Princeton Help Please.

Steve Collins · From: Alaska, USA

I am test driving a sIlver-face non-reverb Princeton, probably a '69. It sounds real clear and nice on the attack but as the tones decay there is a pronounced electronic fizzle. The problem is worse with 6-string than with PSG, which is not what I would have expected. Quick diagnosis? The amp is available for immediate sale, $400, but the guy wants an answer tomorrow. Tubes have been replaced and have less than 30 hours on them. I feel confident doing a cap job if that is all it takes, but I don't feel confident diagnosing the issue here. Thanks, Steve

Jon Light (deceased) · From: Saugerties, NY

Do you have a good amp tech who you trust? Because I have to believe that at this price and with competent tech work, you will still have a great deal. Worst case scenarios of transformers and/or speaker replacements still would have you in good $$ shape as long as you have someone who won't take you down a rabbit hole chasing after the problem.
Or so it seems to me....

Michael Pierce · From: Madison, CT

Steve, I hope this works out for you. I have the same amp, just a year younger (1970). Fantastic sound. A nice added feature is that the baffle is easily switched out if you want to put a 12" speaker in it.

Steve Collins · From: Alaska, USA

That was actually my other question Michael. At first glance I thought "hmm, looks like there is just enough space for a 12 in there". What did you put in yours?

Tim Marcus · From: San Francisco, CA

I'd skip it. Non reverb Princetons can be had for about that same price - this one would require some work and that problem may be tough to track down.
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Michael Pierce · From: Madison, CT

Steve, re your question on a 12" speaker, I go back and forth between a 12" Weber California and a 10" Tone Tubby (a recommendation I got from this Forum). I believe I got the replacement baffle from Mojotone for about $50.

Ken Metcalf · From: San Antonio Texas USA

Hold out for a reverb
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Chris Boyd · From: Leonia,N.J./Charlestown,R.I.

I'd pass and hold out for a reverb too... I got Larry Rodgers to make me an aged baffle cut for a 12" in my '66 Princeton Reverb... Tried lots of speakers and ended up using a Jensen P12N (RI)...
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Stephen Cowell · From: Round Rock, Texas, USA

Sounds like a speaker to me... try subbing a replacement. I'd offer $300 as-is, s a good deal at that price.
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Steve Collins · From: Alaska, USA

Thanks for the input here guys. I am still really torn about this purchase. The seller dropped off the original tubes (RCA & CBS), which I tried just for kicks, they sounded great, but the fizzle is still there. I will try a different speaker.
When folks say these "can be had for a comparable price", or "you can hold out for reverb", I cringe a little for living In Alaska, where these statements are somewhat less true.

Bill Hatcher · From: Atlanta Ga. USA

sounds like a voice coil dragging. hear it when the vol is real low at the end of the note. try another speaker

Steve Collins · From: Alaska, USA

Well, I picked up the amp for 350, it is such a honey I couldn't resist. The speaker is not the issue, ruled that out. I have been too busy to tinker in the last few days, but I did order a new filter cap can, and I will be installing that and some tubes of known quality very soon.

Dave Mudgett · From: Central Pennsylvania and Gallatin, Tennessee

You did great - rarely do I see 69 silverface Princeton Amps for anything like $350. There are a lot of possibilities, filter cap is a good place to start.

These non-reverb Princetons are one of my favorite small amps. They take preamps/reverb-units and even modelers very well.

Stephen Cowell · From: Round Rock, Texas, USA

If it's not the speaker it could get complex/expensive... output transformer, tube sockets... or both. I don't really correlate 'fizzy' with filter caps or tubes.

I just had to order a new power transformer for my brownface Princeton... fool before me put in a much hotter tranny, voltages up near 450VDC... blue molded caps turning brown! Mercury Magnetics to the rescue... on a side note, I ordered the MM PT from WeberVST thinking I'd save a few bux... didn't realize it takes *3 weeks* to get a MM transformer from Weber... much faster/better to just order from the source, great folks at MM. Weber was nice about cancelling the order as well... they only order from MM once a week, it seems.
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Tim Marcus · From: San Francisco, CA

fizzy could also be a hairline fracture in a resistor caused by the fiber board flexing over time. Could be a pain to track it down

it could also be a high frequency oscillation - which could relate to that ancient filter cap. I agree that is a good place to start.

