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Princeton Reverb Circuit - Late 70's vs Blackface/early Silverface

mingo

Member
Messages
430
How close is a late 70's (1978) Princeton Reverb Circuit (with the pull volume taken out of the circuit) to a Blackface and/or early Silverface circuit. What's different?

Can this be converted to blackface?

Does anyone have the circuit number, or schematic for a late 70's silverface Princeton Reverb??

Thanks
 
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mark norwine

Member
Messages
17,163
it's a 12W amp. How much more headroom could you possibly hope to eek out of it?

On *ANY / ALL* amps, "headroom" lives in the PT, specifically it's VA rating [i.e. ability to maintain voltage while supplying lots of current]. The PT in a PR doesn't have any juice.

You may want to believe you can make it into a Hemi Cuda, but under the hood it's pure Pinto. Want a bigger amp? I'm afraid you need ot buy a bigger amp.
 

mingo

Member
Messages
430
it's a 12W amp. How much more headroom could you possibly hope to eek out of it?

On *ANY / ALL* amps, "headroom" lives in the PT, specifically it's VA rating [i.e. ability to maintain voltage while supplying lots of current]. The PT in a PR doesn't have any juice.

You may want to believe you can make it into a Hemi Cuda, but under the hood it's pure Pinto. Want a bigger amp? I'm afraid you need ot buy a bigger amp.
I'm not expecting a lot, and don't want a bigger amp, was just curious.

So bigger power transformer! Thanks for the tip. I'm going to be getting one in this week i think.
 

mark norwine

Member
Messages
17,163
bigger PT....yes.

But that's not the only requirement.

Again: think Pinto under the hood. Pull it & drop in a 426 Hemi! Tons of power now!

But can the stock tranny take it? Can the driveshaft handle the increased torque? Can the U-joints? Differential?

A higher power amp requires a lot. At minimum, a new OT, too.
 

SamBooka

Member
Messages
2,222
On *ANY / ALL* amps, "headroom" lives in the PT, specifically it's VA rating [i.e. ability to maintain voltage while supplying lots of current]. The PT in a PR doesn't have any juice.
.
Often i hear more knowledgable people than myself recomment 12ay7s or 5751s in V1 for "more headroom" of fender amps. How does this affect the PT? I have never tried it (to me headroom means something completely different)
 

mark norwine

Member
Messages
17,163
A lower-gain preamp tube doesn't affect headroom whatsoever...

All it does is change "where on the volume control" the (power amp section of the) amp runs out of headroom (i.e. starts to breakup). Instead of breaking up at "5", you might break up at "7".....but ultimately you're hitting the very same headroom limit.

Lower gain preamp tubes to "get more headroom" is psychoacoustics, at best.
 
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TweeDLX

Member
Messages
3,756
Back to Mark's car analogy, using a lower gain tube is like going to a much smaller tire. The speedometer will say you're going really fast, but everyone else is passing you...
 

mingo

Member
Messages
430
Got it.

Does anyone know if this can be blackfaced? And if the circuitry is close?
 

RedRock

Member
Messages
3,656
If you want a more powerful Princeton Reverb, David Allen,
on his Allen Amps website, has upgraded drop-in PTs and OTs
for this amp. You can use larger output tubes than 6V6s with his
PT because it can deliver a increased current on the 6.3v heater line.
And, of course, there are a lot of 10 inch speakers on the market
capable of handling higher wattage.

There is very little difference between the Blackface and the Silverface Princeton
Reverb amps. The very early ones used a 5AR4 rectifier for a short time; this was changed
to a 5U4 which was used on all following models. A 2000pf cap to ground on the reverb return was added
to the Silverface amps. That's about it for the differences.
 
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Messages
69
I have a late 70s (pull boost) PR.
There are only a few minor differences between it and earlier PR iterations.
I was pretty easy to blackface - just a few minor component swaps. The biggest deal was reconstructing the portion of the reverb circuit that was omitted in order to do the boost.
The thing that made the most difference was replacing the stock VOL pot with a good audio taper pot. The stock push/pull pot is linear taper & it sends the preamp into full on OD at about 3 on the vol knob.
 
