BF Princeton Reverb clone - Too much fuzz. - Solved

[Lectroid]

My latest build is a mostly-stock AA1164 Princeton Reverb.  I used Hoffman's board and layout with the exception of changing the Fender 12AT7 reverb circuit.  I replaced it with a 1-tube reverb using the 12DW7, also changed the mix resistor from 3.3M to 470K.  Schematic below.  Using that reverb circuit let me leave off the fourth 12AX7 tube, which has boosted B+ voltages slightly.

The problem is the amp's sound.  It's got a fuzzy distorted sound at anything over low volume. It sounds like tube-driven overdrive, a little bit fuzzy with a softer, rounded sort of distortion, a tiny bit fizzy.  Like John Mayall/Yardbirds sounds played on a mono 45.  The overall volume is good, the controls all change the sound like they should. Increasing the guitar's volume increased the amount of distortion.

Voltages: Most are close to Fender's AA1164 schematic, but two areas look wrong to me:

1.  The Reverb driver's cathode voltage looks low to me. Actual reverb signal seems strong, can drive a 6" speaker easy.
 
2.  The cathodyne PI voltages are pretty far out of balance - 193 vs. 46V, another reason I suspect it.  Also, on the scope I see two small kinks in the output of the PI that aren't there when the signal leaves the 1st or 2nd stage coupling caps.  Cathodyne image below.  For now I'm assuming that's the distortion I'm hearing.

So far, I've swapped out every preamp tube a couple of times and swapped power tubes and rectifier tubes (now 5Y3),  none of the changes made any difference. Today I'll double-check all the voltages and component values on the PI and surrounds and keep poking around.

Any thoughts or advice would be welcome.

TubeName	Plate	Screen	Grid to cathode
6V6GT	398	379	-34.3
6V6GT	398	378	-34.25
12AX7 - PI	193		46
12AX7 - 3rd preamp	166		-1.1
12DW7 - rev. driver	375		-2.38
12DW7 - rev. recvry	157		-1.24
12AX7 - 2nd preamp	163		-1.12
12AX7 - 1st preamp	167		-1.06

PS nodes -------- A: 402 B: 379 C: 310 D: 241

[ac427v]

I'm not aware of Tubenit using a 1 Tube reverb circuit with a cathodyne PI. You are traveling in uncharted waters so expect some experimenting!
Here is my thinking:
The PI voltages look good to me. They are within 10% of the voltages reported on the Fender schematic.

The cathode of the driver triode needs revising. Fender specifies a 2.2k resistor to bias two 12AT7 triodes. That would equal 4.4k resistor for a single 12AT7 triode. Change to a larger resistor to increase the cathode voltage.

The Princeton Reverb has a third gain stage before the PI. The resistor mixing wet and dry signals needs to be different compared to Tubenit's reverb in Long-Tail Pair amp. Change that 470k back to something close to the original 3.3 Meg. That will eliminate overdriving the following triode with too much dry signal.
I call this "my thinking" because that is exactly what it is. My closest experience is building a 1 1/2 tube reverb in a plexi clone. That required a lot of experimentation to get decent sounding reverb. Good luck!

[Lectroid]

@ac427v

Thanks for the insights.

First thing I should say is that I hooked up the reverb tank yesterday and instantly heard reverb.  It should be putting out more verbs, but it is working.   

Your suggestion about raising the cathode voltatge is a good one.  I'll swap resistors until I get to something like Fender's value of 8.0V for the reverb driver cathode voltage.   Re: the mixing resistor: tubenit suggested I drop it down to 470K or even 330K.  His reasons made sense to me at the time.  link: https://el34world.com/Forum/index.php?topic=32462.msg359941#msg359941.

Still wrestling with my main problen, the distorted output.  I think that's happening in the cathodyne PI, but haven't found the problem yet.  Any thoughts on what might be causing this could be a big help.  I'm running out of things to test.   

[Latole]

All of us know the original Princeton don't fuzz.

You may do some mistake when building it. ?

Show us schematic as build. Reading what you do is not enough.
The original one :

[Lectroid]

Just to cap this off:  I had a bad solder joint on the 3rd stage preamp's plate.  After that, the amp's sound cleaned right up.  Thanks again.