SF Princeton Reverb: *nasty* attack distortion. . .

[drasp]

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!!!

Oh, some pix might help, 'eh!? (can certainly provide larger versions of either, more detailed shots, etc.)

Before:



After:

[pdf64]

Try a grid stopper on the reverb driver (say 47k-100k) and / or remove its cathode bypass cap.
Is the power amp gain sufficient, eg has the 47 ohm NFB tail resistor drifted up?
Fender BF reverb pre-amps tend not to overdrive smoothly but the power amp does, does so earlier, and so dominates.
Pete.

[Jazz P Bass]

I would do a few voltage checks to see what is happening when V2 is in circuit & again when it is out.
If the B+ is being pulled down with the tube in, your reverb transformer may be suspect.

[gbono]

Have you scoped the output? I bet the output stage is clipping = "splattery" distortion. What tube are you using for the phase inverter?

[drasp]

Thanks all for the suggestions!!!

pdf64 - both sound like good suggestions, want to dig a bit deeper for bad/failed/drifted parts or a "solution" before modifying/adding parts, ya know?

Jazz P Bass - will check that, you did see I replaced the reverb transformer with a brand new one, no change between the old & new transformers. . .

gbono - have not, I'm a hobbyist (as far as amps go), no scope. The PI I'm running now is the old GE that was in the amp when I got it, but I've swapped in known good tubes in all preamp positions w/ no change.

[drasp]

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.

[Jazz P Bass]

What happens if you totally disconnect the reverb tank?
Does the reverb control still mess up your signal?
If so, then the return half may be the issue.
Test the wiper of the reverb control for Vdc.
If there is any, that .003 cap may be leaking.

[drasp]

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.

[drasp]

UPDATE: the amp is dramatically improved from when I first got it and really sound just fantastic up until ~6ish on the volume knob. At that point, when I play with heavy pick attack it still falls apart - sounds, to me, like the power section just isn't keeping up. The problem still goes away entirely when V2 is pulled, BUT today I realized that it ALSO goes away when the volume pot is maxed out!

Seems this just HAS to be a clue I'm not smart enough to solve! Anyone?

[audiopete]

I can't see the rating difference or location or lack of a bypass cap as being the problem here. The beauty of a scope is you can find out exactly where the distortion is being produced and concentrate on that area of the circuit.

You sure you aren't getting parasitic oscillations in the reverb circuit that are robbing your sound when you turn up? Check the lead dress around the reverb wires/tubes/components. Keep plate wires away from grid wires. Someone has been in there mucking around before so anything is possible.

What happens when you pull the rca from the output side of the tank and play?

[audiopete]

I looked closer - try to move the white and off white wires away from each other between V2 and V3 near the circuit board. Ones a plate wire and one is a grid wire.

Also there was a 50uF bypass cap because the two sections of the reverb driver are in parallel and have a shared cathode capacitor.

[drasp]

audiopete - thanks a TON for taking such a careful look at the problem!

Will get the chassis out & play with the lead dress to see if I observe any changes. If that DOES turn out to be the issue, you'll have to explain to me how turning the treble knob all the way up, even with v2 in & the reverb cranked, makes the problem go away 100%. . .totally baffled at that!

[g1]

how turning the treble knob all the way up, even with v2 in & the reverb cranked, makes the problem go away 100%. . .totally baffled at that!

This will sometimes occur with oscillation problems. The fact that it is happening helps confirm that the problem is likely an oscillation. Sometimes the pot that causes this to occur will be in the associated circuit, sometimes it only needs to be physically near the oscillation.

[drasp]

VERY cool! Not going to have time tonight, but hopefully tomorrow can dig in. . .

[audiopete]

Parasitic oscillation is a result of gain, frequency and lead dress (or component layout). If the gain is enough, the frequency band correct and the lead dress too close together, signal with couple via magnetic induction to an earlier part of the signal. Anything you change that changes gain or frequency (which is also a gain change) will affect the oscillations. Turning up the treble reinforces higher frequencies and raises signal gain in the circuit. Turning down tone controls dumps signal (and gain) to ground (eventually).

You want to have any parallel wires as far apart from each other as possible. Any wire that needs to cross another ideally should be done at 90 degree angles to prevent coupling.

While you are in there, pull all the green heater wires up into the air straight above the tube sockets to get the heater hum as far away from everything else as possible. Follow the green twisted pair from the power tranny to the first socket. Pick one wire and follow it along to all the other sockets and make sure that wire connects to the same heater pin on every socket. If it gets reversed somewhere, flip it with the other one and continue following to make sure all the heaters are in phase. You may not have a heater hum problem but this will make sure. Just saying cause that lead dress is a little unruly....