Another thing that every Princeton on earth needs is a grid stopper on the input to the PI tube grid - that will really smooth out the distortion character of the amp
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Steve Collins · From: Alaska, USA

Stephen and Tim, thank you, your input is valued and noted.

Steve Collins · From: Alaska, USA

I have caps and tubes ordered and on the way, can't wait. Tim, the additional resistor you refer to on the PI grid, would that be pin 7 on V2 (the 12AX7)? What is the recommended value? Thanks, bear with me as I learn.

Stephen Cowell · From: Round Rock, Texas, USA

Pin7 is on a buffer section... pin3 is the input to the PI. These are normally 1500ohms and applied right at the grid... take the wire off and solder the resistor (with 1/4" of lead, bent in a hook) right to the pin... then solder the wire (again, no more than 1/4" of lead) to the resistor. 1/4watt should be OK, most folks use 1/2watt to match the rest of the amp.

You can put stoppers on the other grid too... I'm tempted to do this to my brown Princeton now that Tim has chimed in on this... it has a very piggy clipping characteristic, so bad that I'd ordered a new OT for it. Perhaps this is the fix! Might even be useful on the final tubes as well... that's where you find them on most bigger Fenders.
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Steve Collins · From: Alaska, USA

Great info Stephan, thanks.

Tim Marcus · From: San Francisco, CA

the Phase Inverter grid stop should connect from the 1M resistor on the fiber board to pin 7, which is the grid of a 12AX7

I use 330K on my Milkman amps, because I have a heap of that value in Allen Bradley and use both 470k and 1M values elsewhere. Anything between 220k and 1M should do the trick

this will certainly help make a buzzsaw amp breakup turn into smooth sweet buttery love.
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Stephen Cowell · From: Round Rock, Texas, USA

I'm afraid I misread the schematic... Tim's right, of course.

His recommendation of a higher stopping resistor is more of a blocking distortion remedy rather than a parasitic stopper... good policy either way. If you put the resistor right on the pin then it will be a grid stopper as well. Both the buffer and the PI have pin 7 as grids... the final PI grid has the 1M resistor going to it, as Tim pointed out.

Blocking distortion happens when the grid goes positive and the time constant of the circuit prevents quick recovery... the high value of resistor prevents the circuit capacitance from charging up and 'blocking'. Aiken, as usual, is the final word on this, although most of the article concerns the final tubes, the following is germane:

http://www.aikenamps.com/BlockingDistortion.html

Near the bottom:
" Look for the problem in the preamp stages. Blocking can (and does) occur just as bad on RC-coupled preamp stages that are overdriven. The solution is similar. Add large value series grid resistors (100K - 470K), reduce coupling capacitor values to the minimum required for the desired low frequency response, add interstage attenuators to limit the amount of grid drive to the next tube, reduce the size of, or eliminate, the cathode bypass capacitors, as they also contribute to the blocking due to the long time constant associated with the recovery from a transient signal, or add a diode clipper bounding circuit to prevent the grid from being driven too far."

My new Mercury OT is coming tomorrow... I'll try Tim's suggestions before I install it, I hope I can send the new OT back and save some money! Good luck Steve... not sure how applicable all this is to your problems.
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Tim Marcus · From: San Francisco, CA

when I do this I usually take out the short piece of wire and add the resistor in - right to the pin from the fiber board. Put a little heat shrink to cover up the metal leads and it will look real nice. Worth trying - it takes 2 minutes to install and does not otherwise affect the tone/value of the amp at all
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Steve Collins · From: Alaska, USA

Well, I can't say I wasn't warned. New tubes and filter cap did nothing to remedy the sizzle in this amp, except maybe make it clearer. Would the signal caps be the next suspects on the list?
I know, I know, take it to a tech.

Stephen Cowell · From: Round Rock, Texas, USA

Also look at tube sockets... check for carbon tracks, especially on the final tubes.
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Tim Marcus · From: San Francisco, CA

it could also be a flaky coupling capacitor

in general, I try to avoid amps with oddball issues like this because they can be very stressful.

The good news is that it is likely not the transformers or the chassis - you can always gut the whole thing and put all new everything in there and I bet the noise will go away - kind of extreme - but sometimes you need to cross the streams
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