What is this mod in my 1972 Princeton Reverb?

[yaryaryar]

Hello.

I just picked up a 1972 Princeton Reverb yesterday, and today I'm poking around inside just to check on things. It seems to be mostly original, except for this this one out of place cap, which I assume is a mod? Maybe?

A photo is attached, but I'll try to describe it: There's a 22uf/50v cap running from pin 3 of V4 to the ground lug on the vibrato pedal jack. It doesn't look original to me, and I'm not sure what its function could be.

Any thoughts?

[Leo_Gnardo]

Nothing to worry about. That cap's in parallel with V4's cathode resistor, part of the vibrato oscillator circuit. At some point Fender started wiring them this way rather than put 'em on the circus board, and yours looks like a replacement for the original part which was probably a white Mallory cap.

[pdf64]

It looks like the cathode bypass cap has been replaced http://bmamps.com/Schematics/fender/..._bf_AB1270.pdf
If it's the only non-original ecap, then perhaps the original failed?

[Chuck H]

Layout looks a little off. Hard to be sure without more board to look at. That cap is probably a "repair" done by someone troubleshooting the tremolo. Perhaps trying to solve for ticking. Looking at the eyelet for that tube pin I can just make out where someone may have snipped out the old cap and there's only one component lead leaving the eyelet when there should be two. A resistor and a cap in parallel to another eyelet that is grounded. the cap would have been very much like the one oddly placed in your amp. In t's current installation it's serving the same function electrically. So, not a tone mod. Perhaps a function mod or just a weird repair.

[Tom Phillips]

Nothing to worry about. That cap's in parallel with V4's cathode resistor, part of the vibrato oscillator circuit. At some point Fender started wiring them this way rather than put 'em on the circus board, and yours looks like a replacement for the original part which was probably a white Mallory cap.

I'm with Leo. I agree that his explanation is spot on. Attached is a photo of the original Fender cap placement circa the early 1970's.
Tom

[gui_tarzan]

I've seen several of those in SF amps I've worked on.

[elipsey]

These princeton reverb pictures are giving me G.A.S.!