Reverb driver circuit causing fartiness?

[Euthymia]

I'm trying to finalize a custom build for a client who really liked the sound of my hot-rodded Princeton Reverb style amp with master volume (but no reverb).

The problem I've run up against is the friggin' reverb circuit. My earlier development was done without the actual reverb circuit, but the client wants reverb.

I figured it would take a while to integrate the reverb effect, but what the heck, the amp's heritage is a circuit that has reverb, so just graft it back in and see what happens.

Turns out plenty bad happens. The amp sounds great as long as the master volume is dimed, but any attempt at getting distortion from the preamp goes to cutoff-farty-city as soon as I stick that 12AT7 driver tube in.

Pull the 12AT7, and the circuit goes back to sounding great (if you like distorting 12AX7's, and I do).

Figuring that there's some grid blocking going on, I've padded down the signal going into the 12AT7, adjusted the cathode resistor down and up, and the damn thing will NOT stop with the fartiness.

Any suggestions?

[Enzo]

Isolate the problem. Does the problem remain with the reverb turned down to zero? That is, does the reverb drive affect the tone no matter what, or just when the reverb is mixed in? Does leaving the reverb drive tube in the amp but disconnecting the pan make a difference? ( unloads the driver tube)

I am looking at the AA1164 drawing. If something else is closer, what is it?

First thing to my mind is power supply. The reverb driver here is drawing power off the screens node. Does yours come off the same node? If the reverb draws from the same node as a preamp stage, decoupling issues could be at work.

Scope the B+ rails throughout to see if any respond different with the reverb tube in place. Does it alter the B+ voltage at any node?

Does the reverb work and sound OK? Possible problem with reverb driver stage.

How about the mix resistor? Did you stick with the 3.3M and tiny cap?

Scope the amp output and also the reverb drive output looking for RF oscillation. Any of that going on and it can sap the sound good, but you of course can't hear it directly.

How is your layout? ANy chance the relatively hot signal of the reverb drive is coupling into another nearby stage out of phase and causing some cancellation?

[Arthur B.]

What the undesirable distortion comes from after the reverb? A reverb signal that has been distorted will give the characteristics you describe. Putting the reverb later in the signal path will alleviate this.

[Euthymia]

Isolate the problem. Does the problem remain with the reverb turned down to zero? That is, does the reverb drive affect the tone no matter what, or just when the reverb is mixed in? Does leaving the reverb drive tube in the amp but disconnecting the pan make a difference? ( unloads the driver tube)

The problem doesn't care what the reverb return control is set to, or even if the reverb tank is connected or not. Doesn't care if the reverb recovery tube is installed.

It only appears when the 12AT7 driver tube is installed and the preamp volume is cranked.

I am looking at the AA1164 drawing. If something else is closer, what is it?

AA1164 is closest at this point.

First thing to my mind is power supply. The reverb driver here is drawing power off the screens node. Does yours come off the same node?

Yep, I have it connected to the screen node.

Scope the B+ rails throughout to see if any respond different with the reverb tube in place. Does it alter the B+ voltage at any node?

Not significantly. Maybe a volt or two.

Does the reverb work and sound OK? Possible problem with reverb driver stage.

The amp sounds great until the preamp stages are cranked. Reverb sounds fine.

How about the mix resistor? Did you stick with the 3.3M and tiny cap?

I've dropped to 2M2 with a 10pF.

Scope the amp output and also the reverb drive output looking for RF oscillation. Any of that going on and it can sap the sound good, but you of course can't hear it directly.

Will do. I still find it tricky to find RF oscillation; any tips?

How is your layout? ANy chance the relatively hot signal of the reverb drive is coupling into another nearby stage out of phase and causing some cancellation?

I've poked the wires around and while other things can be induced (hum, motorboating) with bad wire routing, this problem is not affected.

Thanks!

[BK-Amps]

Is your reverb driver/return power supply node decoupled from the previous stage's node?

BK

[Euthymia]

Is your reverb driver/return power supply node decoupled from the previous stage's node?

BK

I have the reverb driver on the same node as the power tube screens.

The reverb return is on the same node as the first two stages. Just like the Princeton Reverb.

[Tom Phillips]

This is a strange problem.
One comment with reference to your statement: "The problem doesn't care what the reverb return control is set to, or even if the reverb tank is connected or not. Doesn't care if the reverb recovery tube is installed."
With a stock Princeton Reverb there would be NO signal thru the amp if the reverb recovery tube is not installed. Maybe this is a clue. You probably have some other deviations in your wiring that is causing this problem. Hopefully, careful checking will turn it up.
Good luck,
Tom

[Enzo]

Get the amp solidly doing the bad thing, then scope the plate of the reverb drive tube. With no signal happening, the plate ought to be sitting at some steady DC relatively clean, and with signal the clean wavy line of the waveform. RF will look like a band of light rather than a thin straight line like DC or a clear signal. If you then speed up the trace, the RF will resolve into a wave form. I am not looking for parasitics here, I am looking for all out RF oscillation.

For that matter, look for it anywhere in the amp.

As opposed to guitar fartiness, what if anything does sending a sine wave through it tell you?

What happens if you ground the grid of the driver tube? What efect does that have on the symptom?