Weakness for my Princeton

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by stantoine, Jul 8, 2006.


  1. stantoine

    stantoine Member

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    Sorry if this issue has been recently addressed... I'm new here (I did skim through the recent threads and found some pertinent info, but i'm afraid i must persist :).

    I recently bought a late-seventies Princeton that appears to have been sitting in someone's basement/garage pretty much since...well, the late-seventies. It has some light surface wear, but the "Fender Special Design" logo on the original tubes is still a bright orange and i had the distinct feeling that i might have been the first to crack open the chassis since it left Fullerton.

    Having said this, while the amp does have a wonderful clean tone, it seems to be a bit feeble when pushed... It's just not very loud and doesn't break up much at all, even with humbuckers. Also, and i'm not sure if this is just my imagination or not, what volume there is when i first dime it seems to taper off after a couple of minutes.

    So here's what i've done, so far:
    -new Weber 10" Alnico speaker
    -upgraded OT to 25w Deluxe
    -replaced all 25uf/25v cathode caps with 22uf/50v parts
    -upped the PI voltage via the Stokes mod.

    However... No, i have not replaced the Mallory can filter cap... It sounds dumb, but it's hard to find those can caps these days and i don't really want to make a mess by installing a bunch of individual caps... I will if i have to but, (and this is the purpose of my question here) is there anything else that could be causing the amp to have a weak output? Am i forgetting something?

    (Or is this just the way they're supposed to sound?... I bought a brand-new Princeton Reverb in 1979, and though i know the circuit is a bit different, i swear it was way louder and broke up like crazy when cranked, even without the pull-boost engaged. This Princeton i have now doesn't even seem as loud as the Blackface Champ i had a few years back.)

    Any advice will be much appreciated,
    Rob
     
  2. Blue Strat

    Blue Strat Member

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    Yeah, I have a weakness for mine as well....oh wait, that's not what you meant ;)

    Princetons, without reverb, are less loud and less distorted when pushed than their cousins Princeton Reverbs. What you're hearing may be normal.

    Be sure to at least check, if not adjust (by installing a bias pot or trial and error with different bias resistors) the bias and check all the tubes. If you have a known good set of tubes, swap them into your Princeton and see what happens.

    Though it's not likely to be the cause of low volume (in this case) the can caps are available for about $30 from Antique Electronics.

    If you don' mind losing tremolo, it's possible to modify that amp to add the missing extra gain stage found in Princeton Reverb amps.
     
  3. danieldroukas

    danieldroukas Member

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    Old Tele Man ... you don't play two guitars at once? Amateur.
     
  4. stantoine

    stantoine Member

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    Thanks for the advice, Mike! I'm gonna try the things you mention... I will probably also try installing a temporary set of new filter caps and if it does make any kind of noticeable difference then at least i will know that a new can cap is a, uh... sound investment.

    rob
     
  5. krash

    krash Member

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    did you try replacing the tubes?

    I have a late 70s SF Princeton Reverb (now modded of course, rather significantly) that is way louder than my GDS 18W combo. It has a 12" speaker but it can't make THAT much difference. My friend has one just like it but with a Celestion 10" speaker and it is also very loud when cranked. I'd think there's probably something wrong with yours :)
     
  6. Blue Strat

    Blue Strat Member

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    This brings us back to the original post (Princeton, not Princeton Reverb) and several that follow.
     
  7. fullerplast

    fullerplast Senior Member

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    I agree with Mike that the diffence you are hearing is the difference between the reverb version and the non-reverb version. The non-reverb does not break up much and generally sounds anemic in comparison to the 'verb version. Best you can do in a non-invasive way is to get a very efficient speaker. You've done the Stokes and Paul C mods, added the Deluxe OT which is all I would do circuit wise. You could get a new baffle and go with a 12" Celestion Blue or Weber ceramic blue dog for a bump in efficiency. Make sure your power tubes are good, maybe try some JJ 6V6's to get closer to a 6L6 performance.

    In the end though, I suspect what you really want is a Princeton Reverb.
     
  8. Blue Strat

    Blue Strat Member

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    As before, he can "steal" the tremolo triode from V2 and use it as an additional gain stage to match the performance of the reverb amps.
     
  9. Reeek

    Reeek Member

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    You certainly need to replace ther filter caps. I wouldn't run it much anymore if it were my amp just in case. You have a little gem there :)


    You may find an in stock multi-can cap at Antique Electronic Supply www.tubesandmore.com/ that is either right on or close enough to use.

    However, you can go to Vibro World www.vibroworld.com and order a custom value multi-can cap that will fit perfectly and work as designed. I've done it dozens of times with my somewhat extensive vintage tube amp collection. It isn't period looking but that doesn' detract from that amp which is collectable for it's tone for sure. What I mean is the Vibro World multi-cans are almost chrome like and although they aren't period looking, they sure are pretty to look at. If you go this route, make sure you get a higher voltage capacity since the more voltage the cap can handle the longer the cap will perform for you (generally anyway) because it's the surge at power up that can hit the filter caps pretty good. With voltage capcity, the more the merrier with filter caps as far as I'm concerned.

    The other option is to have discrete Atoms installed inside the chassis if they will fit somewhere (someone here should know if that's possible) then just leave the old Mallory there for looks.

