Help Troubleshooting 60's Deluxe Reverb AB163 Amp

[jlatrace]

Thanks Ted, very helpful explanation. The vibrato does work, I have replaced v5, reflowed solder joints in the entire vibrato and phase inverter circuit and cleaned the tube socket contacts, no change.

I am going to re-check voltages to make sure I didn't miss something.

Okay rechecked voltages and found something new. The plate voltages on the 6v6's are low. Here is what I get:

reading B+ at the + side of the filter caps (under the doghouse), I get:
B+1 = 448
B+2 = 446
B+3 = 357
B+4 = 295

However the readings on the 6v6 pins 3, 4 & 6 are:

v7:
pin 3 = 443
pin 4 = 445
pin 6 = 446

v8:
pin 3 = 443
pin 4 = 445
pin 6 = 446



Ps - sorry for typo in title (AB763).

[jlatrace]

Rechecked voltages, plates are low

Okay rechecked voltages and found something new. The plate voltages on the 6v6's are low. Here is what I get with all tubes in the amp and fully powered and warmed up, line voltage at 120 vac, biased at 22 MA:

reading B+ at the + side of the filter caps (under the doghouse), I get:
B+1 = 448
B+2 = 446
B+3 = 357
B+4 = 295

However the readings on the 6v6 pins 3, 4 & 6 are:

v7:
pin 3 = 443
pin 4 = 445
pin 6 = 446

v8:
pin 3 = 443
pin 4 = 445
pin 6 = 446

Any ideas?

Ps - sorry for typo in title (AB763).

[Chuck H]

Those look alright to me. I'm going with Bruce and thinking it's a cold solder joint somewhere. I know you've resoldered some stuff already. But you should re flow more. I've usually heard this behavior with bas contacts on sockets (lifting pads for board mount and oxidation for chassis mount) but you've covered that. A long second would be a bad ceramic cap.

[jlatrace]

Those look alright to me. I'm going with Bruce and thinking it's a cold solder joint somewhere. I know you've resoldered some stuff already. But you should re flow more. I've usually heard this behavior with bas contacts on sockets (lifting pads for board mount and oxidation for chassis mount) but you've covered that. A long second would be a bad ceramic cap.

Thanks. Yes I am going to suck out and re-solder all ground points. I had a grimlin in my 1965 BFDR, I never figured out exactly what the issue was but it was corrected when I lifted the board & re-did all the grounds in the entire amp.

Also somebody has worked on the amp in the past in a less than professional way. I'm going to re-do that work too. Probably not the problem, but it is really bugging me to see this amp so hacked up. By the way, I did find a bad ceramic in the phase inverter section. It wasn't the culprit but after replacing the ceraminc the voltages dropped across the board (from 448 to 441 B+).

I did additional research and learned that BF amps are known for their screen voltages exceeding their plate voltages by a few volts (ohms law, choke dc resistance, etc, I guess I should have known that).

I'll report back when I figure ou the problem. Everyone has been very helpful and now it's time for me to dig in and find the problem.
Thanks,
Les

[52 Bill]

A couple of points here. Have you tried to physically induce the problem by hitting, tugging, tapping, on different parts of the board? I thought that you said that flipping the standby would do it, so I was wondering if the cause was mechanical or electrical.

The vibrato in this amp does not alter the output tube bias. It has an optoisolator that grounds out the signal from the second preamp circuit.

[jlatrace]

Well I found the problem, it was a bad standby switch. Found it by metering almost every stage and noticing that when the noise happened, the 6v6's at times lost all voltage except for the bias voltage, which remained steady. Also the preamp section would go dead (just like flipping the standby switch-hum).

Pulled the standby switch, confirmed it was bad, installed a new one & the amp is as good as new.

I also cleaned up an earlier plate resister and cathode elctrolytic change out that was messy. I had some NOS AB 100k resisters, so I installed them. I also changed out the .001 ceramic cap, it was bad (I probably fried it troubleshooting). Sucked solder from all grounds. cleaned them and reflowed solder. Cleaned the board with denatured achohol. Cleaned and tightened all tube sockets too.


Thanks to everyone for the assistance, I truly needed and very much appreciate the help.

[Jazz P Bass]

Good catch

[Ted]

My apologies for misleading you about the tremolo operation. i should pause and think before I type. If you're interested its worth comparing your amps schematic with that of the AA964 Princeton Reverb to see the different ways they do it.