Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: fdesalvo on November 29, 2022, 07:30:09 pm

Title: SF Princeton Rev: distorting with reverb tank plugged in
Post by: fdesalvo on November 29, 2022, 07:30:09 pm

Hello!  Long time and I hope alls well. Have a 72 Princeton reverb on the bench that’s distorting heavily with the reverb tank plugged in and reverb level above 4. Level acts like a gain pot lol.  Amp arrived with a decent cap job completed. Thought maybe the screen filter cap node was going south in the ce cap but it’s fine. Voltages look great with no leaking coupling caps or conductivity on the board. Has anyone seen this before?


Cheers!

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: Latole on November 30, 2022, 03:42:34 am

You need a scope to see how look the input signal at the reverb tank and what s look a the reverb tank output.

Clean at the input and distortion at the output ; replace the reverb tank
Distortion at the input; check amp stage before.


Try new cable; any hi-fi rca cable do the job

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: sluckey on November 30, 2022, 05:29:51 am

Does the amp distort with the reverb footswitch turned off?

Does the amp distort with the reverb pot set to zero?

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: fdesalvo on November 30, 2022, 08:13:08 am

- No scope access, I’m afraid.
- I’ve already tried with known good test tank and cable set.
- Tube change didn’t help
- I’ll see if the amp distorted with the reverb at 0, can’t recall offhand. I believe it sounded pretty clean with the level set to 0.
- Haven’t tried with footswitch plugged in. Have been testing without




Will update Ty!

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: sluckey on November 30, 2022, 09:06:43 am

Quote from: fdesalvo on November 30, 2022, 08:13:08 am

- Haven’t tried with footswitch plugged in. Have been testing without

Well use a gator clip lead to ground the footswitch center pin. Any distortion?

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: fdesalvo on November 30, 2022, 09:45:53 am

I removed the cathode bypass cap on the recovery stage and it resolved the issue. Strange times at the shop. I’m going to install smaller cathode bypass caps on the preamp to trim some additional blub. Ty all!

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: Latole on November 30, 2022, 12:49:42 pm

Quote from: fdesalvo on November 30, 2022, 09:45:53 am

I removed the cathode bypass cap on the recovery stage and it resolved the issue. Strange times at the shop. I’m going to install smaller cathode bypass caps on the preamp to trim some additional blub. Ty all!

Bypass caps give boost to a tube

Title: Re: SF Princeton Rev: distorting with reverb tank plugged in
Post by: fdesalvo on November 30, 2022, 01:21:48 pm

Indeed. Removing that and cleaning up the lead dress around the recovery tubes did the trick.