Fender Princeton Reverb - "imbalance" in PI: specifics?

[Alex_SF]

Hi --

I have a silverface Princeton Reverb that I'm trying to find out some more information on regarding a couple of suggested mods.

First, I've read that the phase inverter has an inherent "imbalance" that correcting will allow you to get a little more power out of the amp. However, none of the articles I've read that mention this have specified exactly what components or connections create this "imbalance," or what would need to be changed in order to "balance" the PI. Does it have something to do with the two 220K resistors that connect to the grids of V5 and V6? Or is it something different?

Second, there's an article suggesting placing two 470 ohm screen grid resistors on the power tubes, similar to what bigger Fender amps had. I see these two resistors on a blackface Bassman schematic and layout: what exactly would adding these to a Princeton Reverb do?

Thanks!

Update: schematics for the PR and Bassman are here:
http://www.ampwares.com/amp.asp?id=74
http://www.ampwares.com/amp.asp?id=27

[Bruce / Mission Amps]

Hi --

I have a silverface Princeton Reverb that I'm trying to find out some more information on regarding a couple of suggested mods.

First, I've read that the phase inverter has an inherent "imbalance" that correcting will allow you to get a little more power out of the amp. However, none of the articles I've read that mention this have specified exactly what components or connections create this "imbalance," or what would need to be changed in order to "balance" the PI. Does it have something to do with the two 220K resistors that connect to the grids of V5 and V6? Or is it something different?

Second, there's an article suggesting placing two 470 ohm screen grid resistors on the power tubes, similar to what bigger Fender amps had. I see these two resistors on a blackface Bassman schematic and layout: what exactly would adding these to a Princeton Reverb do?

Thanks!

Update: schematics for the PR and Bassman are here:
http://www.ampwares.com/amp.asp?id=74
http://www.ampwares.com/amp.asp?id=27

NO, don't speculate.
The imbalance discussed is not much of an imbalance so this mod is not Earth shattering and you will not get much more power, if any additional power, out of the amp.
It will stay very slightly cleaner a little longer but the absolute power output is not changed very much.
Anyhow,
do a search for the paul C mod
It is an old school way of biasing a triode that gives a little better output balance in the cathodyne driver.
And, it works better in fixed bias amps such as the Princeton Reverb.
But, I can tell you that using the 2meg with the 1Meg resistor, talked about all over the NET, .... does not do the mod justice.
After you have read about the paul c mod, ... try it, but go back later with a 3m3 + 1M resistor combo instead of the recommended values.
I prefer the 3m3 with 1m value although I suspect a larger value works very well too.

Sure, you can add the 470 ohm screen resistors if you want.
It won't hurt anything and the additional voltage drop across them with 6V6s is minimal.
A 6V6 doesn't draw very much screen current anyhow.
A better mod is to find the 1K 1 watt carbon comp resistor on the eyelet board (on main filter cap side of the board and in series with an 18K resistor) and replace it with a 1k5 2 watt metal oxide resistor.
Next remove the tiny OT and replace it with one for a tweed Deluxe or equivalent.
Even an inexpensive, reissue Deluxe Reverb OT (yes it is the wrong primary
impedance) sounds better in these amps.
Don't run the power tubes too hot in these amps as the power tranny is small (about 70ma) and one of the only power trannys from Fender that I have had to replace with any noticeable regularity with tweakers trying to get more power out of the amp.

Added
And make sure you only use a 5U4GB with a silver face Princeton Reverb... do not succumb to the temptation to run a GZ34.

[ncboy654]

What would be the adverse of running a GZ34 in a SF? I added a progressive power switch and put one in and seems to be fine, bass is a little tighter with it.

[Bruce / Mission Amps]

What would be the adverse of running a GZ34 in a SF? I added a progressive power switch and put one in and seems to be fine, bass is a little tighter with it.

In case you missed the other thread:

I'll have to reread what I said regarding not using a GZ34 in a Princeton.
The only thing I can think of right off the top of my head is:

1.) with a GZ34 in a silver face PR, the B+ will be very high and stock, they do not have an adjustable bias supply.
2.) with too high of B+ comes too high of power tube, DC idle current.
3.) the power tranny is a wimp in these amps and that spells a possible power transformer failure with high idle current plus a bigger OT that can pull more power from the power tubes.

[GibsonLover]

Imbalance is good. It adds a little second-order character with only a minor penalty in clean headroom. If you want to dial it out, you have to work on the PI, bias balance, get a balanced OT, and pay for matched tubes. Then you'll have a nice Hi-Fi amp that'll go to 6KHz. It's not worth it.

[boyt_bpc]

Balanced Phase Inverters are snake oil. An amp tech I used to use did an experiment on balanced Phase Inverters, nobody could tell the diference between balanced and unbalanced. Also, nobody picked balanced as soundit better.

[Bruce / Mission Amps]

YIKES... Not snake oil....

I don't know where you got that idea but balanced phase inverters are NOT snake oil and a push pull amp sounds like ass if it is very unbalanced.
I'm afraid that was an opinion based on less experience then many others have.
When you get a little further down the road with your tech skills you'll see that your statement was quite a bit overblown.
The truth is that many players ...can ... tell the difference in tone between a well balanced and mildly unbalanced phase inverter.
Good or bad sounding isn't the point because you might love an amp that makes every single guitar sound like a thousand bumble bees in a paper sack.... but I can tell you from MANY many years of doing this that the vast majority of players prefer a well balanced phase inverter over one that is not! ha ha

Anyhow, you didn't say what amp the test was done on.

