Princeton Reverb doesn't overdrive

[markmil]

I built a Princeton Reverb from schematic, (AA1164) and I'm not able to get a nice overdrive sound. Variations to the circuit include: a 1/2 ohm resistor connecting to 6V6 cathodes to ground, (to measure bias current), pot & divider network to adjust bias current, 3 spring reverb tank, 1M resistor to GND at the 12AT7 reverb driver's grid replaced by two 470k ohm resistors to cut the drive in half and a 12" Celestion Blue Alnico speaker. Problem is it doesn't overdrive until the volume is past 3/4 and when it does it doesn't sound good. The clean sound is excellent. The bias current is set to 23mA with a pair of NOS RCA military 6V6's, (they run much hotter than the EH units I originally purchased drawing ~30mA bias current at the same bias voltage for 23mA with the EH tubes). I ended up dividing the drive to the 12AT7 in half after tracing a harsh sounding distortion to connecting the reverb tank. I suspect I'm still getting this effect. The reverb sounds like you're playing inside a well pipe at full tilt but is OK at low settings. Even with the reverb tank disconnected I don't seem to elicit much overdrive in the sound. The plate voltages seem to be pretty close to those listed on the schematic, maybe a little higher, (PT is wound for 115V primary but I've measured our outlet voltage at 124VAC). Any ideas why I'm not enjoying the sweet sound of 6V6's being overdriven?

[Chuck H]

Are you measuring 23mA for each tube or both tubes? And, with your half ohm resistor your math is voltage across the .5ohm resistor divided by .5, correct?

[Jazz P Bass]

You could try changing the value of the feedback resistor.
Up, down, I don't know.
Trail & error.

[Chuck H]

The thing most likely to cause ugly overdrive IME is instability. Layout, lead dress and ground scheme are critically important when overdriving.

[markmil]

Thanks for the feedback

Hi Chuck,
The 23mA should be the bias current through each tube. I set the bias current for a reading of 23mV across the 0.5 ohm resistor, (23mV / 0.5ohms = 46mA), and assume that with a matched pair of 6V6's that there's 23mA through each tube.
Right now I'm more inclined to start putting the circuit back to the original spec, (rather than changing the feedback resistor value), to see if one of my "improvements" is causing the problem.
My layout uses a single point ground. After completing the build I did have an oscillation problem that I traced to a signal wire running along the chassis which I moved and have not had a problem since. I think the ugly part of the sound is coming from the reverb driver still. I think I'm going to try using the proper 2-spring reverb pan and then see what happens with the overdrive.

[tubeswell]

Right now I'm more inclined to start putting the circuit back to the original spec, (rather than changing the feedback resistor value), to see if one of my "improvements" is causing the problem.

Good thing to do - if you have a problem with it. If you have any tweaked bits of the circuit, then untweaking them and getting the 'right' voltages everywhere would be a good starting point to replicate the original. Not that tweaking the circuit is bad mind you, its just that the original amp has been tried by many and valued for what it is for a reason.

The BF fenders were arguably not intended to be hi-gain amps. Rather, you can see from the way in which the successive circuit topologies of different eras of Fender amps evolved, that Fender intended BF to be the next thing (after the brownface amps) on the way to undistorted nirvana. They inherently do not do hi-gain as good as something like a SLO100 or PV5150.

If the problem is weird oscillations, hum, buzzing etc, then attend to the lead dress and the tubes first.

If the problem is 'blattiness' when the amp is driven into clipping, you could try a large (470k +) grid stopper at the grid pin of the cathodyne stage. Similarly, keeping 1k5 to 4k7 grid stoppers at the 6V6 grids can also help. (Although these are both non-standard BFPR circuit parts).

I think the ugly part of the sound is coming from the reverb driver still. I think I'm going to try using the proper 2-spring reverb pan and then see what happens with the overdrive.

Does the ugly sound only happen with the reverb 'on'? The reverb driver should work perfectly well in the stock configuration (albeit that the BF reverb is typically overkill)

[markmil]

Even with the reverb level turned all the way down the ugly sound was still present at high volumes. I realized after awhile that only when the INPUT to the reverb tank was disconnected the ugly sound went away. I made this discovery after taking the chassis out of the cabinet several times to probe signals. The ugly sound went away on the bench but came back every time I put the chassis back in the cabinet.

[tubeswell]

Can you post any decent resolution (gut shot) pics?

[Chuck H]

How is the cable mounted in the cabinet. Common practice is to let it just hang in there. If you have it strapped up perhaps it's strapped close to something that's introducing positive feedback.