Noisy volume pot, gah

[Diverted]

Hello all,

First post in a while.
I am stumped. I have a Princeton Reverb build (no tremolo) just completed that is fine except for a crackly, whooshy static that appears when I turn up the volume. Buzz also increases with volume. I measured DC on the vol wiper and at volume 0, DC is a few mv. It then gets quiet until about 12 o'clock, then starts up again. By 4 o'clock mv reading is it's highest, about 30mv. From there it backs down and the amp returns to silent operation dimed.
Problem disappears with V1 and V3 pulled. I tried reading for leakage with V1 pulled but there is nothing on the downstream side of the 250pf, .05 and .1mf tone caps.

When I ground V3B grid (pin 7) sound, and problem, vanish.
I have tried the usual ... chopsticking adjusting lead dress. The wiring is squeezed into a small space but I think I've done a decent job keeping wires separated. I also used shielded on inuts, wiper to V1B grid, and on jumper from downsteam side of .0033 cap off the V3B plate to phase inverter input.

Only changes I made in the amp are a few layout changes, putting the tone caps at the pots, and using a 6AV6 in V4 phase inverter.
Here are some high res shots of the photos. If you need more, please let me know. And if you can help, thank you!

https://imgur.com/a/ncR84nG

[Diverted]

Here's another photo of V1-2 pot wiring. Thanks.

[shooter]

this schematic close?
https://el34world.com/charts/Schematics/files/Fender/Fender_princetonreverb_b1270_schem.pdf

Problem disappears with V1 and V3 pulled.

When I ground V3B grid (pin 7) sound, and problem, vanish

so V1 by itself doesn't eliminate problem?

except for a crackly, whooshy static that appears when I turn up the volume.

typical things would be a parasitic oscillation
sketchy tube sockets

[Diverted]

Yes that's the schematic apart from no tremolo.

I looked into parasitic oscillation this morning but didn't get anywhere what would you suggest?
As far as dodgy sockets go, they seem OK to me but I'm no expert. They're belton's and the pins all seem to be seated tightly ... no noise or other issues when I give the tubes a nudge.

Thanks.

[Diverted]

Pulling V1 eliminates the volume and all other noise.

[stratomaster]

As far as dodgy sockets go, they seem OK to me but I'm no expert. They're belton's and the pins all seem to be seated tightly ... no noise or other issues when I give the tubes a nudge.

Your V2 socket (reverb driver) looks pretty well cooked/melted. Your plate connection has a solder run that looks in danger of shorting to chassis.

I think you need to really go over everything and scrutinize your workmanship.  Watch for collateral damage due to soldering iron positioning. Plan your approach and escape carefully. Replace overheated components, etc.  All these things add up to a much better end product.

As far as whooshing sounds and DC I have experienced more and more preamp tubes with DC on their grids due to internal leakage.  This is noisy and upsets the bias of the triode.  Try swapping in a new tube and seeing if your DC measurements change under identical conditions.

[Diverted]

Thanks Shooter for your assessment.
This was a tricky chassis, deep with not much angle to hit all the spots easily. It made running the heaters difficult because I do that last. But apart from some melty bits on PVC wire and socket edge, all joints are good and it's functional with solid conections if not the neatest thing I've ever done.
I've swapped tubes ad nauseum but still getting pretty much the same results.

[stratomaster]

Look again. Burnt and melted sockets compromise the insulative properties of the substrate. This run is very close to shorting HT to chassis.

[Diverted]

Thanks for the assessment Stratomaster. I saw that but honestly, I don't think V2 is my issue. The photo is misleading. The lead is probably 3/16" above the chassis. The melted bit between the pins is unsightly but again, shallower than it looks. Also pulling V2 has no impact on the volume noise either way, so the problem I would think has got to be coming from elswhere? Thanks.

[shooter]

this happens with nothing plugged in??

[Diverted]

Yessir, correct. Issue present with or without an instrument plugged in. Thanks.

[shooter]

As far as whooshing sounds and DC I have experienced more and more preamp tubes with DC on their grids due to internal leakage.  This is noisy and upsets the bias of the triode.  Try swapping in a new tube and seeing if your DC measurements change under identical conditions.

Thinking along those lines, except V1A's grid leak "missing" or sketchy because of input jacks, the grounds....

I looked into parasitic oscillation this morning but didn't get anywhere what would you suggest?

