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I think I have the answer to your strange LF oscillation in the amp.
I just finished repairs on an HR-1A amp and got the same strange LF oscillation when I switched reverb on. I checked my input power/test connector, which uses a line plug plus 3 wires for ground, G+, and G- attached to a 6-pin socket. One of the G wires had come loose. I reconnected it, and the oscillation disappeared. You must have a bad connection somewhere. I can't imagine anything else producing exactly the same condition. It would appear that the combination of capacitors and resistors between the reverb amp and the output driver forms a LF oscillator if one of the G wires lacks a ground reference.
120Hz background hum at idle with speakers attached is 3.8mV. B+ runs about 20V high. Good luck!
Edit: To be honest, I've never really liked this aspect of Hammond's design -- having to get the ground reference for a power amp driver stage from the organ preamp's output transformer via a ~40' cable and several connectors.
Last edited by David Anderson; 02-26-2017 at 06:55 PM.
I'm David. 'Dave' is someone else's name.