Ampeg V4 Troubleshooting Help

[Diablo]

If the diodes work, why replace them? They don't wear out, go stale, or whatever.

Hum on standby is probably just magnetic coupling between the power and output transformers, it's nothing to be worried about.

Since I don't know how to test the diodes, I thought I may as well just replace them. I figured (maybe wrongly) that a diode could be leaking, passing AC where it shouldn't and making the hum. The PT and OT are on opposite sides of the amp - you think it could magnetically couple at that distance?

[Enzo]

Yes, they can and do couple at distance. The distance from one end of the chassis to the other is not a long way.

If diodes get leaky to the point of passing AC, you will have exploding caps and blowing fuses.

[Diablo]

I did a little more testing. I removed the four power tubes and put the amp in standby mode. The speakers still hummed at the same volume as with the tubes in. I flipped the impedance switch selector from 8 ohms to 4 ohms and the speakers still hummed. I set the impedance switch at 2 ohms at the speakers went dead quiet. I repeated the test and the results were the same. Do you think I'm just getting no magnetic coupling with the shorter length of secondary coil on the OT when set to 2 ohms? I don't understand why it didn't get a little quieter at 4 ohms and yet quieter at 2 ohms. It was buzz or no buzz.

[Enzo]

If it hums on standby through the speakers, then you for sure have magnetic coupling. Can you turn the OT and remount it? Other possibility is ther is still some leakage current flowing.