Silverface Twin Reverb squeal MYSTERY

[brianpb]

I have a Silverface twin here (Master Volume) that is giving me some trouble. The reverb at first refused to change level. It worked, but just couldnt turn it off , or UP. Or down, for that matter.

I resoldered the lugs on the reverb pot and now the reverb SQUEALS when turned up. I can also hear this oscillation a little when turning up the tremolo. Yesterday i turned the reverb up , SQUEAL , and noticed smoke from the screen resistor on power tube 2....or maybe the grid stopper resistor underneath. It is hard to say. I plan to replace all the screen resistors even though they measure OK.

So far, i have measured the input and output on the tank, measured the primary and secondary resistance on the reverb transformer, tested for output on the reverb send with an 8 ohm speaker (which sounds fine), swapped out the reverb send and recovery tubes. Tested the reverb cables. The voltages on all tubes, both preamp and power amp, measure good.

With the reverb down , the amp passes signal and sounds ok. With the reverb up it squeals. There is also a persistent low level hum which developed AFTER the first time i heard this squeal. I know i should probably recommend to the owner replacing the caps as regular maintenence but im not sure the filter caps are the problem...and not sure he wants to put the money in. The amp was dead quiet before i retouched the solder joints on the reverb pot.

To me the fact that the squeal caused smoke from a screen resistor should be a big clue..But what does it all mean? Seriously , I would appreciate any help or insights here . I did a search through this forum in which Bruce from Mission Amps recommended a certain value in the reverb circuit, and the amp here matches those values, both the resistor and the cap...i believe it was 2.2k ohms, and 25mfd/25 volts.

[supawah]

I have a Silverface twin here (Master Volume) that is giving me some trouble. The reverb at first refused to change level. It worked, but just couldnt turn it off , or UP. Or down, for that matter.

I resoldered the lugs on the reverb pot and now the reverb SQUEALS when turned up. I can also hear this oscillation a little when turning up the tremolo. Yesterday i turned the reverb up , SQUEAL , and noticed smoke from the screen resistor on power tube 2....or maybe the grid stopper resistor underneath. It is hard to say. I plan to replace all the screen resistors even though they measure OK.

So far, i have measured the input and output on the tank, measured the primary and secondary resistance on the reverb transformer, tested for output on the reverb send with an 8 ohm speaker (which sounds fine), swapped out the reverb send and recovery tubes. Tested the reverb cables. The voltages on all tubes, both preamp and power amp, measure good.

With the reverb down , the amp passes signal and sounds ok. With the reverb up it squeals. There is also a persistent low level hum which developed AFTER the first time i heard this squeal. I know i should probably recommend to the owner replacing the caps as regular maintenence but im not sure the filter caps are the problem...and not sure he wants to put the money in. The amp was dead quiet before i retouched the solder joints on the reverb pot.

To me the fact that the squeal caused smoke from a screen resistor should be a big clue..But what does it all mean? Seriously , I would appreciate any help or insights here . I did a search through this forum in which Bruce from Mission Amps recommended a certain value in the reverb circuit, and the amp here matches those values, both the resistor and the cap...i believe it was 2.2k ohms, and 25mfd/25 volts.

ok, crazy story time. I had a silverface MV twin that had a squealing reverb, everything seemed to be in spec. the only thing that I could see was that the two input resistors in the jack on ch2, the 68k ones, had been changed with metal oxide. I swapped them to carbon comp and the amp stabilized. the lead dress is very critical on those amps with tons of weird parasitic oscillations, in my case the different resistors brought it back into a range that could be handles by the lead dress I guess. maybe something similar on yours?

[brianpb]

Hrmmm. Thats an idea. I did need to pull on the wires a bit to get enough length to resolder to the reverb pot. When i move around those rather long wires going from the reverb pot to the board, the character of the squeal *does* change. Im just afraid to let it squeal too long due that smoking screen resistor...Maybe i'll try some shielded cable grounded at the reverb pot only?

[brianpb]

An update: This evening i pulled the power tubes and traced the hum to a cracked screen resistor. Replaced all those resistors with 470 ohm and the amp sounds good again. I'm back to square one however: the reverb squeals when turned up, and the pot doesn't turn the reverb all the way off. The only way to do that is via footswitch. I repeated the test with an 8 ohm speaker on the reverb send jack. It sounds really good and strong through that jack. So i guess i'm looking for something in the reverb recovery section.....?

[Jazz P Bass]

Make sure the reverb wires are oriented the correct way.