I would park my scope at the 3.3M .02uF cap junction, AC coupled, referenced to chassis ground, NO input, REV OFF, all TS and Vol at zero and verify there is no there there.  Then start playing with knobs 1 at a time 0-10 n see if you start getting an AC signal

[Diverted]

Shooter, unfortunately no scope so shooting blind.
I've lost count of the things I've tried: Unscrewing and moving the rever transformer, thinking it was possibly mounted in a noisy orientation. Nothing. bypassed reverb circuit entirely, hardwiring from tone stack out to gain gain stage just before PI in. Nothing.
Tried new caps in the tone circuit, etc etc etc.

Seeing as it happens even when no input connected, am I looking at a grounding issue? I've checked and rechecked grounds, tried jumpering back and forth between the power supply/first three caps ground and the preamp ground buss, nothing.
Here's a video showing the buzz/noise, with all tone controls at zero. Not quite as loud as it seems in video but plenty loud. It has buzz at volume zero. It then goes away, comes back louder around 2 oclock and gets louder, until it drops off quickly.

[shooter]

sounds like you're letting in "environmental" noise. lights, phones et al.


a sensitive wire has a sketchy ground reference, or "bent" in such a way as to create an antenna

When I ground V3B grid

do V1a

so shooting blind.

I've been known to sell my big-screen TV just so I had a scope, but that's just the geek in me

[Diverted]

I've wanted to get one for years ... actually owned one for a short time.
But I have no background in electronic test equipment and have no idea how to configure and properly use a scope. Nobody around here to teach me, and it's intimidating to try to figure it out. With some stuff I learn better by watching and doing, that is definitely the case with a scope.


Anyway...
Grounding out V1AB, V2AB and V3A makes no difference.
Grounding out V3B eliminates the noise. So it's got to be somewhere in that last gain stage gefore the phase inverter.

[Diverted]

Just trying to keep the thread going, unless there's nothing more to add. Given that you can hear noise with no instruments plugged in and volume at 0, and that it goes away when I ground out V3B grid, can I assume there is a ground issue or something in or just before the last gain stage before the phase inverter? Or should I be looking elsewhere? Grounding reverb circuit, removing V2, grounding V3A, disconnecting B+ from reverb transformer primary, etc., have no impact.

Circuit is same as attached with few changes: No tremolo, 25K mid pot and SS rectifier. First three filter caps are on the power ground which connects to chassis adjacent to PT. Preamp grounds run off a buss bar connected at the speaker jack ground. And phase inverter plate is supplied by the third filter cap, not fourth as built in Princeton Reverb.

Thank you.

[shooter]

Preamp grounds run off a buss bar connected at the speaker jack ground.

move that to input jack ground

[stratomaster]

Grounding out V1AB, V2AB and V3A makes no difference.
Grounding out V3B eliminates the noise. So it's got to be somewhere in that last gain stage gefore the phase inverter.

V3B eliminating the noise implicates everything in the red box. The other grids getting grounded not eliminating the noise exculpates everything in the various other boxes.  What remains is where you look.

You can completely disconnect the signal taps for the reverb circuit from the 3.3M to eliminate that circuit entirely.

You can ground the input side if the 3.3M to rule out the remaining V1 circuitry. 

There's still plenty of places to look, but you've narrowed it substantially.

If you still have the issue after continuing to strategically ground, then look at the negative feedback loop and V3B cathode circuity. Grounding the V3B grid could mask issues there. You can simply lift the 2.7k resistor to rule out the FB. Grounding with NFB can get tricky especially when altering this power/grounding scheme and keeping the PI design.

[Diverted]

Thank you very much Stratomaster. I've already tried some of these things, so my to do list is getting shorter.

When I jumpered V2B plate/coupling cap to third gain stage V3B grid, I just removed everything between those two points, IE the reverb circuit. The noise continued.

Same thing with negative feedback. Removing the NFB resistor from the OT secondary had no effect other than the expected minor increase in noise.

I appreciate the help and the diagram. Will update as I go and hopefully no more hair will be pulled out soon.

[Diverted]

Preamp grounds run off a buss bar connected at the speaker jack ground.

move that to input jack ground

I'm sorry that was a typo. I meant input jack, have no idea why I wrote speaker.

[AlNewman]

Try grounding the wiper of the reverb pot.

[glass54]

Hi Diverted,
Just to divert for a moment 
Any reasonable, not expensive (affordable) 60-100MHz modern LCD Scope would do.
There is a MAGIC Button ( ) that I occasionally resort to, see attached, saves a lot of headaches. This can be you new friend.
They come in various brands, so do a little surfing and let us know. I'm sure between the guys here and some YouTube probing, you could learn and be comfortable with an Oscilloscope. The family could shout you a Christmas present? 
(This is my "GO TO" workshop oscilloscope)
All the best with the amp.
Kind regards
Mirek