Thanks very much for this. The drop in DC is/was on the order of minutes, not seconds. I had some success with a .1uf capacitor right on the input jack. I don't know if that's a dumb idea, but it pretty much works, so I think I'm going to leave it at that.
Now I am still struggling to figure out the problem with the hum on the reverb, which I think has to be somewhere after C13, as the VR4 pot affects the amount of hum. I can't find fault with any of the capacitors in that circuit, so I am really at a loss.
By the way, and just out of curiosity, is there any other transistor in the circuit that can be replaced with a JFET without any other alterations? This Q1 was a huge improvement.
No, replacing other transistors in the circuit with JFETs requires redesigns of their biasing networks and in some cases redesigns of feedback loops. In any case, it wouldn't help much if at all; JFETs are great for low-level stages driven by high impedances, and the input stage is the only one that fits that description.
Do you still have hum when you short the reverb unit's output?
If it still hums, is there any way you can reduce the loop area formed by the reverb unit, R47, C13, Q5, R26 and C14, for example by taking the negative side of C14 loose from the PCB and giving it a dedicated wire to the ground side of the reverb unit output? Is there any way to increase the distance to the power supply transformer or to rotate it?
Do you still have hum when you short the reverb unit's output?
If it still hums, is there any way you can reduce the loop area formed by the reverb unit, R47, C13, Q5, R26 and C14, for example by taking the negative side of C14 loose from the PCB and giving it a dedicated wire to the ground side of the reverb unit output? Is there any way to increase the distance to the power supply transformer or to rotate it?
I'll make a note of that for future projects. 😎
With the spring tank output (reverb return) grounded, the hum is the same.
I could try rotating the power transformer, but when I test this outside the amp chassis, the tank is already a considerable distance from the transformer. Bringing the output side of the spring tank close to the transformer does make the hum much louder.
C14 is here - I could try grounding it separately.
It's hard to imagine that the reverb hummed this much when the amp was new - I assume it's a fault somewhere.
