Reverb Oscillation

[TimmyP1955]

There's a possibility it could be a bad cathode bypass cap on V4.
Unplug the tank and see if it still oscillates.

Yep, I've seen this before. Removing the cap made it much worse. New cap and all was well.

[Tom Phillips]

Have you tried a different tank and verified that the problem is independent of the tank?

[PfeifferElectronics]

Wanted to give everyone an update... First of all, thank you so much for all the help. Pulling the tank out of the cab eliminates the problem with the oscillation/feedback, but the reverb is still "too hot". there is too much reverb. Also I noticed that the reverb driver tube is hotter than the rest of the pre-amp tubes. I also noticed that the tank is for the stand alone reverb unit where it is mounted differently. The tank is a 4AB3C1C. I think the original spec for a pro reverb is a 4AB3C1B. I'm thinking I need to order a different tank. Thoughts?

[PfeifferElectronics]

Yes, and it sounds much better with a different tank.

[Jazz P Bass]

The last letter indicates the plane in which the tank is oriented when mounting.

DIGIT #7 - MOUNTING PLANE
A = Horizontal Open Side Up
B = Horizontal Open Side Down
C = Vertical Connectors Up
D = Vertical Connectors Down
E = On End Input Up
F = On End Output Up

[pdf64]

That's normal, it's a minipower amp, there's 432V plate to cathode at 1.9mA per triode ~1.7 W total, compared to 0.55 W total for V6 http://bmamps.com/Schematics/fender/...a165_schem.pdf

[Chuck H]

Already mentioned earlier. I just want to keep it in play. The recovery triode does a lot of amplifying. There's still a possibility that the recovery stage is microphonic until a new tube is tried there.

[Tom Phillips]

...The tank is a 4AB3C1C. I think the original spec for a pro reverb is a 4AB3C1B. I'm thinking I need to order a different tank. Thoughts?

As JPB said in post #20 that last character is just the mounting configuration.
Also note post #15 where g-one talked about the four suspension springs that hold the inner reverb spring tray assembly to the outside case.
You can reconfigure a 4AB3C1C to be a 4AB3C1B by moving the suspension springs to different holes in the outside case. The idea is to suspend the inner tray roughly in the center of the horizontal and vertical mounting plane to prevent undue reverb crashing.
However, I don't think that is the cause of your stated problem.

[g1]

Yes, and it sounds much better with a different tank.

Of same or different type? What was the number on the tank that sounded better? What brand?

[pdf64]

As mentioned, even back when, different tanks of the same spec / part # can sound unbelievably different.

[PfeifferElectronics]

the tank that I am using has no numbers on it. It is accutronics. the one that had a problem was a MOD tank. Anyway, different tank, all is good. I never realized that reverb tanks have personality, that is each one is unique. Thanks for the help!

[Chuck H]

Indeed! Each tank is unique in it's specific frequency range for each spring, as well as sensitivities, interaction between the springs, resonance with a particular amp, etc... If one has enough tanks around it's a very worthy effort to try them all with any given amp to find the one that performs best with THAT particular amp. I've even had some limited success tweaking a tank to work with a particular amp by making minor spring length adjustments. It's possible to tune the springs so they don't resonate out of control with a given amp. I don't recommend trying this since I did eventually ruin that tank My point is that the resonant sensitivities of the tank need to cooperate with the amp it's installed in. The very best solution is to have several tanks to try out. I've never had that luxury myself.

[nickb]

An old thread I know, but it seemed a good place to add my findings on reverb related oscillations on the 65 Priceton reverb, just in case anyone came searching for answers,

I found two different kinds oscillation. The first happens when you crank the reverb, the treble and dial in the volume, it bursts into full power instability around 24KHz. This was fixed by adding a 470pF cap in parallel with the existing 220pF on the RCA socket for the signal back from the reverb.

The second sounded very much like acoustic feedback from the speaker to the tank, but it wasn't. I could run into a dummy load and still get the problem. The volume and reverb needed to be well up to get it to happen. I found that if I lowered the load impedance I could stop it. That told me that the feedback was coming from the output stage. I tried variations of dress of the leads to the output transformer with zero effect. I started to look the the speaker jacks. I found that if I removed the negative feedback I could stop it in in tracks, surprising since that would increase the loop gain. Suspecting the negative feedback wire was coupling capacitively to something, I tried moving the feedback resistor off the PCB to right next to the jack so attenuating and coupling from the wire. This had no effect so it must be the current though R7, the 47 ohm feedback divider resistor. I lifted the ground side of that and hunted around for a different ground that didn't add hum. The ideal place turned out to be the -ve end C26. Problem solved. I have no doubt that changing the tank could significantly affect and even cure this issue, but only because it would have moved the resonance to a place where the loop gain was lower.

How this helps someone else with similar issues.


Piccie of mods:


PS: I upload the wrong image at first. Updated.

[SoulFetish]

An old thread I know, but it seemed a good place to add my findings on reverb related oscillations on the 65 Priceton reverb, just in case anyone came searching for answers,

I found two different kinds oscillation. The first happens when you crank the reverb, the treble and dial in the volume, it bursts into full power instability around 24KHz. This was fixed by adding a 470pF cap in parallel with the existing 220pF on the RCA socket for the signal back from the reverb.

The second sounded very much like acoustic feedback from the speaker to the tank, but it wasn't. I could run into a dummy load and still get the problem. The volume and reverb needed to be well up to get it to happen. I found that if I lowered the load impedance I could stop it. That told me that the feedback was coming from the output stage. I tried variations of dress of the leads to the output transformer with zero effect. I started to look the the speaker jacks. I found that if I removed the negative feedback I could stop it in in tracks, surprising since that would increase the loop gain. Suspecting the negative feedback wire was coupling capacitively to something, I tried moving the feedback resistor off the PCB to right next to the jack so attenuating and coupling from the wire. This had no effect so it must be the current though R7, the 47 ohm feedback divider resistor. I lifted the ground side of that and hunted around for a different ground that didn't add hum. The ideal place turned out to be the -ve end C26. Problem solved. I have no doubt that changing the tank could significantly affect and even cure this issue, but only because it would have moved the resonance to a place where the loop gain was lower.

How this helps someone else with similar issues.


Piccie of mods:


PS: I upload the wrong image at first. Updated.

nice work, I saved it in my 65 prin/rev reissue folder.
Interesting that the optimum connection for noise was not -ve node of C29, which is the filter supply for the stage input for the feedback. Just wondering if fender used a higher impedance voltage divider for the feedback, limiting the current through that point, would've improved the noise issue?
Regardless, nice fix, and appreciate you sharing it.

[nickb]

nice work, I saved it in my 65 prin/rev reissue folder.
Interesting that the optimum connection for noise was not -ve node of C29, which is the filter supply for the stage input for the feedback. Just wondering if fender used a higher impedance voltage divider for the feedback, limiting the current through that point, would've improved the noise issue?
Regardless, nice fix, and appreciate you sharing it.

It is interesting. I think the problem is an unintended side effect of R60 in the ground between the power and preamp. The feedback current has to flow through this to get back to the output transformer.