Super Reverb AB763 - Fizzy Distortion & Oscillation

[bbmade]

Finally powered up this 1966 Fender Super Reverb after checking all the resistors for tolerance, doing a filter cap job and a few other standard things (checking resistor values, cold solder joints etc). 

Once powered up, I checked all of the voltages while on a dummy load and everything looked great.  When I finally played through it, there was a horrible fizzy distortion and occasional intermittent squeal without playing.  I checked all the tubes I was using and ended up putting all new JJ's in, same issues.

I found more cold solder joints and noticed the board was really noisy around the coupling caps coming off V2.  I cleaned the underside of the board and fixed other cold solder joints, and cleaned up excess solder and flux.  Once put back, the noise at the coupling caps was gone and I installed new shielded grid wires grounded on one end.   

I checked the bias and noticed it was super cold.  As I adjusted it while playing, it seemed to come alive but, the oscillation came back.  It seems the oscillation is part of the bias supply.  I changed the 100uf/100V cap to a 47uf/100V and had the same issue.  I put another 47uf/100V from the wiper to ground and still no better.

When I put a signal generator on it and used my scope, the wave form looked good, no cross-over distortion or anything.  When just using the scope to try and figure out the oscillation frequency, I saw about 20Khz average

I've never come across an issue like this, and can't for the life of me figure out what I am missing.  I have not had a reverb tank nor tried tremolo the entire time, and those controls have been all the way down while troubleshooting.

I was determined to figure this out on my own but am up against my own limitations yet again.

Thank you in advance, this forum has helped immensely over the years.

[Platefire]

You might try disconnecting you NFB from the speaker Jack to see if that changes anything. If it stops when you disconnect NFB, you may needto reverse/switch your OT plate wires, then re-connect your NFB to speaker jack.

[bbmade]

You might try disconnecting you NFB from the speaker Jack to see if that changes anything. If it stops when you disconnect NFB, you may needto reverse/switch your OT plate wires, then re-connect your NFB to speaker jack.

I could live a thousand years and never have figured this out.

This amp had a non-original OT when I got it which was damaged in shipping.  I had a donor 1965 Super Reverb chassis that someone absolutely butchered attempting a Dumble build.

When I installed the OT, the blue wire was longer than the brown so I soldered it to the sockets each wire reached (from the donor chassis).  It never occurred to me this would come back to bite me. 

I removed the NFB from the jack and the problem was gone.  I reinstalled it and then swapped the secondary side of the OT at the jacks and all is well.  Tomorrow I'll re-check the bias, touch a few things up and move on to the reverb to see if the tank works.  Light at the end of the tunnel isn't a freight train thanks to you and this forum.

I'm convinced I'll never be half as smart about amps as the people in this forum.  My only regret is not asking about six hours in labor ago.

THANK YOU

[Platefire]

Well I'm glad that worked for you. I've done the same thing in times past and when you described what it was doing, I thought it was worth a shot. As far as amp knowledge, it just comes in time as you stay with it and glean what you can along the way

[stratomaster]

If you're feeling froggy, try my heater elevation via bias tap mod.  I show a layout in this thread that comes from my 67 Super Reverb. It'll drop right into your 66.

https://el34world.com/Forum/index.php?topic=30961.0

[bbmade]

If you're feeling froggy, try my heater elevation via bias tap mod.  I show a layout in this thread that comes from my 67 Super Reverb. It'll drop right into your 66.

https://el34world.com/Forum/index.php?topic=30961.0

Is this to make things quieter?  I’m not clear on the motivation.  Probably won’t try it on this amp but certainly open to it on the next Fender restoration.

[stratomaster]

It's heater elevation for hum reduction. All the typical motivations for heater elevation apply.

This is just a convenient way of implementing it without affecting the HT.