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FDP Forum / Fender Amps: Vintage (before 1985) / Distortion in 1966 Fender Princeton Reverb


united states

Mar 22nd, 2017 06:52 PM   Edit   Profile  

I have distortion on the bass notes of my 1966 Fender Princeton Reverb. It almost sounds fuzzy on the bass notes especially the low E string. The high notes seem fine. Below is what I have done to diagnose it.

• changed electrolytics including filter caps, filter caps changed last year, i checked for DC current leakage on the ground with the amp on and all the caps are fine
• checked all capacitors with the amp on to see if DC current was leaking in any of them.....it's not. these are vintage mallory blue molded caps. i don't have an ESR meter.
• checked the tolerance of the resistors (in the circuit) and they all seem to be in spec....they are all vintage carbon comp
• lowered the v1 cathode cap to 1uF just to see if it's that flubby Fender low that is typical.....it was still there but less present.....it's definitely in the lower registers
• changed output transformer (i had an extra one) just to see if it was going bad......same result
• changed the signal caps on v1 to .022 with 5uF on v1 cathode....distortion still there but less.
I'm completely at a loss. I don't have an oscilloscope or I would do that next. I was just hoping someone may have had experience with this and could guide me? The reverb makes it much worse but it's there even with the reverb unplugged. I checked all the voltages with the schematic.....it's all there. What could be distorting in the circuit? A resistor that has gone south? A bad Mallory cap? I changed all the plate resistors last year to NOS 1 watt CC's (Allen Bradley).....typically that is popping and crackling when they go bad (I think )...it's not that. The amp distorts on the low end. help!



Mar 22nd, 2017 07:19 PM   Edit   Profile  

Have you crossed the amp over to another speaker to rule out the speaker?


It's not just good..

...it's good enough.
Mar 23rd, 2017 05:59 AM   Edit   Profile  

You didn't mention tubes. It seems unlikely you haven't with all the detailed work you've done, but have you tried changing any of them?


united states

Mar 23rd, 2017 06:53 AM   Edit   Profile  

Yes. Sorry. Tried different speakers and swapped all tubes. I may change the slope resistor. It has the be early in the circuit because it gets worse with the reverb circuit added. The 100k is original. I'm starting there for now.


Cincinnati, USA

Mar 23rd, 2017 10:15 AM   Edit   Profile  

Changing the slope resistor sounds like you think the amp is healthy and the tone stack needs to be tweaked.

Was it ever undistorted earlier in your ownership and then it changed over time?

Are the bass notes distorted at low volumes?

Have you checked the DC voltages and compared them to the schematic?


united states

Mar 23rd, 2017 02:31 PM   Edit   Profile  

D.C. Voltages are fine. Amp has been slightly distorted in bass notes in the past but this is worse. I should say replacing the slope resistor. Not changing the value. The signal goes through that resistor. Just wondering if it is distorting. It also passes through a 3.3M...may change that as well. A friend suggested cleaning all the tube sockets and tightening them. I may go there next.

Contributing Member


watch sunrise from the bottom of the sea
May 5th, 2017 08:37 AM   Edit   Profile  

I repaired a Princeton Reverb that had an intermittent distortion issue caused by the treble pot. Tried cleaning it first but replacement resolved the issue. Anecdotal but worth mentioning.



May 5th, 2017 11:24 AM   Edit   Profile  

Usually what I do at the point you are at in a relatively simple amp where you have checked all components (including swapping tubes and running to an outside speaker/cab) is to simply tweak the tone stack to reduce bass using the Duncan Tone Stack Calculator as a guide. Hopefully you can reduce bass enough to eliminate the distortion while maintaining balance.

My 1979 PR is heavily modded. I have no qualms doing this to an old amp to make it sound good.

Contributing Member

I've got my own

double-cross to bear
May 6th, 2017 07:04 AM   Edit   Profile  

Are you seeing the distortion on a scope?

I ask because if not, you may be chasing a cabinet-induced noise caused by vibration of something loose, after you've checked off the usual electronic causes.

Contributing Member

I've got my own

double-cross to bear
May 6th, 2017 07:04 AM   Edit   Profile  

d p !

(This message was last edited by Peegoo at 09:04 AM, May 6th, 2017)

Contributing Member

I tried to think

but nothing happened!
May 7th, 2017 12:33 PM   Edit   Profile  

I had replaced the electrolytic caps on my SF PR. I too had a faint distortion which I couldn't find. Finally sent it to a famous FDP gent who found (using an ESR meter apparently) that one of my new caps was bad.



May 8th, 2017 08:46 AM   Edit   Profile  

Yes one of the first things I do is to simply run the amp to a separate cab and speaker to rule out "tube rattle" and "cab rattle" issues.



Jun 7th, 2017 03:11 PM   Edit   Profile  

Stokes mod addresses this as well



We had the hit but Van got the money
Aug 3rd, 2017 12:04 AM   Edit   Profile  

Try replacing the reverb transformer. Common fail on a PR, causes the exact issue you noted.

FDP Forum / Fender Amps: Vintage (before 1985) / Distortion in 1966 Fender Princeton Reverb

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