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Hi,
As you know these amps are notorious for this problem. A friend brought one where after 15 min of operation without signal one pair of tubes would start overheating due to bias shift. From what I read this PCB version is one of the worst.
I followed some of the guides in the net. What I did so far.
1/ replaced all bias resistors with metal film 1W
2/ replaced all PI resistors (except for the 1M ones) with metal film 1W and CF 1W (plates).
3/ replaced PI coupling caps and fizz cap (WIMA)
4/ milled around pins 5 of power tube sockets (the 5k6 ones look like they are touching power tubes' pins but they don't)
5/ tried another set of tubes without change
Now the bias drift is much slower but it's still there and on the same side (left pair of tube). Over the course of 10-15 mins the bias would shift 1Volt up and idle current would go from 75mA to 90mA.
Most puzzling however is PI plates' voltage drift. When switching the HV on pin6 is 255V, pin1 278V. Over the time (10-15min) pin6 voltage is dropping to 199V and pi1 voltage goes up to 330V.
As a side detail pressing the Mute button affects the bias voltage only with couple of mAs although I changed the caps going to the Line out circuit as suggested in another thread.
Any ideas and comments would be appreciated.
As you know these amps are notorious for this problem. A friend brought one where after 15 min of operation without signal one pair of tubes would start overheating due to bias shift. From what I read this PCB version is one of the worst.
I followed some of the guides in the net. What I did so far.
1/ replaced all bias resistors with metal film 1W
2/ replaced all PI resistors (except for the 1M ones) with metal film 1W and CF 1W (plates).
3/ replaced PI coupling caps and fizz cap (WIMA)
4/ milled around pins 5 of power tube sockets (the 5k6 ones look like they are touching power tubes' pins but they don't)
5/ tried another set of tubes without change
Now the bias drift is much slower but it's still there and on the same side (left pair of tube). Over the course of 10-15 mins the bias would shift 1Volt up and idle current would go from 75mA to 90mA.
Most puzzling however is PI plates' voltage drift. When switching the HV on pin6 is 255V, pin1 278V. Over the time (10-15min) pin6 voltage is dropping to 199V and pi1 voltage goes up to 330V.
As a side detail pressing the Mute button affects the bias voltage only with couple of mAs although I changed the caps going to the Line out circuit as suggested in another thread.
Any ideas and comments would be appreciated.
I have plenty of it without ill behaved TSL100's.
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