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FDP Forum / Amp Mods, Repairs, and Projects / SF Princeton Reverb *nasty* attack distortion. . .

drasp

USA

Apr 4th, 2012 05:27 AM Edit Profile Print Topic Search Topic
I picked up a mid '70s Princeton Reverb on my local CL a short while back & have been enjoying getting it up & running. Came to me with some nasty hum, a swiss cheese CTS from a mid '60s Super, a dead reverb tank & some REALLY yucky "splattery" distortion on anything above medium pick attack, even at medium volumes. . .

I had the Reverb tank re-built, re-capped the amp (new 20/20/20/20 can + 22/500 on the first stage, Atoms for all the 25/25s, replaced bias cap) + all the "usual suspect" resistors. I've also been playing with other 10" speaker options. Have FIVE to play with at the moment, my current favorite being an old C10N with a recent re-cone by Weber. Put in a set of JJs 6V6s - the amp had all old glass when it came to me, but the GE 6V6s were obviously tired, the JJs brought quite a bit of life to the party. I have swapped in known good tubes in all other positions & feel pretty comfortable with the old GEs in V1-4 & the 5U4.

SO, now the reverb works, the speaker sounds fantastic, the amp idles quiet and all is well in Princetonland EXCEPT, the attack distortion is still present and TOTALLY bumming me out. . .

I did some "interneting" and saw mention of the reverb transforming crapping out, especially in the middle/later SF amps, so I pulled V2 & VOILA - the amp sounds lovely. Can dime it & there the attack distortion is completely gone. . .put V2 back & its nasty again. SO, thinking I'd found the problem, I put in a new reverb transformer (ClassicTone, was available locally), no change. Still sounds great with V2 pulled, awful with V2 in place. I've swapped in two other AT7s there and neither makes a bit of difference.

SO - given all of this - what do I need to be looking at next? Any help is greatly appreciated, of course!!! ;)

amphead4

Cincinnati, USA

Apr 4th, 2012 07:02 AM Edit Profile Print Topic Search Topic
Is this a pull-boost circuit?

Looking at the AA1164 schematic, I don't see how pulling V2 would affect the dry sound. Is the distortion only in the wet signal? That would make more sense to me.

If V2 affects the dry signal, look for a mod where the V2 plates (or the reverb trasformer secondary) are connected to something not on the schematic.

RDR
Contributing Member
***

I tried to think

but nothing happened!
Apr 4th, 2012 08:45 AM Edit Profile Print Topic Search Topic
I had the same problem, changing the reverb tranny didn't help. Turned out one of the 25/25 caps I replaced was bad.

jhundt

Netherlands

Apr 4th, 2012 09:58 AM Edit Profile Print Topic Search Topic
very interesting. I had a similar problem which was finally solved by simply changing the reverb drive tube. But you have already tried that...

maybe you have to try replacing components in the reverb circuit, beginning with the 25/25 bypass caps since RDR said that worked on his amp.

Hammond101
Contributing Member
*

So. Cal. USA

Apr 4th, 2012 10:04 AM Edit Profile Print Topic Search Topic
Gotta agree with RDR. Sounds like a bypass cap.
(This message was last edited by Hammond101 at 10:06 AM, Apr 4th, 2012)

drasp

USA

Apr 4th, 2012 11:13 AM Edit Profile Print Topic Search Topic
Excellent. I've actually read of others doing a "cap job" only to find eventually that they'd replace an OLD bad cap with a NEW bad cap. . .D'oh! Will report back. . .

edit: Oh, *NOT* a pull-boost, I assume its an AB1270, 5U4 rectifier, no tail on the logo, stock 3-prong, etc. (tube chart is missing)
(This message was last edited by drasp at 11:18 AM, Apr 4th, 2012)

Hammond101
Contributing Member
*

So. Cal. USA

Apr 4th, 2012 11:43 AM Edit Profile Print Topic Search Topic
"I've actually read of others doing a "cap job" only to find eventually that they'd replace an OLD bad cap with a NEW bad cap. . .D'oh! "

It happens every day. In my last customer build I found a bad Sprague filter cap prior to installing it. I test all componenents (caps & resistors) prior to installation & bench run the PTs.

drasp

USA

Apr 4th, 2012 12:03 PM Edit Profile Print Topic Search Topic
Hmm,. . .interesting update (maybe) I just played some more with the amp & found that I don't actually have to PULL v2 to fix it, at this point, I can simply turn down the reverb level to 1 & it sounds great! Doesn't matter if I have the reverb turned "on" or "off" with the footswitch, if the 'verb knob is at its min position, all is well. . .

