Help troubleshooting a Princeton Reverb Build

[Dr Tony Balls]

Hi All,

I'm building an AA164 Princeton Reverb and having some issues with noise/shutoff at elevated volumes. I'm building to Fender's schematic and using my own layout, modifed from Fender's. Everything works and sounds good at lower volumes, it just goes south as you turn up. Here's what I have going:

TESTING WITH FULL AMP: As you turn up the volume control,at some point a ragged distortion starts to happen, particularly with a higher input signal (i.e. as you dig into the strings harder). If you turn it up even more this becomes a squeal/oscillation, This is exacerbated by maxing out the treble control, i.e. it happens earlier if it is all the way up.

TESTING WITH REVERB DRIVER REMOVED: At some point I decided to test it with the 12AT7 (V2) removed and got different results. Now there was no oscillation, but if both the Treble and Volume controls are all the way up it goes silent. I should mention that I am testing without the reverb pan connected.

Deviations from stock:

- 470K grid stopper to PI adeed, per Rob Robinette suggestion
- 6.8K resistor in tone stack replaced with 25K Mids pot
- LarMar master volume added
- shielded wires used for girds of V1

Here's the Fender schematic i'm using: https://schematicheaven.net/fenderamps/ ... aa1164.pdf

And here's my layout, let me know if there are questions:



Anyone have ideas what could be the problem or where to look? Am I potentially looking at multiple problems given that its different with the Reverb Driver installed?

THANKS FOR ANY INPUT.

[johnnyreece]

Have you tried disconnecting the NFB? That'll tell you if your OT primaries are backwards.

[Dr Tony Balls]

Have you tried disconnecting the NFB? That'll tell you if your OT primaries are backwards.

Yep, and no change. Here's what i've done since the original post....

Made sure that the Master Volume has no effect on anything. Ive tried it with the Master cranked and the problem is the same.

WITH V2 INSERTED AND TURNED UP TO OSCILLATION:
Ground grid of V1A - no change
Ground grid of V1B - silent, no surprise as this is essentially turning the volume off
Ground grid of V2A/B - silent
Ground grid of V3A - silent, no surprise as this is essentially turning the reverb pot down
Ground grid of V3B - muted but still squealing
Ground grid of V4A - no change, tremolo oscillator
Ground grid of V4B - muted further, but still squealing


V2 REMOVED, SINE WAVE INPUT, WATCHING ON OSCILLOSCOPE AS I TURN UP THE VOLUME AND WATCHING THE WAVE AS THE OUTPUT SIGNAL CUTS OFF
Grid of 4B: Sine wave grows as volume is increased till it cuts out at the same time speaker output cuts off.
Grid of 3B: Same.
Plate of V1B: Same.
Grid of V1B: Same.
Plate of V1A: Sine wave does not cut out when speaker output cuts off.

So this tells me its gotta be in the tone stack/volume control somewhere.

I tried chopsticking with the treble and volume all the way up (i.e. when the signal dies) and notice no change.

I've reflowed all my solder joints between V1A and V1B and no change.

I tried replacing the treble pot and not change. Can try the volume next.

The ONLY thing that produces a change is plugging in to the low input, but I think that's just attenuating the signal enough where the change doesnt happen.

[johnnyreece]

Okay, another dumb question...have you measured for continuity on the volume pot across the sweep? I mean, is it possible that the end of the track is dead when it gets turned all the way up? Have you tried rolling preamp tubes to see if maybe you've got a noisy one?

Edit: You mentioned shielded grids, but what about the leads going to the bass/treble controls? Also, did you try chopsticking when it was oscillating, or only when the sound was gone?

[Dr Tony Balls]

Okay, another dumb question...have you measured for continuity on the volume pot across the sweep? I mean, is it possible that the end of the track is dead when it gets turned all the way up? Have you tried rolling preamp tubes to see if maybe you've got a noisy one?

Edit: You mentioned shielded grids, but what about the leads going to the bass/treble controls? Also, did you try chopsticking when it was oscillating, or only when the sound was gone?

I tried swapping the volume pot for another and got the same result. And yes, tried replacing each tube with a new one and no changes.

No amount of chopsticking anything so far has gotten any change to happen.

[johnnyreece]

Got any gut shots? Oh, I didn't notice whether you addressed whether the wiring to your pots was shielded or not.

[Dr Tony Balls]

Pics!

Board and control panel:







Board and tube sockets (the jack between V2 and V3 is for footswitch):







And the reverb transformer connections and send/receive RCA jacks are kinda hidden beneath the V2 socket:




At this point ive isolated the problem to the tone stack, as im seeing it on the scope at the grid of V1B, but not at the plate of V1A. Gonna try disconnecting it and bypassing it soon and checking results.

[johnnyreece]

I see that you've got the wires to your reverb control tucked in with your tone stack wires. Maybe try separating those? Good looking build!

Edit: Lots of long leads, too. I know it looks nicer that way, but maybe trimming some extra wire length will help.

[Dr Tony Balls]

I see that you've got the wires to your reverb control tucked in with your tone stack wires. Maybe try separating those? Good looking build!

Thanks! Not my first rodeo, which is what makes this so puzzling. At this point i'm testing without V2-V4 even installed so its hard to believe the reverb control wires would matter. Without V2 and V3 all the reverb looks like to the rest of the amps is a 500pF/1M to ground on one side of the 3.3M resistor and a 470K to ground on the other side. All that wiring is completely devoid of signal right now.

[sluckey]

Compare your V2 wiring to Fender's wiring. Your jumper between pins 1 and 6 goes directly across the socket while Fender's jumper loops around the socket, close to the chassis. Make your jumper look like Fender's jumper and see if some of your issues disappear.

[johnnyreece]

Sorry, didn't mean to sound like I was insulting you. I'm aware you've been at it a while. Just starting where I would start were it mine; with simple, common things.

[Dr Tony Balls]

Compare your V2 wiring to Fender's wiring. Your jumper between pins 1 and 6 goes directly across the socket while Fender's jumper loops around the socket, close to the chassis. Make your jumper look like Fender's jumper and see if some of your issues disappear.

Good observation, and will definitely address that as needed, but without V2 inserted will that really make a difference? I will try temporarily disconnecting that and checking.

Sorry, didn't mean to sound like I was insulting you. I'm aware you've been at it a while. Just starting where I would start were it mine; with simple, common things.

NO INSULT TAKEN! And yeah I know...its always the simple things but that's why I need to talk it through with others who might be seeing what i'm overlooking.

[Dr Tony Balls]

Compare your V2 wiring to Fender's wiring. Your jumper between pins 1 and 6 goes directly across the socket while Fender's jumper loops around the socket, close to the chassis. Make your jumper look like Fender's jumper and see if some of your issues disappear.

Good observation, and will definitely address that as needed, but without V2 inserted will that really make a difference? I will try temporarily disconnecting that and checking.

Tried this with no change.

[johnnyreece]

Glad I didn't ruffle feathers. Tone is hard to read. I still suspect something in the tone stack/volume control wiring. Have you pulled out the bundle when chopsticking? having lots of wires running parallel, while looking nice and tidy, is sometimes a recipe for noise. If that doesn't do anything, it might be time to post some voltages for more clues.

[Dr Tony Balls]

Issue resolved! The problem was the board being too close to the jacks. I put some shielding tape on a piece of card stock, grounded via an alligator clip, and put it between the jacks and the board and that solved the distortion on the scope. I was able to pull it out and watch the distortion reappear. Thanks to everyone that was helpful providing a sounding board for problem solving ideas! Now to design a permanent shield....