Princeton Non-Reverb 12-Watt Amp

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Princeton Non-Reverb 12-Watt Amp

Postby search4sound » Sat Jul 28, 2012 12:49 pm

Just got an unmodded '73 or '74 Princeton non-reverb combo amp. Anyone have experience with one of these? Mods? Can't find a complete schematic for non reverb silverface model. Has vibrato, would love to use the extra vibrato stage without vibrato.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby Smolder » Sat Jul 28, 2012 2:07 pm

Nice amps... I've heard them referred to as a "small twin". I don't believe you can remove or turn off the term for added game like most fenders. The term is tied into the bias portion of the circuit. The paulc mod is nice... I personally do not like a 12" speaker or using 6L6's in princetons. If you want that get a bigger amp. I did shift the tone stack to a very Jerry like dual .022 caps. I also picked up a K110 for it. On 10, it's the same volume and similar break up as my princeton reverb on 7.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby search4sound » Wed Aug 01, 2012 4:02 pm

Thanks, I see what you mean about the bias being tied to the trem. I presume one could remove the trem where the bias connects to the intensity pot (essentially remove this pot so the bias voltage goes from the diode directly to the 220k resistors connected to the power tube input grids). At any rate, I do like the idea of adding a bias trimpot.

I read somewhere that one reason why tremolo (aka vibrato) is removed from Twin Reverb and other amps with similar trem circuit is that this circuit can load down the circuit creating suboptimal tone. Any truth to this?

In the case of Jerry's TR mods, I assumed the reason for removing the trem was to free up the tube to be used for an extra stage. Maybe it was both? Related to this, I wonder if anyone has the vibrato mod and has A/B'd the normal channel (stock) and the vibrato channel (extra stage, no vibrato). How do they compare?

In the case of the Princeton, I read that the way the trem is connected does not load down the circuit. So I presume that I can turn down the intensity to 0 and the vibrato should sound the same as the normal channel?

+1 on the tonestack mod. I'll do the cathode bypass caps on the triode stages as well. I suppose I should consider replacing all the electrolytic caps, that amp is almost 40 years old.

Thanks for you recommendation not to upgrade the speaker to 12" or increase the gain - I've long wanted a push pull amp that sounds good at lower volumes. So do you still get shimmering Garcia clean with a 10" speaker?

What about preamp tapping the Princeton? Anything keeping me from doing that? Is it done the same way as with a TR?
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Re: Princeton Non-Reverb 12-Watt Amp

Postby Smolder » Thu Aug 02, 2012 6:33 am

search4sound wrote:I read somewhere that one reason why tremolo (aka vibrato) is removed from Twin Reverb and other amps with similar trem circuit is that this circuit can load down the circuit creating suboptimal tone. Any truth to this?

In the case of Jerry's TR mods, I assumed the reason for removing the trem was to free up the tube to be used for an extra stage. Maybe it was both? Related to this, I wonder if anyone has the vibrato mod and has A/B'd the normal channel (stock) and the vibrato channel (extra stage, no vibrato). How do they compare?

In the case of the Princeton, I read that the way the trem is connected does not load down the circuit. So I presume that I can turn down the intensity to 0 and the vibrato should sound the same as the normal channel?

+1 on the tonestack mod. I'll do the cathode bypass caps on the triode stages as well. I suppose I should consider replacing all the electrolytic caps, that amp is almost 40 years old.

Thanks for you recommendation not to upgrade the speaker to 12" or increase the gain - I've long wanted a push pull amp that sounds good at lower volumes. So do you still get shimmering Garcia clean with a 10" speaker?

What about preamp tapping the Princeton? Anything keeping me from doing that? Is it done the same way as with a TR?


I don't recall reading that Jerry disconnected the trem stage. I'll have to check Waldo's age and see. In any event, I've put a pull pot on the intensity of the trem on a few bigger fenders and it is an effective and simple way to increase gain (and volume). On the princeton, you can turn intensity all the way down... simpler and sometimes more effective method would be to use a shorted rca jack in the footswitch slot. By default on that amp the trem is on. The shorting jack will turn it off. Not the same as removing it though.

The jbl d110 (or K/E) is a three inch voice coil... slightly different tone from the 12 or 15", but close enough for what I need.

The electrolytics in the cap can are said to last longer than others, but if it were my amp, I would not leave the originals in. The other electrolytics that are on the board will effect tone... the tantalum capacitors that alembic used will sound different, and that's an important part of Jerry's tone.

I have not looked at the princeton for a preamp tap... but can't immediately think of any reason it would not work... just in a mono version.

best of luck!
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Re: Princeton Non-Reverb 12-Watt Amp

Postby search4sound » Thu Aug 02, 2012 3:56 pm

Yes, Waldo's site shows how when the TR is used as a preamp with McIntosh for amp, the vibrato channel is used but the vibrato itself is removed from the circuit.

http://www.wald-electronics.com/preampmods.html

I'm not sure if this frees up the vibrato triode to be used as an extra gain stage, or if it is completely out of the circuit and the exctra stage I've heard referred to is actually the triode associated with the reverb that gets used in the vibrato channel. Now would thet be a 12AX7 or a 12AT7? And does the dry signal before the reverb get amplified before the signal splits off to the reverb, or does the signal split off and then only the reverb signal is amplified before being mixed back into the dry signal?

