Princeton Reverb questions

[New2ampsman]

I have a drip-edge Princeton Reverb S/F which I believe is a 1968/69 amp. Some sort of transition period where S/F still had some B/F parts? Anyway the tube chart is AA764 which seems to be a Champ type circuit not a Princeton Reverb. It indicates a GZ34 rectifier but the AA1164 circuits for the Princeton show a 5U4GB tube.
I had it professionally serviced about 6-7 years ago and basically it's worked fine but recently I noticed it seemed to break up at around 4-5 on the volume and it seems like the front of the note is suppressd for a fraction of a second then it comes back in at the correct volume  (if a little overdriven) with definite break up. I recently built a tube amp from a kit and it has been a lot of fun learning about tube amps. Now I realise the old Princeton may have some issues which I'd like to fix myself.
I suspect what I am hearing is rectifier sag - I'm running a 5AR4. First thing I've checked is the heater voltages and they're running at 6.85V ac. I understand that the power transformer is supposed to deliver a precise 6.3Vac so is this significant?
Also many of the carbon comp resistors seem to be quite a bit higher than the values shown on the AA1164 circuit even allowing for a 10% tolerance.
Any suggestions about what sequence I should follow for sytematically troubleshooting and bringing this great old amp back to specs?
Thanks

[tubeswell]

1st check the DC idle voltages throughout the power supply (B+) rail and the tubes.

You may want to think seriously about re-doing the filter caps.

I wouldn't worry about the CCs at this stage (if it were me) - check the other stuff first

[New2ampsman]

Ok thanks tubeswell,
I'll do that - the main filter cap totem was replaced by the service guy just a few years ago and I have assumed it was OK but some of the voltages do seem off what they should be? The
I think all my tubes are in good shape but I wouldn't really know how to verify that without just changing out each tube in turn and I don't have new replacements for all of them.
I guess I'm just going to have to keep reading books, checking voltages  :smiley:and trying to see what seems really way out and go from there?

[dude]

I wouldn't think the 5AR4 would have much sag, but the voltages probably did drift up over time.  I'd check the B+ rail down the line and change the resistors to get your voltage in the proper range as tubeswell mentioned.

IMO, old coupling caps in the tone circuit can effect the tone (I think it's more a matter of inexpensive caps used than the age), you may want to replace a few with better caps. I had a Princeton that sounded kind of muddy when it started to break up 4-5, I changed out some of the coupling caps along with the filters and checked the "bias of the 6V6s" which was off, correcting that bias cut out the mud and made the amp more crisp.

al   

[Fresh_Start]

Tubes?  Especially power tubes - I'd try a substitute pair of power tubes and then, one by one, test substitutes for each of the preamp tubes.  Tubes are wonderful, but they ain't perfect!

Also, if the resistors for the phase inverter are way out of spec, that might have undesireable effects like you describe.  I recently "blue printed" a Blackface Princeton Reverb and found that many of the components had drifted (resistors up in value & capacitors down, generally).  Wanting to change as little as possible, I only replaced the bass & mid caps in the tone stack, plus what should have been a 3.3 meg resitor in the reverb mixing circuit.  The owner now has a Bass control that actually does something, and the signal going to the PI is at the level it should be instead of much quieter.

But I'd start with the power tubes...

Chip