I have some issues with a reverb in a Selmer Vanguard 15W amp. I have seen that this happens quiet often aswell in other Vanguards and need advice how to improve the reverb.
The main issue is that the reverb has a feedback problem, but also some noise on the highest level. The ringing tone slowly increases whithout increasing the reverb volume. The feedback starts with revern volume around half.
I had to do a speaker change and now it has a celestion in it, which has a bigger magnet than the original Goodmans speaker and after doing the change the feedback started. And going back to the old speaker is not an option, since it had cone rub. The original tank is built without internal mounting springs and seems to be mounted pn spring to the tank and to a wooden box. It is marked E8 and sems to be a 8/12500 ohm tank. I replaced this tank with a MOD tank with the same impedance and feedback was gone.
What i noticed though is that the cathode resistor on the V3B ECC83 preamp tube (that is after the recovery tube 6BR8) was in real, and surley from factory was only 220 Ohm. In the schematics it is supposed to be 2.2 kOhm. This doesn't seem to be right? I tried chaning that to 2.2 kOhm wich is in the schematics and the fedback comes back!
The tank is mounted on rubber grommets that comes with the tank, vertically on the side of the cabinet which after testing is the least noisy position.
The original tank has in- and output grounded and tank chassi as well. I tried to lift the input ground and have that isolated as well, but that didnt give much improvement.
Using only 220 ohm cathode resistor on V3B does not seem right, or am I missing something? What else could be done to improve this circuit to tame feedback and noise?