Hi,
A friend of mine bought an used F-50 and asked for retubing. After changing all tubes besides the well known hiss issue in these amps I also noticed that the clean channel has a pronounced hum (the more you turn the gain up the more hum you get). I narrowed down the problem to the input stage V2B. When grid (pin 7) is grounded the problem disappears. When the 1M5/180p combo is shorted the hum is almost completely gone. Tried different tubes in V2 position without any improvement.
I was also suspecting bad filtering of the relay supply but adding more filtering didn't help. Drive channel doesn't have the hum.
So is this 1M5/180p hum a "built in" issue in these amps or I'm missing something? Any ideas would be appreciated.
F 50.pdf
A friend of mine bought an used F-50 and asked for retubing. After changing all tubes besides the well known hiss issue in these amps I also noticed that the clean channel has a pronounced hum (the more you turn the gain up the more hum you get). I narrowed down the problem to the input stage V2B. When grid (pin 7) is grounded the problem disappears. When the 1M5/180p combo is shorted the hum is almost completely gone. Tried different tubes in V2 position without any improvement.
I was also suspecting bad filtering of the relay supply but adding more filtering didn't help. Drive channel doesn't have the hum.
So is this 1M5/180p hum a "built in" issue in these amps or I'm missing something? Any ideas would be appreciated.
F 50.pdf
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