Fender Supersonic 60 reverb whistle

[mikeydee77]

A Fender supersonic 60 which came to me with a faulty reverb. Turns out that Reverb works but has a horrible whistle ~12khz appears at about 2-3 on the dial regardless of guitar input connection or not.

• oscillation is very evident on V10 plate and this can be stopped by grounding its grid
• Plate / Cathode resistors look ok
• I have tried ruling out leads and connectors. The whistle is certainly effected by pressure or hand grounding on the connectors but it won't stop it all togehter
• Reverb tank feedback coupling doesn't seem to be the issue (not really feedback sound) but I have tried using a different tank and both give the same fault regarless of whether the tank input connection is present.
• Tubes have been swapped and voltages measure ok

I did contact Fender in case there had been any tech notes on this amp but they (Fender GBI) were not aware although they did comment that it is a very sensitive amp.

wondered if anyone has some ideas or inspiration?

• My next thoughts for when my energy level is restored
• I noticed that the 100 watt version has 220pf caps between pretty much all of the reverb connections.
• Was wondering about trying a 5-10k stopper on V10 grid.
• Plate / Cathode resistors look ok but might try replacing them

Super-Sonic_60_Combo_schematic_Rev-A.pdf

[Leo_Gnardo]

Yes something to reduce bandwidth all right. On the recovery tube V10 a stopper R may help; don't forget to use a carbon comp R. I see they "doubled up", don't see why that would be necessary. Should also be easy to cut the drive some by removing C93, I'd try that first.

[g1]

And you have tried it with the tank out of the cab?

[elipsey]

Yes something to reduce bandwidth all right. On the recovery tube V10 a stopper R may help; don't forget to use a carbon comp R. I see they "doubled up", don't see why that would be necessary. Should also be easy to cut the drive some by removing C93, I'd try that first.

Leo, may I ask why carbon comp is prefered here?

[mikeydee77]

And you have tried it with the tank out of the cab?

Hi G1, thanks for replying.

Yes tried various combinations either to get benefit of amp chassis being screened or to isolate from cab vibrations. Currently tank is on the bench but have tried on the shelf on the floor - close and far doesn't seem to be a proximity thang.

[pdf64]

I wonder if the HT ecaps are getting tired, no longer effectively decoupling? Try tacking a known good one across each in turn.
Other than that, yes, grid stoppers seem to be noticeable by their absence; fitting them is basic good practice really.

[g1]

As well as the HT caps mentioned above, maybe try known good caps across C93 & C95.

[pdf64]

Also is there continuity from the body of the tank to the amp chassis?

[Leo_Gnardo]

Leo, may I ask why carbon comp is prefered here?

Stoppers usually trying to limit hi frequency response, filter out RF noise. A low pass filter with R being the stopper, C the miller capacitance of the triode. R's that are made of coil structures (metal film, metal oxide film, carbon film) can act as inductors with unwanted results. Carbon comp are cylinders of partially conductive material (carbon & clay similar to what's inside a pencil), not much induction there. Also best practice to mount the stopper right on the tube socket, close as possible to the tube. Always a larf to see RI Fenders & other amps where they put the stoppers on the circus board, and typically use a spiral-cut carbon comp resistor: what's the point?

[mikeydee77]

Hmm . . . it could be me (or at least my test set up).

A younger person in the house just asked me to turn the whistling off (I couldnt hear it lol) so I switched from a speaker to my dummy load which also has a little attenuated monitor output. At the same time I replaced C93. When I powered back up the whistle is gone (great I thought it was the cap).

But when I switched back to speaker, the whistle is back - very strange.

So just done a little more experimenting and what I find is that with the speaker connected, if the speaker lead is within just a few inches from the reverb connector then the whistle occurs. If it is on the dummy load, then the whistle is stopped. All my cables etc don't change when I flip between loads as the dummy load box that I have is a piece of home brew with four 25W 8R resistors and a toggle switch for flipping between the speaker and resistor load.

I guess something about the difference in characteristics between inductive and resistive load is enough to increase the chances of oscillation occurring.

[Leo_Gnardo]

I guess something about the difference in characteristics between inductive and resistive load is enough to increase the chances of oscillation occurring.

Sure thing, the effective resistance of your speaker increases with frequency. And when you bring its leads close enough to the reverb circuit it's enough to set up an oscillation. You can shield one from t'other, or reduce the bandwidth in your reverb circuit so that it won't oscillate at 12 KHz.

And with comments about "whistling" from the young'in, you know why those urchin-dispersal schemes involving hi frequency speakers can work.

[mikeydee77]

Thanks for everyone's help on this.