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I got this amp with a "squeal" problem. Sure enough, tightening the reverb connectors so they weren't so sloppy on the tank was one issue, re-soldering the 470/5w resistors eliminated the rest. However, the damned thing hums fairly loudly on the drive channel once you turn the master up past 2. The more master you give it the louder the hum gets. Not loud enough that you can't play over it but it's not normal in my mind. It hums in the clean channel as well but much less and it also changes with the volume and drive control on the clean channel.
Removing either of the pre-amp tubes kills the hum. Swapping the tubes doesn't change it. Plugging into the power amp in jack totally eliminates it so it has to be in the pre-amp.
There were a lot of shaky solder joints which are common in this amp from what I've seen myself and read online, so I re-soldered all of the tube, flat cable and any other connections that looked odd. After reading several other threads here and other sites I replaced the pre-amp filter cap and the rectifier diodes but the hum persists. I have good voltage on all power pins, -70v bias, and a good ground cable to the input jacks.
I don't know what else to look for, but I will add that the customer has been running a 16 ohm speaker in this for some time, even though it's supposed to have an 8 ohm speaker. I don't know if this contributed to the hum problem or not, but it seems like it would have been putting a heavy load on the OT.
Removing either of the pre-amp tubes kills the hum. Swapping the tubes doesn't change it. Plugging into the power amp in jack totally eliminates it so it has to be in the pre-amp.
There were a lot of shaky solder joints which are common in this amp from what I've seen myself and read online, so I re-soldered all of the tube, flat cable and any other connections that looked odd. After reading several other threads here and other sites I replaced the pre-amp filter cap and the rectifier diodes but the hum persists. I have good voltage on all power pins, -70v bias, and a good ground cable to the input jacks.
I don't know what else to look for, but I will add that the customer has been running a 16 ohm speaker in this for some time, even though it's supposed to have an 8 ohm speaker. I don't know if this contributed to the hum problem or not, but it seems like it would have been putting a heavy load on the OT.
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