Oscillation in 70's Fender Quad Reverb

[telecaster]

I've got an oscillation when I turn the reverb up past 7 (I know..... only Dick Dale runs it that high). It is the 100watt version, non-ultralinear, I can post the schematic if you'd like. I have isolated the problem to V4, the reverb recovery tube and specifically the "A" side. It oscillates with the tank disconnected, with the channel volume down and the reverb control above 7. I worked through R.G. Keen's geofex debugging tips for "squeals", trying different tubes, lead dress changes (in fact I rewired the two long reverb pot connections), and checking all grounds. I reflowed all the solder joints around V4, then I tried a new bypass cap - .002 on pin 2 of V4A to ground. I have checked all the resistors, and they are all in spec, as are all the DC voltages. I'm running out of ideas - I'd appreciate your thoughts or suggestions on how to find and fix this problem. Thanks!

[g1]

Reverb hums and squeals can often be related to the tanks. Is it the stock tank? You mentioned checking grounds, that is critical, but a non-stock tank can have the wrong grounding scheme. Have you tried moving the tank or turning it upside down? Also make sure to try it without the footswitch.

[Chuck H]

Did the amp always do this? Maybe you never noticed for whatever reason?

At some point Fender started adding 220pf caps from the reverb recovery triode grid to ground. I assume this was done to stop oscillation. Since the problem exists with no input I'll bet six beer it's a lead dress problem causing oscillation. You could try a few more lead dress or even ground point changes. If that fails then try the 220pf cap if it's not already there. If it is you might try adding a 10k stopper right on the grid pin.

[telecaster]

It is a customer unit in for repair, so it is possible that the amp was doing it before - I didn't run the reverb up to 10 before working on it (retube. recap, plus replacement of the reverb drive cathode resistor, which had burnt). I always run all the controls to max before sending it out, and that's when the problem appeared. The reverb tank is original, they were mounted on the side in the Quad Reverb, but I'm not sure that matters because the oscillation exists even with the tank unplugged. I don't have the footswitch plugged in on the bench, so it is not in the picture. The amp did have a .002 cap in parallel with the 220K resistor on V4A's grid (pin 2) to ground. I even replaced the cap thinking maybe it went bad, but the oscillation remained. I even grounded the V4A grid pin with a clip lead, and the oscillation remained. I'll keep trying lead dress - there is a mass of wires up front behind all the control pots that are difficult to move or work with, and the 2 long reverb control leads plus a couple to the Master Volume run there.

[Chuck H]

Sounds awful. But it sure seem like it is a lead dress problem. IIRC that is a later silver face amp, yes? Probably some of the worst lead dress I've ever seen is in Fenders of that era.

[soundguruman]

OF COURSE the thing oscillates with the reverb unplugged. Why would it NOT oscillate?
You have the input of that stage OPEN and unterminated.
You have a bad reverb, broken wires in the reverb pan or circuit, or bad reverb cables, oxidized connections...
OPEN and INSPECT the reverb tank. The wires break in those things! The coils can become damaged or open circuited.

[Jag]

He said,

I even grounded the V4A grid pin with a clip lead, and the oscillation remained

. So the way I read that is, with the pan REMOVED and the grid shorted to ground he's still got oscillation. Not that there couldn't still be issues with the pan.

So if you take V4 out, the oscillation goes away? Does the reverb work or is it intermittent? Did you retention the V4 socket? Maybe there isn't a solid connection to the pin?

[Alex R]

well if it still oscillates with the v4a grid grounded that cuts out any possibility that anything is feeding back before that grid. So that means v4a isn't causing the oscillation. That's strange and I'd want to check that the v4a grid truly is grounded, and the amp still oscillates with the reverb control turned up. If so that leaves the possibility that v4b, the mixer stage, is oscillating when its grid ground reference gets up towards its maximum. Maybe that's a bad filter cap, classic cause of oscillation. I'd be tempted to try jumpering in a filter cap above the v4b plate resistor, see if that cures the oscillation, if so change the 20uF 500v cap on that stage.

[Chuck H]

Even with the grid grounded it can still be a lead dress issue stemming from the coupling from V4a to V4b. As the reverb level control is advanced, the impedance on that lead increases. If you've ever seen the inside of a late SF amp (one with trimmings like two channels, reverb and trem) it's really not a stretch.

[Alex R]

yeah the twisted leads to and from the reverb control prime suspects, agreed.

With osc. problems I spend a lot of time poking around with a filter cap grounded with a croc lead though, quick way to check what damps the squealing out.

[Jag]

Yes, I agree about V4B and the lead dress. Disconnecting the pan and grounding the grid of V4a should eliminate those as the source.

[Chuck H]

Just so everyone can see what I'm talking about... I found this image of an open SF rev/trem chassis:

[pdf64]

How about the cathode bypass cap on the shared cathode resistor, V4?
If that develops high ESR then oscillation can result at high reverb settings.
Pete.

[SmartAlex]

Change the tube. A poor connection to the reverb tank like a bad cable will cause oscillation. A grounded grid will not oscillate if the circuit is working properly. The late 60's / 70's amps were prone to oscillation because of the new plastic wire and poor wire dressing. CBS put caps throughout the amp to prevent oscillation. You may have to put a cap across the 220K reverb return jack or you just may have a different problem. Please describe oscillation? Squeel? Is the oscillation in the audio range? Do you have a scope? In my experience the problem is usually related to the reverb tank wires or an open in the tank. I've also seen a bad 7025/12AX7 cause the same problem. Some tubes develope crosstalk between the two sides of the tube.

[SmartAlex]

"reverb drive cathode resistor, which had burnt"
I'm surprised to hear that the cathode resistor burnt. What caused that? Was the tube shorted? Check the voltages and filter cap board.
Most of the reverb circuits will oscillate if the reverb return jack is left open.