There is an obvious difference between balanced and unbalanced phase inverters but the Princeton, (or any amp with a cathodyne driver/phase inverter) .... is not very unbalanced anyhow, otherwise it would never be used.... so there is no magic bullet when going to the fixed bias approach.
With respect to the cathodyne driver/phase inverter, balancing is a subtle thing that might not be as obvious to the casual player or someone not intimately familiar with the amp in the first place.
These are not very powerful amps and power tube distortion is right around the corner with not so loud sound levels so I think better balance here sounds a little better when driven hard.
If you use a dual trace O'scope and put your channel A probe leads on one coupling cap and channel B on the other, it is visually very clear to see,
when the phase inverter gets driven hard enough, one side is distorted and the other is still clean.
Going a little further with the drive level (and deeper into clipping from that point) will eventually get the other side to distort but the two "distortions" are quite different looking and there in lies the additional tonal shift.
Also, because of the difference in impedances of the cathode side vs the anode side of the cathodyne driver triode, the power tube reacts differently and that makes a different tone too.
One of the other problems is that right around that same time, the power tubes are ready to distort too so when nudging it a little further, there is a mix of amp distortions.

[boyt_bpc]

Hi Bruce,

Fat Willy at NBS in Denver was the amp tech I was talking about that did the experiment.

Boyt

[Bruce / Mission Amps]

Hi Bruce,

Fat Willy at NBS in Denver was the amp tech I was talking about that did the experiment.

Boyt

I rest my case.. the most un-opinionated tech on the planet!! ha ha ha

[boyt_bpc]

I rest my case.. the most un-opinionated tech on the planet!! ha ha ha

I'm not going to argue with anybody claiming that Willy is opinionated, but jeez can he make an amp sound good. Also, I've never bought anything from him that didn't work perfectly, which in the world of vacuum tubes is a pretty impressive thing imo.

[lain]

Power supply

What effect does changing the 1k1w to a 1k5 2w have?

[Bruce / Mission Amps]

What effect does changing the 1k1w to a 1k5 2w have?

Very little in tone but the additional resistance gets the screen voltage of the power tubes slightly lower so as not to be too high with respect to the plate voltage... plus the 2 watt Mtl Ox resistor won't burn like the carbon comp one will and it handles the current better then the 1 watter.
I replace blown up 1W 1K resistors in Princeton Reverb amps all most as much as bad power tubes.
The 2 watt Mtl Ox will last longer and maybe never burn open.

[hasserl]

Balanced Phase Inverters are snake oil. An amp tech I used to use did an experiment on balanced Phase Inverters, nobody could tell the diference between balanced and unbalanced. Also, nobody picked balanced as soundit better.

Perhaps what was originally described as snake oil was the higher priced balanced tubes often sold by tube dealers for use in the phase inverter position. The problem being that in a cathodyne type PI, as used in a Princeton Reverb (as well as a few other amps) the balanced triodes will do nothing to balance the PI, as this style of PI only uses one triode. And in other PI styles that use two triodes there may be some other imbalance in the circuit, so that a balanced tube may or may not result in a well balanced PI.

Then again, maybe that wasn't what was intended, just trying to give the benefit of the doubt.

[GibsonLover]

It just goes to show what you're really after in sound. Balanced or not, the sound could be exactly what you're after depending on what you play. I scoped my 6G2 Princeton's PI, and the the signals were exactly the same in amplitude. Shifting the bias with the Paul C mod didn't seem like it would produce a favorable effect. Look around on the Paul C mod, and you'll see a love hate relationship there. Maybe some need it, some don't.
Further to the point of 'balance' itself, P-P or otherwise, I have three amps from the 40's with paraphase inverters that sound fabulous. They've been universally praised by professionals for decades for their clean jazz sounds, yet are inherently imbalanced. They are obsolete now of course, because LTPI circuits are a 'better' design, also sound good, and go that extra step to help achieving more power which became the trend.
Little tweeks for the purpose of more power are a mistake in my book. The increase you get in power by modding a stock amp are usually minimal and at the expense of the amplifier's original character. As Bruce indicated with the Princeton, the transformers are not up to it anyway. If you want something louder, buy a louder amp. After all, how many guitar players do you know only have one guitar? Balance your PI if you want, but I wouldn't expect much except maybe a small amount of increased headroom.

[SoK66]

Hi --

I have a silverface Princeton Reverb that I'm trying to find out some more information on regarding a couple of suggested mods.

First, I've read that the phase inverter has an inherent "imbalance" that correcting will allow you to get a little more power out of the amp. However, none of the articles I've read that mention this have specified exactly what components or connections create this "imbalance," or what would need to be changed in order to "balance" the PI. Does it have something to do with the two 220K resistors that connect to the grids of V5 and V6? Or is it something different?

Second, there's an article suggesting placing two 470 ohm screen grid resistors on the power tubes, similar to what bigger Fender amps had. I see these two resistors on a blackface Bassman schematic and layout: what exactly would adding these to a Princeton Reverb do?

Thanks!

Update: schematics for the PR and Bassman are here:
Amp Guide » Fender Princeton Reverb (Blackface)
Amp Guide » Fender Bassman (Blackface)

This appears to be the Dan Torres stuff for PRs. You can ignore completely, Dan likes to make a lot of money. The so called PI "imbalance" is miniscule and imperceptible, screen resistor duties are handled by the 1K 1w SCREEN resistor on the component board.

Try doing the Stokes mod on your amp (do a Google) and see if you like it. A simple one wire mod that increases PI plate voltage. Increases clean headroom & punch. Some very knowledgeable amp techs believe the stock circuit was an intentional "mistake" to prevent the PR from being too close to the DR in performance.