Modding a '62 Princeton Brownface

Hey guys,

I am thinking about modding my Princeton (incredible amp, and all original right now) to add a bias pot. The '62 Princeton is fixed bias. This mod requires drilling the chassis. Do you think this is a bad idea?

I never plan to sell it, and it's in excellent condition but not a museum/collector piece. Probably an 8/10 specimen. I modded it by putting on a 3 prong chord already.

Thing is I don't want to pay a tech to bias it each time I swap the 6v6 tubes, and I don't know how to bias it myself.

Thoughts?

Thanks!

[Steve DeRosa]

A three-prong cord is a reversible mod - drilling the chassis is not...

If you absolutely must install a bias pot, have your tech mount it invisibly, inside the chassis, and in such a manner as to be reversible somewhere down the line...

If it were me I'd get hold of some good JAN tubes from the '50s/60s, have them properly biased, use the amp sparingly, keep it as clean as possible, and probably never need to retube it again during my lifetime - and if you must have "that sound" I'd sooner buy either a DIY kit or one of the early-60's Fender-clone boutique amps from Allen, Vintage Sound, etc. ...

I actually have some (4, matched pairs) 60s JAN RCA matched 6v6 tubes.

How difficult is it to put those in myself and bias it?

Can you recommend a clone that sounds like a brown princeton? I will look into those.

[Al Acuff]

I used to have a brown Princeton no reverb. Great amp! Wish I still had it. FWIW my advice is don't modify yours. Keep it stock (the 3 prong plug is no big thing).

If you have the urge to tinker with an amp and breathe some flux why not build yourself a nice clone amp. Build it stock first and then experiment with mods.

Building an amp is a fun way to learn. There are a variety of kits available from Mojo and Weber ranging from simple builds to very advanced. A tweed Champ or Princeton makes a good first project.

Originally Posted by Al Acuff

I used to have a brown Princeton no reverb. Great amp! Wish I still had it. FWIW my advice is don't modify yours. Keep it stock (the 3 prong plug is no big thing).

If you have the urge to tinker with an amp and breathe some flux why not build yourself a nice clone amp. Build it stock first and then experiment with mods.

Building an amp is a fun way to learn. There are a variety of kits available from Mojo and Weber ranging from simple builds to very advanced. A tweed Champ or Princeton makes a good first project.

Thanks for the feedback.

So, I do know how to solder well, but in terms of reading schematics I'm not versed in that.

What would be a good company to go with in terms of instructions?

I'd also consider a pre built amp that sounds similar to the brown princeton. (agree, great amp!)

[DukeX]

Lil Dawg ChocoPrince. Jim can do any mods you want. There's a lot of info on him over at the Telecaster forum.

[Mr. Jelly]

Really? If you don't like the amp get a different one but don't mod the '62 Princeton. This actually sounds like a troll. Anything you want to do to it is only going to degrade the amp and the sound. Move on.

[Nama Ensou]

Originally Posted by Steve DeRosa

A three-prong cord is a reversible mod - drilling the chassis is not...

If you absolutely must install a bias pot, have your tech mount it invisibly, inside the chassis, and in such a manner as to be reversible somewhere down the line...

This, exactly this.

Take care of that baby, and both these mods, the one you've already done and an internally mounted bias pot, will both be reversible, which is good.

[MC5C]

I don't understand why you would want a bias adjustment pot added to that circuit. That 6G2 circuit uses an output stage bias modulation tremelo circuit, and the tremelo intensity pot actually sets (and changes) the output 6V6 pair's bias. Anytime you use the tremelo function, the bias of the tubes changes. Something like a Bandmaster does the same effect by modulating the signal level to the phase inverter, and has an adjustable bias pot for the output stage. I sincerely would not bother even worrying about biasing tubes in your Princeton, they knew what they were doing when they designed it like it is. They under-biased the tubes so they can change the output volume by varying the bias voltage and never get the tubes to a place where they are over-biased. It's part of the sound and charm of that amp. http://www.guitarplayer.com/miscella...-tremolo/22018

Point of trivia - tremelo is a repeated variance in volume. Vibrato is a repeated variance in pitch. The Princeton has a tremelo channel, the Bandmaster actually calls the same effect "vibrato", not sure why they changed the name of the effect. At the same time they had the Stratocaster (and other guitars) that had a "tremelo" bridge that is actually a vibrato system. Now, of course, the terms are interchangeable.

Originally Posted by MC5C

I don't understand why you would want a bias adjustment pot added to that circuit.

Only because I want to change out the 6V6 tubes and don't want to pay a tech to bias it each time.

You think I can just swap different ones in and not worry about bias?

Is the only downside of that the tubes might burn out faster?

[muscmp]

don't mod that amp, please. do some tube rolling both with the preamp and the output tubes. once you find a combo that works, either take it to your tech for a long term biasing or learn to do it yourself. there are companies that make bias tools so you can do it yourself if you ever need to do it again. but, the cost of the bias tools will be about the cost of the tech doing it. i built my own bias kit and do my own.

play music!

[MC5C]

Yes, in my opinion you can swap in new tubes and not adjust the bias. A lot of this is opinion, there are multiple right answers. If you add a bias trim pot inside the amp (no holes) then you gain the ability to tweak the bias - this can tweak the tone of the amp, or optimize the power output of the amp. The reasons I suggest that you can install new tubes and not bias them is several-fold - it's been this way all it's life and in 55 years it's going to have had a number of output tubes installed, and not adjusted. Second, the legendary tone of the tremolo circuit is dependent on the bias range of the existing circuit. If you start modifying that circuit, you stand a chance of changing the tone. Finally, it's only a 10 - 12 watt amp. The plate voltage is low compared to 20 watt 6v6 amps, so the tubes are barely cruising. It would be unusual to get a pair of modern tubes so out of spec as to cause plate current issues in that circuit. But this is only my opinion...

Thanks, guys. I decided I won't mod it. But I have to figure out something. It's $60 each time the tech adjusts the bias. That's his minimum bench fee. We live in pricey area.

Do you have a link to instructions on how to do this at home? Or a book I could get at the library that explains how to do it. I googled it and found nothing.

Originally Posted by MC5C

It would be unusual to get a pair of modern tubes so out of spec as to cause plate current issues in that circuit. But this is only my opinion...

Your entire post makes a lot of sense. Fender must have figured people would swap tubes without biasing when they made this. I guess my concern is it sounding sub optimal because I don't know how to bias it.

So, I've been using vintage tubes in it.

Do you think going with new, modern tubes in the 6v6 section would alleviate any potential problems? I could do that for the 6v6 and leave the 12axy7 as a vintage RCA. Say I put new tubes in it, and there is a bit more voltage with them. Is the only downside of not biasing it the tubes burning out faster? Is there risk of harming other parts of the amp?

[clintj]

Biasing that style of amp requires a few steps. Measuring of the current bias point, seeing if it needs adjustment, then soldering in a new resistor in the bias circuit to adjust the bias voltage followed by checking the work done. More complicated than a trimpot for sure.

Bias vary tremolo has an interesting side effect, though. If the bias point is too far out of tolerance (high or low), the tremolo becomes weaker or even non-existent. In other words, these amps can tell you when it's about right just by listening, which is how I do the final adjustments on my bench.

I'd hesitate in modding a collector's piece like that (I don't consider updating to a proper, safe cord a mod), but it is possible to mount a small trimpot invisibly within the chassis if you choose. I did it on my tweed Bassman build.