The wire to the amp that makes noise when touched is the return (it goes in the OUT of the pan)

[brianpb]

Indeed, the reverb cables are correct with respect to in / out. I confirmed that by reading resistance. Kind of unrelated, but it really bugs me how the fender chassis is labelled. "Reverb IN" on the chassis goes to "reverb in" on the tank, and same for the "OUTS". Only piece of gear I can think of that's labelled as such. Since when do outputs get connected to other jacks labelled "output"? Can't they just have labelled it "reverb send" , which would then go to the "reverb in" on the tank?? Grrrr

[km6xz]

The 100k reverb pot is a convention potentiameter configuration so the output it a ratio between low side and high side based on the relative position of the wiper. So if that ratio never can never go to 100% low, there will be a low level output signal. The ground has to be truly ground for it to shut off the signal at the wiper. Look for a few ohms or higher from the low side terminal to ground, or dirt or mechanical damage that prevents the wiper from rotating fully to the low side. A good conformation, other than testing the resistance of the wiper to ground when turned fully towards the low side would be ground the wiper to the chassis with a small screw driver to see if the signal goes away.

Send and return are relatively newer terms that were not used in the early 70s so it is natural that conventional signal line terminology be used. Connecting a wire between two same named terminals is standard in electronics, for example a connection labeled B+ connects to every point similarly marked B+

[cyclone]

test the reverb transformer windings with a meter, should read very low value. (Discharge caps first!) If the primary goes open or high resistance it will make the reverb squeal like a banshee. If not that check the reverb stage cathode bypass caps, an open or high reading one can cause said squeal. At least thats been my experience with the Twin and Plush (virtually identical) circuit.

some of the oscillation problems with old fenders that people think are wire mojo are caused by dried up cathode bypass caps. Higher than stock values, I've seen the stock 25uf ones go to 100uf and higher. The caps often are swelled and split. thing is, the amp can work well and sound great with these failing caps, with unpredictable exceptions and susceptibility to oscillation and imminent failure. The tone WILL change if you replace them. Thats why some of the old school guys won't let you change them. That being said, on newer white wire fenders, its more likely rats nest wiring mojo!

[brianpb]

I think i mentioned having done a test by driving an 8 ohm speaker with the reverb "input" (send) jack. It sounded really good and strong. My impression is that this pretty much rules out the reverb transformer, as it is essentially a mini output circuit and the transformer turns ratio makes the higher impedence of the 12at7 work with the lower impedence of the reverb tank input. I have measured between the primary and secondary and the readings were something like 2.2k primary and between 2 and 4 ohms at the jack. Seemed normal to me....Thanks i really appreciate the responses so far.

[brianpb]

Well......It was the coupling cap on the 12ax7 reverb return. Pins 3 and 8 i believe go to a cap... 25uF, 25 volts. I replaced it with a similar part and everything is magically fixed. I googled "fender reverb squeal" and, in conjunction with my own tests, came to the somewhat educated conclusion that it had to be in the recovery section. Having found more than several references to that coupling cap, i just swapped it out. BOOM , all problems fixed. i have to assume the cap went bad and was leaking signal all the time to the reverb pot, causing a little bit of reverb constantly, and not allowing the pot to do its job. Thanks to everyone who responded. It's nice to bounce ideas around, rather than pondering alone in the basement with a soldering iron and one hand behind the back. Haha.

[g1]

Good you got it figured out. That would be called a cathode bypass cap, not a coupling cap.

[brianpb]

Right O

[jvm]

I've had the same cathode cap issue on several Silverface Fender Super reverbs.

[silvertone8]

thankyou so much for this thread. i had a 1978 silverface twin reverb,master volume, 135 watts. the reverb knob at 4 made a horrible squeal. thanks to this thread it now works beautiful. i replaced the 25mf-25 volt electrolytic cap that came from pins 3 and 8 of reverb recovery 12ax7 and presto!! the squeal is gone and the amp works great!

[Chuck H]

They're very old caps that suffer a failure mechanism in that the electrolyte crystalizes with age and especially in disuse. If the cap shorts when that happens the affected tube has 0V bias. That can probably impress reverb into the signal path even with the reverb knob at zero and would certainly make the circuit unstable.

Considering this... There are OTHER electrolytic caps in those amps that have the same failure mode. Almost invariably they are skipped when a "cap job" is done. Most important are the bias caps. Many DIY'ers don't entirely "get" the bias circuit, so they leave it up to a "shop" to fix the amp when the power tubes explode and take the OT with them

Moral of the story: If you're doing a cap job on a "working" amp you should replace ALL the electrolytic caps. Not just the big ones. Next to the tubes they are the OTHER components that need to be replaced periodically (like every twelve to twenty years). New caps perform great, but don't last like the old ones IME. Well, there ARE caps that are long life rated but they don't show up in guitar amps much, so...

If the amp is a "collector" then leave it broken and leave the original caps in there (or at least save them if they haven't burst open already). If the amp works for a living then it HAS TO have good caps. And old electrolytic caps are seldom good ones.