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.

[nandrewjackson]

I found a broken trace on the circuit board. V4B was not getting +VDC. I fixed that.


 I stuck with nonconventional 1uF coupling caps after the P.I.


Also, I changed the "mid" resistor value down to 2K.


Also, I added a 3 way NFB resistor switch that gives either 2K, 6K, or 100K settings.


Also, I upped the added V4B grid stopper to 475K as suggested by stratomaster.


I really hit the sound I was going for.
This is an FSR combo with 12" speaker. Now it really can do the wide, deep, clear fender twin sound.
The quirky princeton growl is long gone, but that was the point.

[stratomaster]

Those PI couplers are a terrible idea.

The Princeton is fully bypassed at all stages, so there is no meaningful low end attenuation outside of the Bass knob.  The blocking distortion effect is proportional to the time constant of the PI couplers and grid leaks. Yours is 10x higher than the design already notorious for blocking distortion.

The stock corner frequency is about 5 Hz. Yours is about 0.5 Hz and the low E fundamental is about 82 Hz.  You are gaining nothing but problems with this practice.

See here: https://www.aikenamps.com/index.php/what-is-blocking-distortion

An excerpt:
In addition to an increase in crossover distortion, if the time constant of the AC coupling is large enough, and the transient waveform is of low enough frequency, a transient distortion known as blocking occurs.  This happens when the transient signal quickly pushes the clamped grid waveform down far into the cutoff region, and there is a finite time that is required for the grid waveform to recover and float back up to the correct bias point once the transient signal is removed. Until the bias comes back to the correct point, the output stage is effectively cut off for a major portion of the signal.  This results in a choppy, "farting" sounding distortion.

Fender amps are particularly susceptible to this because of the large values of coupling capacitors on the grids of the power tubes (0.1uF). Blackfacing your Super Reverb can actually make the problem worse, because you change the grid bias feed resistors from 100K to 220K, which increases the time constant of the AC coupling to the output tube grids. You will note that most Marshalls use 0.022uF coupling capacitors and 100K resistors, which gives a much faster time constant.  In addition, the preamp stages have a much more rolled off low frequency response. This is why they sound tighter when played wide open.

[nandrewjackson]

I do appreciate your input, stratomaster.  I'm not intentionally choosing ridiculous components.


After fixing the broken trace, and changing the PI grid stopper to a higher value, the amp sounded so darn good I put it back together and called it done.


It really has a deep/wide/clear tone like a twin reverb.


I only play at 70dB or lower, there are neighbors behind 2 walls of my living room.
The amp really sounds good.
If I pull the chassis any time soon again, I'll try some 0.1uF and 0.02uF coupling caps after the P.I. and if they don't change the tone I'll go with them.


Again, thanks for your input, I do appreciate the sciences of the amps.