Hum in the boost circuit of a silverface MV Twin Reverb.

[Mark]

I’m still working on a Silverface Master Volume Twin Reverb with the boost switch. It initially had a blown power transformer which I got rewound, then I noticed the board was conductive which I mentioned on this board. This was allowing the tremolo/vibrato oscillator to bleed into the audio (P.I. stage) I found the board was the culprit, I got the wax off the top but it made no difference. I have the components currently suspended in air and there is a bit of thumping but it’s vastly reduced and turning intensity down reduces the thumping.

I thought this would be it and I could close the amp up. I find that when I pull the boost switch with the master on ten I get hum. The boost switch takes an output from the secondary of the reverb transformer and inputs it into a triode and finally to the master volume control. I lift the wire attached to the send socket of the reverb secondary and the hum goes away. I swap 12AT7’s and no difference. I earth the input to the 12AT7 which is attached to the reverb transformer and no difference. The power supply caps are brand new, the choke measure 92 ohms which is what I expect, the ac voltage on the rails after the choke is 0.1vac so that’s negligible. I tried lifting the earth of the secondary if the reverb transformer and as you’d expect the hum goes away. I did try and earth the secondary in different locations and it made no difference.

Any thoughts on this one?