SO, I retested some tubes, tried a spare set of RCA cables for the reverb tank, tried a spare tank, no luck in any of that, but I thought I'd mention this as it DOES change the conditions of the problem a bit.

willie
Contributing Member
***

Walton County, GA.

We Fixee good long time you like
Apr 4th, 2012 12:11 PM Edit Profile Print Topic Search Topic
Almost certainly a bad cap in the reverb circuit.

We too get high ESR caps as well as some out of tolerance and even shorted and open caps in new orders of caps from most all mfg. Same goes for tubes. We most recently received an order for 10 ECC81/12AT7 and 9 of the 10 tested bad enough that they couldn't be used.

willie
musicmenders.com
(This message was last edited by willie at 12:11 PM, Apr 4th, 2012)

amphead4

Cincinnati, USA

Apr 4th, 2012 12:16 PM Edit Profile Print Topic Search Topic
Check the coupling cap that feeds the reverb pot to see if it's leaking DC. If it is, turning up the reverb is putting DC on the grid of the next stage, wrecking the bias.

drasp

USA

Apr 7th, 2012 10:34 AM Edit Profile Print Topic Search Topic
I can't say its 100% quite yet, need to bias the fresh JJs I just put in & I think there is room for improvement in the preamp glass I'm using, but I'm WAY closer to where I need to be. . .

The solution!? Take a look at the pix above - notice anything funny about the # of cathode bias caps? Uh-huh, the 25/25 that should go to pin 3 on v2 was MISSING. Now, I can't say for sure that the amp was all original, obviously someone added a 20uf cap where one section of the original cap can failed - long ago from the looks of it, but I was pretty darned sure I was the first to solder on the eyelet board - the joints looked original. That said, they weren't 25/25s, they were 50uf/25V Mallorys, which doesn't fit any version of the PR schematic I can find. Also, the layout on the cathode bias cap for V4 seemed weird (right on the tube socket to ground @ the reverb RCA jacks) - I moved that back to the board as well & replaced it with an atom. SO - going to go back to the original reverb transformer, which it now seems was just fine (will see) and poke around a bit more to see if anything is missing/weird.

rfrakes331K
Contributing Member

USA

RonHalen Jokingly He Says
Apr 7th, 2012 01:20 PM Edit Profile Print Topic Search Topic
Seems like I remember reading that a 50uf/25V cap is used to make the power supply more robust. In Princeton mods.

amphead4

Cincinnati, USA

Apr 7th, 2012 03:48 PM Edit Profile Print Topic Search Topic
Given that cathode bypass caps increase stage gain, I'm having trouble seeeing how a missing one would cause harsh attack distortion.

But finding all miswired and rewired "features" of your amp is a good thing.

SoK66
Contributing Member
****

USA

We had the hit but Van got the money
Apr 12th, 2012 07:56 PM Edit Profile Print Topic Search Topic
I had this exact symptom on a '79 SFPR. Turned out to be a bad reverb transformer. Drove me nuts. Not uncommon failure on some mid-70 & later SF amps where the pull boost uses the driver. CBS Fender reduce the cathode resistor value and eliminated the bypass cap, driving the little reverb tranny harder and causing eventual failure. A suggestion is to change the resister value to the AA1164 (BF) spec and add the bypass cap.

amphead4

Cincinnati, USA

Apr 13th, 2012 06:39 PM Edit Profile Print Topic Search Topic
I can't figure out how a bad reverb transformer impacts the dry signal.

SoK66
Contributing Member
****

USA

We had the hit but Van got the money
Apr 14th, 2012 01:27 PM Edit Profile Print Topic Search Topic
AS I read it, taking the reverb tranny out of the signal cleans it up. Same symptoms I had. That he still has the racket with a new reverb tranny eliminates that possibility. He's noting that there are "missing" and incorrect cap values in the reverb circuit, which is common post '76. We've had success conforming those elements back to AA1164 schematic values.

FDP Forum / Amp Mods, Repairs, and Projects / SF Princeton Reverb *nasty* attack distortion. . .

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