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mingo

Member
Messages
430
The thing that made the most difference was replacing the stock VOL pot with a good audio taper pot. The stock push/pull pot is linear taper & it sends the preamp into full on OD at about 3 on the vol knob.
Where does it overdrive with the new audio pot??

Thanks
 

guitarcapo

Senior Member
Messages
2,326
A lower-gain preamp tube doesn't affect headroom whatsoever...

All it does is change "where on the volume control" the (power amp section of the) amp runs out of headroom (i.e. starts to breakup). Instead of breaking up at "5", you might break up at "7".....but ultimately you're hitting the very same headroom limit.

Lower gain preamp tubes to "get more headroom" is psychoacoustics, at best.
I disagree. A lower gain preamp tube hits the power tubes less hard. It
creates different distortion characteristics than just a volume change.

Mods I recommend:

12" speaker (I like a Jensen C12N)

Stokes mod. It increases voltage to the phase inverter circuit and in my opinion made the amp sound far better.

3 spring, medium decay reverb tank with a lower gain reverb tube like a 12AY7
 

Cue Dog

Member
Messages
270
I have a late 70s (pull boost) PR.

The thing that made the most difference was replacing the stock VOL pot with a good audio taper pot. The stock push/pull pot is linear taper & it sends the preamp into full on OD at about 3 on the vol knob.
Is this true for the pull-boost SF Vibrolux Reverb's as well? This might explain why mine breaks up at 3-4....
 

mingo

Member
Messages
430
Is this true for the pull-boost SF Vibrolux Reverb's as well? This might explain why mine breaks up at 3-4....
My amp is at a tech's shop right now getting the volume pot replaces, and also some resistors/caps replaced inside, i'll let you know if this fixes this issue.
 

teleman1

Silver Supporting Member
Messages
15,123
I have a pull vol PR. Its really a spectacular amp. Listen to mark Norwine. It blows me away all the mods folks try to do to every amp they get. Leo put tons of thought into these amps. A larger speaker will shake the tubes even more and not sound as good as a separate cabinet. And the dang cabinet is designed for a 10 " You want more power? Get an different amp. THe already folklore of this amp is based on the original setup.Change caps, use a different speaker, get NOS tubes, outside of that, your changing the amp. True PR lovers don't go braggin to their friends about all the mods they did to the timelss PR. Its usually stock. The way it sounds best and was meant to sound.
 

mingo

Member
Messages
430
I think everyone thinks I want a bigger amp, I really don't. This amp sounds as distorted through a 12" speaker as it does with the 10"

It's plenty loud for me, it's just overdriving freakishly low on humbuckers and single coils. The overdrive on 3 sounds like it probably should be on 10. I think there's something up with the amp.

It's at a tech right now, hopefully it will get solved, and i'll let everyone know what it is. If it's not fixed after he's done with it, i'll admit i need a bigger amp with more headroom.

teleman1 - could bad caps be causing early distortion?? At what volume level does yours start to break up?? My intentions are to keep it stock, besides the speaker. Oh and the output transformer.
 

phsyconoodler

Member
Messages
4,305
Ok,what humbucker do you have?Some are overly hot and the amp will be driven pretty hard.A vintage low output humbucker will sound better and not drive the amp as hard.

Mark said:"Again: think Pinto under the hood. Pull it & drop in a 426 Hemi!"

Most young guys haven't heard of a Pinto or a 426 Hemi!

The Stokes mod will give a bit more headroom,but I think maybe looking at your guitars might be in order too.How close are the pickups to the strings?Moving them away will help immensely too.
Sometimes some very simple things can make big differences in your quest for tone.

You said:"
It's at a tech right now, hopefully it will get solved, and i'll let everyone know what it is"
There may be nothing wrong at all.See above.
 




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