Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: aabbs20 on August 21, 2019, 11:22:26 am

Title: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 11:22:26 am

Hey all,

I'm finishing up a Princeton Reverb build and I've got a strange issue.  I've built a number of other PRs and have never encountered this problem.  When I turn the reverb control up it works, I hear great reverb, but the overall volume of the amp is decreased significantly.  I've tried different tubes, reverb tank, cables, 10PF and 500PF caps, 3.3m resistor, and bypass cap on reverb recovery with no change.  I've also measured voltages all over the amp while making the problem occur and haven't seen anything suspicious.

One other piece of info...the amp has a NFB switch.  When I cut the NFB, the issue goes away and the reverb works as it should.  When the NFB is engaged, the problem returns. 

I'm sure I've made a mistake somewhere but I'm stumped!

Thanks.

Title: Re: Princeton Reverb Build Issue
Post by: sluckey on August 21, 2019, 12:36:38 pm

Quote

One other piece of info...the amp has a NFB switch.  When I cut the NFB, the issue goes away and the reverb works as it should.  When the NFB is engaged, the problem returns. 

Maybe you need to swap the OT primary plate leads.

Title: Re: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 12:44:57 pm

On my initial startup I heard the classic squeal so I swapped them at that time and it went away.  I just tried swapping them back and heard the loud squeal again.

Title: Re: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 07:30:56 pm

I've tried a few more component swap in the reverb circuit without any luck. Do you guys think it's a reverb oscillation issue or something else that's manifesting itself when I turn the verb up?

Also, the volume drop occurs even when the reverb tank is disconnected and V2 is pulled.  I'm thinking the problem is elsewhere.

Title: Re: Princeton Reverb Build Issue
Post by: shooter on August 21, 2019, 07:49:35 pm

using this for ref;
https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf

based on what you said verify
R 26 28 29 36 37 and make sure they are who they say they are  :icon_biggrin:
(might have to lift ends)

Title: Re: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 07:55:44 pm

Hey Shooter,

They all measure out and I've even swapped out a bunch of them already just in case.  No change.

Title: Re: Princeton Reverb Build Issue
Post by: PRR on August 21, 2019, 08:20:34 pm

Take it out in the sunlight.

The symptom suggests that the 470k is a 47k (yellow versus orange).

Title: Re: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 08:23:45 pm

Thanks PRR.  I put it under a flashlight and the resistor in question is definitely a 470k, I've even tried 2 different 470k resistors in that spot.  I just measured it out to be extra sure.

Title: Re: Princeton Reverb Build Issue
Post by: sluckey on August 21, 2019, 09:52:16 pm

Post some hi rez pics.

Title: Re: Princeton Reverb Build Issue
Post by: aabbs20 on August 21, 2019, 09:59:29 pm

Well...chalk that one up to an ID-10-T error. 

I was just getting ready to post some pics and I thought I'd look it over one last time so I didn't miss any wiring errors.  Fresh eyes helped see an obvious mistake. Turns out I had connected the V3 cathode wires to the wrong cathodes...so the nfb was tied to the reverb recovery. Doh! 

Thanks for your help guys. 

Title: Re: Princeton Reverb Build Issue
Post by: PRR on August 22, 2019, 12:46:13 pm

> the nfb was tied to the reverb recovery.

Thanks for the come-back. Not something I would have suspected.

Title: Re: Princeton Reverb Build Issue
Post by: shooter on August 22, 2019, 02:02:25 pm

Quote

Fresh eyes helped

 :thumbsup:

I'm myopic by birth, sometimes it creeps into mylogic also :icon_biggrin: