Squeal in Amp

[humanbn]

I have a 70's Fender Deluxe amp. When the reverb is turned on only a little bit a squeal occurs. This squeal is loud, louder than the guitar sound coming through if you try to play over it, and constant. Sounds similar to feedback but it has nothing to do with the guitar, happens when nothing is plugged into the amp at all. I have replaced the tubes and the reverb assembly with no result. Anyone have any ideas? Also, there is no reverb effect coming through. None of that room type echo at all. Only a squeal.

[sumgai]

bean,

Are you certain that the lines to the tank are connected to the correct ends? The symptoms you describe tell me that the lines are reversed. Try switching them around. Also, are they in real good condition? This is one area where poorly made replacements won't do you any favors.

HTH



sumgai

[humanbn]

Sumgai,


I have tried switching the leads and it makes the sound either way. The replacement reverb springs are by fender and are suppose to be for the 70's deluxe. Is it possible that a component inside the circuit has gone bad?

[sumgai]

bean,

Depends on what you mean by circuit..........

I just "twigged" on something you said - does the Reverb pot vary the loudness of the squealing?

What happens if you turn up the Channel Volume - does the volume of the squeal also get louder?

Is it possible to turn up the Channel Volume so that you can actually hear the guitar, even if the squealing is still annoyingly loud?



sumgai

[humanbn]

Sumgai,


The volume knob does not have an affect on the squeal. Working the reverb knob does. Raising the supposed amount of reverb does not increase the amount of the squeal but does lower its pitch. When the reverb knob goes below 5 the squeal goes away but there is still no reverb in the signal.

[sumgai]

bean,

Ah, you are now the proud owner of a gen-u-wine Fender Deluxe Oscillator!

Well, there are at least a couple of possibilities. More testing is desirable, but I'll throw out a quicky check, right after this question: Have you changed any of the tubes recently? Particularly the two Reverb stage tubes (V3 and V4)?

The fact that varying the Reverb control changes the pitch tells me that you have a simple RC oscillator. If the tubes are good, then I'd look closely at the 0.003µf cap that's connected to the plate of the Reverb recovery stage. (That'd be pin 1 of V4, check the layout diagram for exact location.) If that cap were to go partially bad, it would set the operating parameters for the following stage to make like an oscillator instead of doing it's normal job of amplifying.

If that doesn't do the trick, there are other things we could try. Or you could just take it to a local amp tech (preferably one that's trustworthy), and have the job done much more quickly than we could do it here.

HTH




sumgai

[humanbn]

All the tubes are new. I'll check the cap, no amp techs for miles and miles. Plus the expense would be lame. I can drain them and all that stuff. I'll post again soon. Thanks again, Sumgai.

[sumgai]

bean,

This isn't a matter of 'draining' a cap, we're not talking about an electrolytic here, one that could retain a charge for several days...... we're speaking of something very small, in comparison, so that particular worry is off the table.

The easiest thing you can do is to measure the level of DC (volts) on each side of the cap, while the amp is on. There should be about 170vDC on the side pointing towards the tube (pin 1), but there should be almost nothing on the other side, a volt or two at the most. If it's more than 3 volts, then you've got a leaky cap.

I should add that this would be the more likely case, but it's not the only way unwanted DC could be getting onto that line...... Tube V2, pin 6, also has 170vDC on it, and that feeds essentially the same point, through a 0.02µf cap. If this one were to fail, partially or fully, you'd get a substantial amount of DC on the line, just like a failure of the 0.003µf cap.

And since you're out of luck as far as a local repair tech goes, we're gonna have to get into this anyways, so you might as well do a voltage check on all the pins of tubes V2, V3 and V4. It's sometimes happens that the electrolytic cap found on the cathode can go bad, but that usually doesn't cause controllable oscillations, in fact it usually just causes major tone loss, or at the worst, a severe loss of gain. The first suspect is the pair of cathodes on V3, which are tied together. Either pin 3 or pin 8 should measure no more than 1.3vDC.

When you've written down all the measurements, compare them to those listed on the schematic, found here. (If your particular unit has the volume boost option, use this schematic instead.)

Kinda silly of me to ask at this late date, but you do have a digital multimeter, right?

HTH



sumgai

[humanbn]

I know Sumgai has left the forum for other things but maybe some one else can help me with this. I finally got around to testing all the caps and pins that Sumgai suggested and they all seem to be fine. around 200v on one side and virtually nothing on the other. Well, to be short, now what? Any ideas?