1966 Fender Princeton Reverb / Sharing A Volume Pot Issue Found

[plexi50]

Hello all. It's been a while since i have been active here.I wanted to share an issue that i had last week with a 1966 Princeton Reverb amplifier.The volume pot would not produce a zero volume at full CCW rotation.
I checked the pot, it was good and tested so with no bad or dead spots on the wiper through full CCWR  (inside of pot carbon wafer and parts)
Grid pin's 2 & 7 of V1 were not shunting to ground through pot CCW rotation on first appearance.
Changed pot for fun. No change. All ground wires intact and not broken. In place and doing there job.
Then i checked the power supply capacitor.
I found at Node D, the capacitor and 18K resistor feeding Node (D) were slightly corroded. I put a new 22uf-450VDC capacitor at that spot and the volume problem was gone. Full CCW rotation working again as it should.
I had seen this same problem a long time ago so it took me a while to remember the solution to the no volume shut off problem.
Think about this: At first glance, what in the heck does a bad power supply node have to do with a Volume Pot that will not shunt and shut off the volume at full CCW? This was one find that i said i would never forget. I did for a while. It does sound strange but that was the problem. Any Questions?

[tubeswell]

Yes, if the pre-amp filter cap is not decoupllng properly, then you'll get interstage feedback shunting the signal from the stage before the vol pot, into the stage after the vol pot, through the power supply rail  (even with the volt pot cut) - because without the filter cap's decoupling action, the power supply dropping resistors just become another part of the load in series with the plates.

[Tony Bones]

My thought too, but the cap would need to be completely dead. 0uf.

[tubeswell]

Yes either dead, or lifted out of the circuit (through the ground lead/side of the cap becoming open)

[PRR]

> The volume pot would not produce a zero volume...the capacitor and 18K resistor feeding Node (D) were slightly corroded.

Good find. Good food for thought.

I simmed-up a highly idealized experiment. TU9 drives a 1Meg resistor (like a volume pot). TU11 grid goes right to ground, like a perfect pot would. This should be "NO signal". Without rail-foolery, sim says -600dB which is as high(low) as it can count.

But put in a typical decoupling filter, then "rot away" the cap return to ground. 10r to 1Meg. Even 100r makes a difference, though -80dB (re: +72dB gain wide-open) is inaudible.

I've been looking at decoupling issues this month and this sneak-by effect totally missed me. While lack of total fade-out is not a killer problem, it seems to plague many builds. Some are just the crappy pots we buy today, but there are other sneak-paths and in a high-gain (any guitar-amp) amplifier small sneakage is amped-up to audible sneakage. I found one totally bizarre response when several decoupling caps share a return path which has gone bad.

[VMS]

Interesting, i had problem with one build where i had to remove the grid stopper resistor that would have been in TU11 to get the volume go to zero.


If not too much trouble could you sim what effect would added grid stopper resistor have. I think i used 100k or even bigger.

[PRR]

> could you sim

Insignificant. 470k tried and any difference is far below audibility. I suspect your amp was more complex; I also doubt the sim models all sneak-paths even inside the triode.

[VMS]

Thank you very much, yes must have been something else going on with that thing then.