69 Fender Princeton Reverb - Vibrato and Tone Stack Issues

[jl_lensman]

I am trying to bring back to live a long neglected 69 Princeton Reverb. The power transformer smoked so I drop in a replacement from MojoTone. All new tubes throughout as the originals looked pretty smoked even though they still worked. The amp works pretty good except for (2) issues.

1) Toggling the Vibrato is backwards - I checked the RCA jack and it is not shorted. If I put in a shorting plug no vibrato, remove the shorting plug gives the vibrato back. The footswitch does not seem to control the on/off function of the circuit. I'm kind of stumped as to what is going on.

2) The bass distorts badly when I turn the bass pot past 5. I guessing that I probably need to replace .047 cap. Does that sound right or do I need to look at something else?

[jl_lensman]

Vibrato problem solved -- still need answer to bass farting out at higher volume

Ok, figured out the vibrato issue. I had only used a shorted RCA plug on the amp vibrato pedal jack and not one on the reverb at the same time. DUH!!!! Fashioned another for reverb and everything works like normal.

Now, the only issue I have is after playing the amp till it is warmed in real good (about 20-30 mins) if I crank the volume up past 6 with the bass set to about 7 or so the speaker starts to flub/fart out hard. Any suggestions to fix? I am bit a newbie so please keep the tech speak simple. I can handle a soldiering iron, read schematics, and know a resistor from a capacitor. I have revived a dead Vibro Champ and "The Twin".

[MWJB]

Firstly, it would be a good idea to check to see if there was specific reason as to why the PT smoked. Have you replaced the electrolytic caps in the power supply, bias supply & cathode bypass?

What plate current are the 6V6s running now? What are the voltages at the 6V6 tube plate, screen & plates, cathodes for the 12A#7 tubes?

Do you really need to turn the bass to/past 7?

[jl_lensman]

I suspect the PT smoked due to age and inactivity. Yes, I did replace the filter cap and resistors that are in line with that as well as couple others that just didn't look right. I didn't see anything really bad (burnt or cracked). After installing the new PT the amp seems fine, good strong vib, loud and response amp the starts to break up around 6 so I am quite pleased. The only thing I changed was the 1M resistor on the vib circuit to a 470k.

This amp really was a basket case. I had to clean up quite a bit of surface rust (my dremel worked overtime), the cabinet needed to have some glueing and wood filler, and the tolex needed to be reglued. The transformers all had heavy rust on them, the chassis some surface rust, and the hardware thoroughly rusted (I had to drill out most of the screws). Amp looks 100% then when I first got it and sounds really sweet.

[Jazz P Bass]

Fender amps have always had an issue with bass notes when the amp is cranked.
You could try lowering the value of the bypass capacitor on the cathode of the first tube.
25uf/ 50V is standard.
Lowering the value anywhere from 15 to 4.7uf will certainly help.