Fender Super Reverb AB763 oscillation reverb

[nonost]

Hi. I'm having problems with an old Super Reverb. I changed all electro caps and a couple tanned resistors. EQ sections and bright switches are working fine.

Normal channel is ok. Although I might say it's not as loud as I was expecting...

Well, the problems are around the Vibrato Channel. I get no tremolo at all. And when the Reverb pot is turn up a bit, I get a crazy pretty loud high pitch. I've pull the reverb driver tube and the problem remains. Also connecting the reverb tank wires doesn't make a difference. Right before the high pitched noise (which is pretty sudden) the tone becomes quite thin.

Could be the bypass 0.1uF cap of the preamp Vibrato channel? Is the only brown fat cap in the whole amp. The rest are the "molded" ones. The brown one has very bad opinions. This one is a bit tanned although It has kept its value...

[Tom Phillips]

Some comments & questions.
Is the tremolo foot switch working properly?
Does the reverb squeal problem happen with the reverb pedal plugged in and/or without the reverb pedal?
Double check the installation connections and polarity connection of the electrolytic cathode cap on the reverb recovery tube.
Can you show us detailed gut shot photos of the current status of the amp?

[nonost]

Hi Tom.

I don't have the original footswitch but I've simulated it behavior with crocodile clips. The reverb does turn on/off ok, but the oscillation happens in both cases. The vibrato however doesn't turn on, it's always off.
The reverb squeal always happens. With the footswitch on/off, and with the input/output RCAs connected or not.
Cathode caps checked.
Yes. I can post some detailed pictures. I'm on it.

I forgot to say that there's some kind of white noise sound even with the volume at 0. In both channels. It doesn't get louder nor quieter.

Thanks!

[nonost]

Pics

[nonost]

Ok it was v5. The half reverb recovery tube and half channel II preamp. It doesn't work fine there. I put it on v1 and it's working fine. Maybe it's an 12ax7 instead of a 7025 because now the channel I is louder.

So I guess it's solved. The white noise it's still there, let's hope it goes away when I'll put everything together.

[Tom Phillips]

Ok it was v5....

That's good news. The circuitry looks to be in excellent physical condition. It's unusual that the 'Bad" V5 tube would work fine in the V1 position. I'm wondering if there was bad conduction at one of the pins that was wiped clean when the tubes were moved. (Just thinking out loud about that.)
Your amp was built in a year when Fender used a brand of carbon composition resistors that tended to drift and/or get noisy faster than the traditional Allen Bradly brand. However, I don't recommend shotgun replacement. The first step I would take to track down the source of the white noise would be to start isolating sections by pulling pre-amp tubes one at a time to determine which section is originating the noise.
A 12AX7 vs. a 7025 is not really significant in a guitar amp so I wouldn't worry about that. If they are modern production tubes then the difference between the two is basically just marketing.

[nonost]

Thanks Tom!

Channel 1 is much louder than Vibrato Channel. The latter needs to be at 5.5 or so to match the volume of the former being at 3. At this volume you can play with a moderate drummer for practice. Maybe the 7025 (v2) of the Vibrato Channel is worn out?

*Knobs go from 1 to 12

I only changed electro caps and some resistors. Grid stoppers and screen grid resistors of power tubes needed replacement since they drifted 100% its value or so. And a couple plate resistors from preamp that were a bit off. Rest of components are within specs.

Really interesting what you said about the resistors. I wasn't aware of that.

I'm gonna close the amp and pay attention to that noise. Hopefully it'll get better. What surprises me is the fact that it doesn't get louder with any knob position.

Cheers!

[g1]

Ok it was v5. The half reverb recovery tube and half channel II preamp.

That sounds like V4. V5 is the tremolo tube.
If tremolo is still not working, that may have something to do with lower volume of vibrato channel.

[Tom Phillips]

...If tremolo is still not working, that may have something to do with lower volume of vibrato channel.

Ah! Good thought.

nonost - Try disconnecting one of the leads of the light bug on the front panel side of the parts board. Let us know if that restores the normal gain of the vibrato channel.

[nonost]

Tremolo is working fine after swapping v5 for v1.

I can pull v4 and see if the Vibrato Channel gains volume.

Thanks a lot

[pdf64]

If the valve in V4 is removed, the fx channel won't pass signal.