Fender 65 Princeton Reverb RI-reverb tank feedback

[xtian]

Well bias was set too hot, and I cooled it to spec (-40v at 200K junction), and I THOUGH that improved the feedback, but now it's back with a vengeance.

[xtian]

Well, think I solved it with another new 12AT7 tube. Whew! Was about to tell owner to return it.

[The Dude]

I've battled several of these, and it seems like the solution is never the same. I remember one of them simply had way too much reverb gain- in the unnecessary realm. I ended up just putting a lower gain tube in the reverb circuit. I don't recall if it was the drive side or recovery. The amp still had PLENTY of reverb and no more howling.

Edit: i was typing during the last post.

[Chuck H]

I was onto the probable microphony of one of the tubes as soon as you mentioned that the tank was isolated from the cabinet with no change. But I hadn't seen this thread before now. I've dealt with too many reverb tanks for a relatively small amount of amps and new ones do tend to be microphonic even with the accepted circuits. But you seem to have isolated the problem.

And Dude is right. Most Fender reverbs have too much gain. Dick Dale never needed that much reverb. But to be fair to Fender the reverb circuit and it's recovery do have to amplify just a few milivolts up to mix levels for the PI in only two stages. So they are inherently troublesome circuits regarding microphony and noise floor.

[Tom Phillips]

Is the speaker frame grounded in that Princeton?

[xtian]

Is the speaker frame grounded in that Princeton?

No. I've not seen a PR (or DR, or TR... or HRD, etc, etc) with a ground wire on the speaker frame.

[Chuck H]

Is the speaker frame grounded in that Princeton?

Do tell on this please. I had a build with a whistle when the reverb was turned up over half way. The ONLY thing that stopped it was grounding the speaker frames. I can't even remember why I tried that but I think I was hoping it could act as a partial shield between the reverb transducers and the PT (which was also problematic for hum) and the speaker coil (which I suspected was related to the whistle). But I had never heard of doing it before or since until you brought it up now.

[Tom Phillips]

Do tell on this please. I had a build with a whistle when the reverb was turned up over half way. The ONLY thing that stopped it was grounding the speaker frames. I can't even remember why I tried that but I think I was hoping it could act as a partial shield between the reverb transducers and the PT (which was also problematic for hum) and the speaker coil (which I suspected was related to the whistle). But I had never heard of doing it before or since until you brought it up now.

I was just going on the same type of memory that you described Chuck. Therefore, I thought I'd ask. Anyway, it an easy test to do but I don't have a technical explanation of why it would make a difference. I have seen old audio systems that had a wire added from the negative speaker terminal of the speaker lead to the frame. I guess that we could assume that any conductive surface that's floating is an antenna that could radiate a signal that causes issues. In a combo amp it could be energized by a stray magnetic field from one of the transformers.

[Helmholtz]

Yes, grounding the speaker frame can help to stop oscillation.
I found that in the 80s and mentioned it in some older thread here.
Some original Jensen speakers found in vintage Fender amps came with a grounded frame (using a rivet on the terminal strip).
These speakers should not be wired in series (Leo knew ).

The speaker frame is capacitively coupled to the voice coil.
Means that HF content of the speaker voltage gets on the frame and is radiated as electrical field as long as the frame is floating.
Grounding the frame prevents this and turns the frame into an electrical shield for the voice coil.
(Electrical shielding has no effect on magnetic fields.)

Also don't forget to use tube shields. Especially the reverb driver tube has a very large plate signal (up to hundreds of volts Vpp) and thus radiates strongly.