Ghosting distortion on note decay 1979 Super Reverb

[judge74]

Hi everyone I am hoping to get a little help with a 1979 Super Reverb I have been working on now for about a month. I picked up this amp recently and it was pretty noisy and the trem and reverb weren’t working. When I opened it up I saw it was pretty much original inside.

So here is what I did:

Replaced all filter caps, bias caps, cathode bypass caps, 470 screen grids (they were burnt) resoldered loose chassis grounds, changed reverb driver cathode resistor and cap to blackface, added trimmer to bias balance pot, added 1 ohm bias check resistors, set basis at 55% dissipation, added 2 100ohm resistors from pilot lamp for heater artificial center tap (the balance pot was broken), replaced open 30k 20w resistor, replaced broken trem roach, replaced the .01 trem oscillator capacitors (found out they were not working after I changed the roach), replaced 47k and 68k after bias balance (they had drifted 50%) removed push pull master volume with all of its extra wiring and a 12K, 1M and .01uf that are there just to run the push pull. I replaced the push pull with plain 1M pot. Resoldered broken wires in reverb pan. Checked voltages against the schematic and everything seemed good.

After all that I have gotten rid of 80-90% of the 120 and 60 cycle hum, The amp sounds really good. However, It still has a little bit of hum, and is not as quiet as I know it can be. The reverb and trem both work. However there is still an odd sounding whoosh of staticky distortion that starts as a note is decaying, it happens at both low and high volume. It happens on both Channels and is not helped by swapping tubes. It almost sounds like it is gated into the decay. Because it is happening on both channels I was thinking, Phase inverter, Power Tubes, or power supply. I did try swapping in a 12at7, 12au7 and a 12ax7 at the PI with no difference.

I went back to the drawing board and tried replacing a few more items that seemed questionable. The 6 100k plate resistors. Swapped both .1 coupling caps after the PI (I used .047 ceramics because I had them handy and thought it might tighten the bass anyway) and the .01 cap between the Master volume and PI. I am looking around the PI because both channels are affected.

Swapping the caps around the PI seemed to fix the problem but it came back after playing for a couple of hours. 

Hoping someone has some other ideas or input I can try to look at

Here is the schematic https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-Reverb-Ultralinear-1978-1981-Schematic.pdf

[pdf64]

A sound or video clip demonstrating the issue may be helpful.
I acknowledge the nasty affects both channels, but does removing the valve in V3 help?
Does it only manifest at high master volume settings?

[judge74]

I'll try to get a video later tonight. I was playing it yesterday and it was making the noise at all volumes. I played it as low as with the volume on 2 and the noise was very apparent. For the same db in the room it does seem to be worse when the MV is up and the preamp is down vs Preamp up and master down. I will try pulling valve 3, I didn't think it would be the reverb driver because turning the reverb up and down doesn't seem to make a difference. I did forget to mention that when the noise started back up was when I was messing around with the tremolo and had the intensity turned almost all the way up. It kept doing it after I turned the tremolo off. I can't imagine how a problem with the tremolo circuit could cause that though, so I am thinking that it is probably unrelated....

In the meantime here is a marked up schematic that shows everywhere I made a change

[Platefire]

I read through your post but don't recall you cleaning tube sockets, pots and jacks with some good contact cleaner. Also try another speaker cab. I always do that just to eliminate it as a possibility

[RadioComm]

Also try another speaker cab. I always do that just to eliminate it as a possibility

+1 Happened to me 20+ years ago on a 1x12 amp. The ghosting distortion was due to a bad speaker. Plugged the amp into a cab out of desperation, and was absolutely stunned when the ghosting went away. Replaced the speaker and that was it.

[judge74]

I don’t have another cab, but what I can do is plug another amp head I have into this cab and see if it makes the noise. I don’t think it’s speakers just because the sound seemed to magically disappear after I replaced some caps, only to come back after a few hours.

I did take a scribe and tighten tube sockets, and I cleaned pots and sockets with some deoxit. Also in the same vein I spent a good bit of time moving wires around with a chopstick. It did help with the hum but not the odd distortion on note decay.

[mresistor]

You didn't say difinitively if you plugged in another amp to test the speaker.  Try it if not.  Try going to 70% max plate dissipation and see.  Did you try a different set of power tubes?, I didn't see that.

[judge74]

Ok I made it home and tried a few things and it seems like I got it.

I pulled a leg up on the bias supply diode and tested it. It was questionable so I replaced it.

I lifted the negative feedback.

I swapped in some old used but good Peavey USA 6l6 tubes.

I set the bias all the way up to 70%. (I think I am going to have to tweak the value of my bias circuit resistor to ground because that’s the lowest I could set it. Maybe replacing the diode changed that.) 

Most importantly on the bottom left speaker I found a little precision screwdriver stuck to the bottom of the magnet and laying against the speaker basket.

Anyway with all those updates it sounds great at least for now. Very punchy and articulate. And less hum so I think that bias diode may have been contributing to the hum.

[tubeswell]

And less hum so I think that bias diode may have been contributing to the hum.

Did you replace the bias supply filter cap as well? When they get old, those white mallory caps on the bias supply are notorious for making hum on Fender Silverface amps

[judge74]

I replaced the bias caps in the beginning when I did all the big filter caps. On the ultralinear they are under the doghouse so it made sense to do them all at once.

[Platefire]

Well we may never know for sure which "try this" did it

The good thing is if the gremlins stay gone and don't come back like theydid last time. Your Super Reverb aught to be good for a long time to come,it's practically all new now

[sll]

Hi, Could you please explain what you meant by the bias supply diode being questionable? Thanks

[judge74]

Hi, Could you please explain what you meant by the bias supply diode being questionable? Thanks

I lifted one leg of it so I could isolate it from the circuit. My understanding is that a diode should read really low resistance one direction and really high resistance the other. This diode read around 1k both directions.

After replacing it the bias voltage now goes higher withe the same trimpot setting. Also the hum is basically gone. My scope is broken so I couldn’t test it but I assume that if the diode was not working quite right it was making a dirty DC which created the hum.