Questions about building AB763 out of Bogen amp...

[Phil_S]

That looks like a FWB with a cap across it. I don't think I've seen this before. What happens to the voltage when you disconnect that cap?

That cap is the first cap (reservoir) in the B+ rail. It's just drawn that way for simplicity.

D'oh! Sorry, just my lack of imagination. Thanks for setting this straight.

[chikov]

I am wondering if 503 Volts on my plates is too much and how shoul I go about lowering that voltage? May be by lowering my Voltage doubler Capacitors' value? any thoughts anyone?

The answer is that it depends on the rest of the circuit and the voltages. Would you like to post a voltage chart, even betterm a full schematic with voltages?

I"m not the most experienced person here. Far from it. I don't see a doubler there. That looks like a FWB with a cap across it. I don't think I've seen this before. What happens to the voltage when you disconnect that cap?


The most expedient way to drop about 40-50V is to use a 5U4GB tube rectifier if you've got a 5V @ 3A winding and a place on the chassis for the socket. There are other ways. You will find discussions here if you search.

No, that old Bogen PT does not have 5V coil I did start a new thread where I want to build an amp loosely built on AA864, but with a tube rectifier and just one channel.

[chikov]

Hello friends... I am stuggling to find the sourse of the 60hz hum coming out of this amp that this thread is about... it is fairly guiet but loud enough to be anoing... Long story short, When I pull my V1 tube, noise stays, when I pull V2, noise completely goes away... I can give you more info... All my grid wires are shielded, the filament wiring is in phase and I wiggled and moved around everything and nothing helps. Does anyone have any suggestions?
Thanks.

[chikov]

BC64721A-67E6-41DB-8486-72E2C8A52A85.jpeg

I decided to post a photo of the amp. It is not a pretty picture - the chassis was too small. Do you think the noise may be caused by a proximity of the filtering caps to the preamp?

[xtian]

It's going to be an issue with grounds. Where is B+ for V2 triodes grounded? Where is the grid leak resistor(s) grounded? What about the cathode resistor and caps?

[Tony Bones]

V2 performs reverb recovery. Could that be a clue?

[chikov]

Thank you guys for your help. I did look for ground loops And found one around input jacks. I connected all the grounds with an additional wire and when they removed it from the amp gotten quieter. I also completely disconnected the Reverb circuit and it helped a little bit too. It is not dead quiet yet buddy it’s much better than it was. I have a 1979 twin reverb on my bench, I will try to copy the wiring on it’s input jacks. I will keep you posted. Thanks again