Charlie,
Sorry to hear this. Was this a stock build? Can you post any pix? It sounds like a short across the tube in order to smoke those resistors. Maybe a short on the socket (since you swapped tubes) or leaky coupling caps? Also check R9 and R10 to ground (should see 475K). Check C1 and C2 and make sure they are open circuit.

Shannon