This amp is really pissing me off. Has a nasty hum and noise on all channels, preamp controls have no effect on it. The noise is kind of a whooshing sound (sometimes some pops) that does reduce after warming up but the hum stays the same. Plugging straight into the effects return it is still there.
I have tried different PI tubes, there seems to be some effect on the hum/noise but it is always there.
I have cleaned all the cable and jack contacts, as I know these are problematic on these amps. Moving wires around with wooden stick couldn't find anything that impacted the hum/noise.
Pull all the preamp tubes other than the PI and the hum/noise is the same. Pull the PI tube and it is dead quiet. Replaced C42-C48 and C57, no effect. Unfortunately don't have high voltage probes for my scope to directly probe around the PI. Using the hum balance can change the noise but not really reduce it much, looking at speaker signal with scope shows that the hum balance can affect 60Hz hum but it seems like most of the audible hum is higher harmonics.
I know that there is some sawtooth ripple on the -56V, even though C46 and 47 were replaced. I was surprised it didn't have hum in the 6L6 section just from that but I guess since it is common mode on a push-pull section it doesn't?
Here is the schematic: https://www.thetubestore.com/lib/the...-Schematic.pdf
Any thoughts on what to do next?
Greg
I have tried different PI tubes, there seems to be some effect on the hum/noise but it is always there.
I have cleaned all the cable and jack contacts, as I know these are problematic on these amps. Moving wires around with wooden stick couldn't find anything that impacted the hum/noise.
Pull all the preamp tubes other than the PI and the hum/noise is the same. Pull the PI tube and it is dead quiet. Replaced C42-C48 and C57, no effect. Unfortunately don't have high voltage probes for my scope to directly probe around the PI. Using the hum balance can change the noise but not really reduce it much, looking at speaker signal with scope shows that the hum balance can affect 60Hz hum but it seems like most of the audible hum is higher harmonics.
I know that there is some sawtooth ripple on the -56V, even though C46 and 47 were replaced. I was surprised it didn't have hum in the 6L6 section just from that but I guess since it is common mode on a push-pull section it doesn't?
Here is the schematic: https://www.thetubestore.com/lib/the...-Schematic.pdf
Any thoughts on what to do next?
Greg
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