1978 Princeton Reverb.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by AL1, Jun 11, 2004.


  1. AL1

    AL1 Member

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    Can anyone tell me exactly what this means :20x20x20x20@450 is this the power supply filter cap or caps, and is it the only cap or caps that I have to worry about as far as shock or electrocution? There are smaller value caps: 25uf-25v and 25uf-50v , do I have to discharge these as well?
     
  2. tonezoneonline

    tonezoneonline Member

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    Yep, those are the supply caps . 20µf won't electrocute you unless the amp is on , may give you a "zat" right after you turn it off is all . Turn it off , leave it 5 minutes , then unplug it . By then they'll have discharged themselves .




    20uf won't electrocute you,it's the 450VDC that will.Clip a lead to pin 1 of V1 with the other end clipped to the chassis on any amp you work on.Not all amps have bleed resistors.The earlier Princeton Reverbs being one of those.
     
  3. John Phillips

    John Phillips Member

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    NO! That is completely wrong. The stored charge in a 20uF cap at 450V is quite enough to KILL you if you're unlucky, even if the amp is off.

    And in the Princeton Reverb, there are NO bleed resistors on the main caps - look at the schematic. The caps will NOT discharge if the amp is left alone for 5 minutes. If they're in good condition they can hold it for days. I learned this the hard way when I wasn't very experienced at working on amps - I got an extremely serious shock from a 50W Marshall that had been off for over a week.


    If the amp is as late as 1978, it will most likely be the last version of the PR, which is slighly different from the AA1164/AB1270 circuit - does it have the pull-boost on the volume control?

    The caps you need to worry about from a hazard point of view are all in the cap can - but if you're planning on doing a filter cap job, make sure you change the bias supply cap too (80uF/70V in that amp probably - if you can't get an 80, 100 is fine). Whether to do the low-voltage cathode-bypass electrolytics (the small 25V ones on the board) is optional - it might improve the tone, but it isn't a reliability or safety issue.
     
  4. Swarty

    Swarty Member

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    ... when potentially lethal advice is given out.

    As for those can style caps, I've found them to be much longer lasting than the axial variety found in larger Fenders. I'd say if you are not experiencing any classic bad filter cap symptoms (120 hz hum, motorboating) to leave it. I don't buy into this "change them every 10 year" farce. Maybe 20-30 years on the axials, but as mentioned, the axials seem to be much more durable (likely due to better seal).
     
  5. AL1

    AL1 Member

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    It was said to be the AB1270 circuit that has been blackfaced and the pull boost left on. If it was in fact blackfaced, shoud the main filter caps have a bleeder resistor or resistors installed? How many bleeder resistors should there be and what value, power rating and type to use for this?
     
  6. John Phillips

    John Phillips Member

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    If it has a pull boost it is not an AB1270. Whether or not it has been Blackfaced (and how thoroughly) is a different question...

    Have a look at the schematic. Right after the second stage (V1B) is a resistor paralleled with a cap - this drops the signal level to match that of the reverb return before both are re-amplified by V3A. In the BF or AB1270 circuits, this is a 3.3M resistor and a 10pF cap; in the pull-boost (post '77) circuit they are a 1.5M and a 22pF, giving less signal reduction. This change is to compensate for the slight signal loss caused by the pull-boost circuit when it is off. You can't fully BF the amp and leave the pull-boost in.

    There's also a change in the B+ chain, the second resistor is lower in the post-'77 circuit and the reverb supply is from the low end of it not the high.

    But apart from that (and another increase in the bias cap value) there's not much difference really - they left the Princeton quite well alone, compared to most of the other amps.

    There are no bleed resistors on any version.
     
  7. Swarty

    Swarty Member

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    FWIW, aside from the rectifier tube about the only difference between a BF and SF Princeton Reverb is the .002uF suppressor cap on the reverb return.
     
  8. Jimi Ray

    Jimi Ray Member

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    Where can I get clear, legable, fender schematics???
     
  9. John Phillips

    John Phillips Member

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    There's a lot of good info (including schematics) here:

    Fender Amp Field Guide

    Aspen Pittman's "Tube Amp Book" also has a lot of Fender schematics, definitely including all three main versions of the PR.
     
  10. tonezoneonline

    tonezoneonline Member

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    Shoot me an email and let me know what you want as far schematics.
     
  11. Jimi Ray

    Jimi Ray Member

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    Thanks for the help guys, John I loaded one for my old princeton reverb from that site but when I printed it I got real blurred resolution. I couldn't read the values!
     

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