    Todd Shock of Wakarusa Amplification did that for me on my 1964 Valco/Airline head that he electonically refurbed for me and it works great. And a plug for Todd . . . this Airline head is the best amp for tone I have ever played to this day and I have some pretty sweet amps including a 1969 BF'ed Twin with the original JBL's. This Airline is the bomb.

    Nice amp you have there ;)
     
  10. fullerplast

    fullerplast Senior Member

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    Yes, he could, but then you are getting into more extensive mods and affecting the value of the amp. You could also re-design the amp and put in a beefier PT, GZ34, 6L6's and make it into a Fuchs mod. From the nature of his questions, I think he was more interested in maximizing performance while staying with basically stock, or easily reversible mods, but I could be wrong.

    Be careful with arbitrarily replacing the stock cap can. They actually last much longer than you would expect and some reports on replacement cans have shown worse filtering than the old stock cans. I have used the Vibroworld cans with good results, but I've also seen many original multi-cans working very well.
     
  11. Wakarusa

    Wakarusa Member

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    Although the reverb version includes an extra gain stage, the non-reverb version is also missing the 3.3M resistor in the signal path that's used to generate the drive for the reverb loop. Unleashing an extra triode stage without adding some attenuation can easily get you an amp with (yes, it's possible) way too much preamp gain.

    Besides, there's more than enough gain available in the non-reverb preamp to push both PI and output section over the edge.

    Some suggestions in no particular order:

    - Maybe I'm reading this wrong, but I'm surprised that you've swapped the speaker & OT, modded the PI, etc. but haven't tried a fresh set of output tubes. There's a good chance the originals are getting a bit tired.

    - +1 on checking the bias of the output tubes - particularly since you've changed the OT. Changing to an adjustable bias setup in these is very easy (takes a 6 lug terminal strip and a handful of components. The 6 lug strip even fits into the existing screw holes for the original bias board)

    - If you want more gain on tap there's a much easier way to accomplish this than stealing the tremolo. On the first gain stage try replacing the 100K plate load with a 220K and the 1K5 cathode resistor with a 3K3.

    - Clean and lubricate the tremolo intensity potentiometer. It's in the bias path so dirt or flakiness will mess with the output section.

    - Measure the plate load resistors and the power supply resistors (the 18K on the filter cap, and the 18K & 1K all the way to the left of the circuit board). Replace anything that's drifted more than 5% from spec.

    - Since this is a 70's vintage amp, look for small ceramic caps between pin 5 of the output tube and ground (usually they're between pins 5 & 8). If they're there, clip them out. If the amp starts to oscillate correct lead dress or put 'em back.

    - Consider installing screen resistors if not present. They'd go between pins 6 & 4 on the power tubes and you'd move the screen supply to pin 6.

    - Consider installing grid stoppers if not present. This time it's between pin 1 & 5 with the grid feed moved to pin 1
     
  12. Blue Strat

    Blue Strat Member

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    Since I'm not telepathic, I don't know what he wants. ;) One thing's for sure, he'll never get what he's lacking from the PR without either a major mod or a pedal.
     
  13. Blue Strat

    Blue Strat Member

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    Adding the extra gain stage assumes that you would add attenuation before it, just like the schematic ;)
     
  14. scottlr

    scottlr Member

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    I have a 66 Princeton, and like yours, it is clean clean clean. Dimed with a Strat it stays clean, with a LP it has an ever so slight breakup. Put my Route 66 OD pedal in front of it, and it sounds great! All I did to mine was to pull the original speaker and tuck it away for safekeeping, and put in a replacement. Mine is 100% original, down to the 2 prong cord. I didn't want to risk blowing the original speaker, in case I ever decide to sell it.
     
  15. Wakarusa

    Wakarusa Member

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    With the original ungrounded cord and stock Fender mains wiring it is more likely that your estate will be selling it (perhaps to pay for your untimely funeral).

    Vintage purity is great if you're going to set it in a case behind glass, but if you're actually going to plug it into a wall a change to a grounded AC cord and correct internal mains wiring is highly recommended.
     
  16. Mr. dB

    Mr. dB Member

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  17. Blue Strat

    Blue Strat Member

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    This is true, but this doesn't really give the desired results. An additional gain stage (discussed earlier) is what's missing. Doing the gain stage mod AND using an upgrade tranny would yield fantastic results.
     
  18. Blue Strat

    Blue Strat Member

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    Well said. When repair customers resist ungrounded power cord upgrades (I refuse to take amps from people like this citing the liability issues) and tell me they won't sue me if they get shocked, I tell them I'm not worried about them suing me. I'm worried about their family and life insurance company suing me because the customer will be dead:)
     
  19. Mr. dB

    Mr. dB Member

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    How many people have actually died from ungrounded guitar amps?

    Really, it is risky, but there's no need to overstate your case. You two make it sound like anyone who uses a vintage ungrounded amp is living on borrowed time, that lethal electrocution is an impending certainty.

    ps: Keith Relf was in his bathtub.
     
  20. Blue Strat

    Blue Strat Member

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    All it takes is one to ruin you day (and you're life when you get sued).
    If the chance is one in 10,000,000 why would I risk it for a $100 amp job? In life, you've gotta play the odds. Those who don't often fail.
     

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