+1 on the tantalum cap, I'm doing that first. Just interested in gain stage from a circuit design standpoint - I have a Trio that I've modded and experimented a lot with - sounds awesome with audio quality 22uF cathode bypass cap, cathode follower bypassed,s and a clean boost pedal, and a MosValve. The Princeton is a fun opportunity to explore another circuit and consider possibilities for mods, plus is more practical as a practice amp.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby Smolder » Thu Aug 02, 2012 7:22 pm

search4sound wrote:Yes, Waldo's site shows how when the TR is used as a preamp with McIntosh for amp, the vibrato channel is used but the vibrato itself is removed from the circuit.

http://www.wald-electronics.com/preampmods.html

I'm not sure if this frees up the vibrato triode to be used as an extra gain stage, or if it is completely out of the circuit and the exctra stage I've heard referred to is actually the triode associated with the reverb that gets used in the vibrato channel. Now would thet be a 12AX7 or a 12AT7? And does the dry signal before the reverb get amplified before the signal splits off to the reverb, or does the signal split off and then only the reverb signal is amplified before being mixed back into the dry signal?




oh right, the pre tap excludes that portion of the circuit. that makes sense... i was thinking you meant disconnecting it in full pre+power amp mode.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby search4sound » Fri Aug 03, 2012 5:11 am

Though the Alembic version of the preamp tap keeps the vibrato in the circui, so taking the vibrato out is optional. Finally looked at the AB763 schem (Twin Reverb) and see the vibrato connects to the audio path before the phase inverters, so it can optionally be kept as part of the tapped preamp. As for using the amp in combo (preamp+amp) mode, I think it is possible to keep the vibrato out of the circuit (i.e. leave the post-intensity pot lead removed), but I think you are right that in the post-alembic version the vibroto gets added back in combo mode (though I don't know if Jerry's TRs were ever used in combo mode).

As for the gain stages, the third gain stage is after the reverb is mixed in, and it is not part of the reverb or the vibrato.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby tatittle » Wed Nov 19, 2014 12:10 pm

I realize I am resurrecting the dead here, but I have a Princeton (1964) that apparently uses the vibrato tube for an extra gain stage...I am wondering if it is pre-amp tapped in another post. I don't have the knowledge to tell you anything about it, but pics are available. I am curious about the reference to biasing as my amp is reading WAY high bias readings on one of those contraptions that goes between the power tube and socket. I was worried about it, but I have used it a lot in the last year without issue, so maybe the reading is just thrown off by the mod to the vibrato?
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Re: Princeton Non-Reverb 12-Watt Amp

Postby Smolder » Wed Nov 19, 2014 12:41 pm

The trem will definitely impact the mA readings when checking the bias, but it will pulse, not be constant. If your Princeton NR is stock, it will not have a bias trim pot. That's a very easy and helpful mod. The trem sounds best (IMHO) when the bias is set in the high teens or low 20's.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby tatittle » Wed Nov 19, 2014 1:05 pm

Thanks Smolder. This amp is definitely not stock. The Trem is disconnected and the pots now control extra gain. The bias didn't waver when I tested it. I need to take it to a tech but I just don't feel comfortable with anyone Ive met around here really (and Im lazy). I am just too illiterate to even begin understanding what is going on LOL. I was beginning to learn some stuff last year but totally set it aside since then so even the little I began to comprehend Ive forgotten now.

I was hoping it had a pre-amp tap already, since I thought I recalled Waldo diagramed it was merely 1 wire going from the circuit board to the jack. But the layout someone else sent me has 2 leads to the circuit board for the tap. There is a 1/4 jack where the original RCA Vibrato pedal was...which I was hoping (pipe dream I guess) was a pre-amp tap. It does seem like the wire to what was the Vibrato pedal jack has been moved...but maybe it is now used to turn the extra gain on and off. Oh well.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby tatittle » Thu Nov 20, 2014 1:16 am

After spending some time becoming acquainted with the Greek known as schematic/layouts I am now wondering if my modified circuit doesn't play with bias of the output tubes to achieve some of its rawer sound. The speed pot gives most dramatic gain (and obviously has modified wire attached to it) but the intensity pot seems connected to what I think may be the bias point of the Princeton 964 circuit. Maybe Ill give that a look next time I hook up meter.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby Smolder » Thu Nov 20, 2014 6:05 pm

Post a pic or two of the inside of the chassis... Happy to have a look.
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Re: Princeton Non-Reverb 12-Watt Amp

Postby tatittle » Fri Nov 21, 2014 4:18 am

viewtopic.php?f=419&t=16546

Clicking on the pic leads to more. Sorry they are a bit fuzzy and not the best.
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