Princeton reverb 65 reissue mods problem

[nandrewjackson]

Hey everyone,  happy new year


I'll post later the schematic and voltages and exactly what mods I did. I'm busy at work and can't do it all right now, I only wanted to start this thread while I have the time.


I've built around 20 tube guitar circuits, AC15, AB763, Champ, JCM800, so I'm not a total newbie, but I'm far from an expert.


I'm hoping there's something glaring that someone can point out real quick.


More later, thanks . . . .   

[nandrewjackson]

Fender schematic

[nandrewjackson]

The mods I Did.


Changed first and 2nd coupling caps to 0.0068uF and 0.01uF respectively,  to reduce lows and overall signal hitting the phase inverter.


Added 33K grid stopper to the phase inverter.


Changed the coupling caps at the poweramp entry to 1.0uF each.  That was to try to recoup any perceived volume loss from the mods earlier in the circuit.


I don't know the math involved, but I've had success changing coupling caps in other circuits for similar reasons before.


So what's it doing?


The amp produces sound.
The EQ, volume, tremolo, and reverb all work as they should.


The sound is very muddy and distorted (not the good kind of distortion) at all volume levels.


The overall volume is way lower than expected. I can turn it to "10" and stay at talking volume.


I will post voltages after I get a chance to record them


Maybe someone here will say "you can't have 1uf at the 6v6 input" and I'll say, ok, that's why I can't find a 6v6 push pull schematic with anything larger than 0.4uF. Or something simple like that.

[nandrewjackson]

I was also thinking that the problem could be because adding 33K on P.I. grid would change the bias of the P.I. because of the 1M resistor that drops down from there to its cathode resistor. 


But, there are many Forum posts about people adding a grid stopper right at P.I. pin 7 like I did, and I couldn't see anyone mention it causing problems.

[stratomaster]

1μF is definitely not the right value for those PI coupling cap.

A first order approximation of the math you referenced is -3dB point = (2*π*R*C) where R is 220k and C is the capacitor in farads.

You'll quickly see that anything larger than around 0.047μF is overkill for guitar frequencies.  There is no boost to be had at any useful frequency by tweaking these any larger.  And you can only cause harm by changing the time constant of the RC circuit and induce blocking distortion.

Change that back to something more reasonable (stock value is fine, and you can go down to around 0.022μF before you hear a noticable change in low end) and see if that fixes the problem.

Also, the 33k gridstopper is doing next to nothing. You'll need 10x-30x to get the intended effect.

[nandrewjackson]

Thanks stratomaster.
I'll see about doing those when I get time to dig back into the amplifier

[stratomaster]

If you're up for digging deep into these amps I've got a running thread here: https://www.tdpri.com/threads/princeton-reverb-reissue-service-observations.1156402/

I'm not done documenting everything I've learned, but I have a goal to get that done soon. Chipping away a little at a time.