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Hey everyone! I'm experiencing the same thing as this poster:
https://music-electronics-forum.com/...illation-noise
Here is what I know:
-the amp has a louder-than-normal hum when the bias lights are in the optimal "green" zone
-I can decrease the hum by increasing the bias (making it more negative) but obviously that isn't "working as intended"
-I can eliminate the hum by shorting either the Slave out or the preamp out jacks
-tube rolling the 12AX7 or 12AU7s makes no change
-no change to the preamp controls makes a difference
-bypassing the 220ohm resistors on the preamp board makes no difference
I'm not seeing any obvious power supply ripple - the amp has had a fresh cap job, all the diodes in the bias detection circuit test okay, all voltages are dead on with the schematic.
I don't see/hear any noise coming from the preamp (test at R33 on the preamp board) which is odd to me since killing the preamp signal at the slave or preamp out jack kills the noise.
Shorting the AC component of the driver 12AX7 pin 1 kills the noise.
I'm not super worried about the oscillation yet as the hum is pretty terrible, but that presents as a high pitched squeal after the amp has been running for some time.
Any thoughts or other interesting things to try? I did also resolder all the problem points while I did the cap job (tube sockets, connectors, AC power board). Thanks!
https://music-electronics-forum.com/...illation-noise
Here is what I know:
-the amp has a louder-than-normal hum when the bias lights are in the optimal "green" zone
-I can decrease the hum by increasing the bias (making it more negative) but obviously that isn't "working as intended"
-I can eliminate the hum by shorting either the Slave out or the preamp out jacks
-tube rolling the 12AX7 or 12AU7s makes no change
-no change to the preamp controls makes a difference
-bypassing the 220ohm resistors on the preamp board makes no difference
I'm not seeing any obvious power supply ripple - the amp has had a fresh cap job, all the diodes in the bias detection circuit test okay, all voltages are dead on with the schematic.
I don't see/hear any noise coming from the preamp (test at R33 on the preamp board) which is odd to me since killing the preamp signal at the slave or preamp out jack kills the noise.
Shorting the AC component of the driver 12AX7 pin 1 kills the noise.
I'm not super worried about the oscillation yet as the hum is pretty terrible, but that presents as a high pitched squeal after the amp has been running for some time.
Any thoughts or other interesting things to try? I did also resolder all the problem points while I did the cap job (tube sockets, connectors, AC power board). Thanks!
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