1964 Fender Princeton Reverb restoration

[xtian]

A client brought me a beautiful '64 PR. Untouched...EXCEPT...someone had removed the reverb tank, transformer, RCA jacks, two tube sockets and all related components. Before:

64pr before.jpeg

The amp played OK in this condition, though the power was anemic, and it wouldn't get very loud. I convinced owner to let me recap, update power cord, and replace the reverb circuit to full function. After:

IMG_1266.jpg

All great now, very nice overdrive, 430 on plates with a JJ 5Y3 recitifer, and running at 11 watts clean output. The reverb is very strong--only need to turn it to 3 for a big 'verb sound. However, when I turn the amp up to overdrive, it's very raspy and gated. As soon as I disconnect the Reverb Send RCA cable, all is good, no ugly distortion. With the reverb tank plugged in, even with the Reverb pot set to zero, the overdrive is really bad. The connection to the tank causes the symptom to appear.

So, did I get the right reverb transformer (Hammond P-T1750A) and tank (Mod® 9AB2C1B)?

[xtian]

I confirmed the reverb tank and transformer are appropriate.

So, anyone know why I'm getting nasty distortion in the signal path with the tank attached, but regardless of Reverb control position?

[ViperDoc]

I used a Hammond 1750A reverb driver on my last PR build and it works well. I put a log taper Reverb pot on mine for a more usable sweep. No idea about your reverb tank distortion. Maybe the RCA jack bus bar creates a ground loop with the chassis? Those RCA jacks are chassis-grounded, yes?

Edit: Not sure, but your gold RCA jacks maybe look isolated to me. Fiber washers?

[xtian]

Yes the RCA jacks are of the isolated type, but I have grounded them to the chassis.

[martin manning]

Very weird… a reverbectomy. The new driver and tank are correct for BF reverb. I’d guess you have parasitic oscillation when the signal level gets high enough. Look at the Fender layout at the reverb driver tube socket, particularly the plate jumper.

[sluckey]

I always wondered why Fender did the plate jumper differently. That may be a factor. Here's a '64 DR pic. PR layout is the same.

[xtian]

I always wondered why Fender did the plate jumper differently. That may be a factor. Here's a '64 DR pic. PR layout is the same.

Yes! I looped that anode wire around the socket, and kept it away from the green output wire. Very good! Issue solved. Thanks, Steve!

[sluckey]

Hey, I was just playing on Martin's lead. Maybe he has a theory about this???

BTW, Hoffman did his PR reverb driver jumpers pretty much the same as you did. Never heard of any issues. Go figure.

[martin manning]

Steve, I suggested moving that jumper because I have heard of it causing problems before. The difference I see in the picture you posted vs. Arron's amp is that the jumper is raised up above the socket on yours. Perhaps it has to be some distance from the tube base, either above it or off to the side.

[sluckey]

This discussion made me dig up some pics of a single channel AB763 DR that I built in 2006. I thought I did my jumpers the same as xtian did. Turns out I put the jumpers up high enough to clear a shield pin in the socket.