SF Princeton Reverb high plate voltage

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Laurent Brondel, Jan 12, 2015.

  1. Laurent Brondel

    Laurent Brondel Supporting Member

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    This is a '77 Princeton Reverb in great shape (non pull-boost). The chassis was probably never pulled out before I did to replace all electrolytics. I also installed a 10k pot to adjust the bias.
    However with 30ma bias I get 485v on the plates! (with a pair of JJ's 6V6 and the original 5U4GB rectifier).
    I swapped the 5U4GB for a 5Y3 and with 25ma bias the voltage dropped to 450v, still high for a Princeton reverb, no?
    I'm thinking of swapping the bias resistor going to the bias pot for a 18k to have a little more bias range (it is already lowered to a 20k), but how high can I go on the bias to drop the voltage on the plates?
    PT is pretty cool to the touch, but the 6V6's and 5Y3 are pretty hot, of course.
    Before I worked on this amp I thought it sounded pretty sterile, with an unpleasant break-up. Now with the 5Y3 break-up sounds great, but of course headroom is diminished. Any advice? Am I endangering anything here?
    The amp came to me with the original tubes, actually original everything, and seemed to have been hardly played at all.
     
  2. SeanF

    SeanF Member

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    a little over 11 watts/tube for 6V6 sounds about right. I'd leave it like that. I think the B+ is a little higher than the schematic just because wall voltages have drifted up since '77, but the tubes oughta be fine with it.
     
  3. Laurent Brondel

    Laurent Brondel Supporting Member

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    Well, voltages keep going up, up to 600v with a bias of 35ma. Not sure what is going on. Could it be a bad electrolytic cap can?
    When changing the electrolytics, I noticed that the 2 100ohm resistors going from the pilot light 6.3v heaters wires to ground were totally burnt, severed in half actually, so I replaced them. Is that odd?
     
  4. twangbanger

    twangbanger Member

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    Your plate voltages should drop a little, not go up when your ma's go up. What is your incoming voltage from your outlet? Is the power trannie original ?
     
  5. SeanF

    SeanF Member

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    ok, that's weird. I'd definitely be looking into replacing the filter caps, but I'm not sure how that could cause your problem.

    yeah, that's really odd, I've never seen that. Both symptoms could be caused by a faulty power transformer. I'd disconnect the PT from everything downstream and do some load testing on it by itself, just to see what it does.
     
  6. Blue Strat

    Blue Strat Member

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    First, replace your meter battery.

    Second, 20 mA is closer to where the bias should be set. 30mA is too high.

    Third, what's your line voltage from the wall?
     
  7. Laurent Brondel

    Laurent Brondel Supporting Member

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    I think this is it… It just died, in the middle of a snow storm now, I'll go get new batteries tomorrow.
     
  8. Blue Strat

    Blue Strat Member

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    ALWAYS suspect your battery first. ;)
     
  9. TheWayfarer84

    TheWayfarer84 Supporting Member

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    I was getting all sorts of odd voltage readings with my meter....opposite effect though...they were in the low 300's. New batteries and everything checked out right where it should be
     
  10. Tone_Terrific

    Tone_Terrific Supporting Member

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    Not rare, imo, but not sure about why that happens.
     
  11. pdf64

    pdf64 Member

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    It's probably caused by a short in a power tube sending B+ current into the heater circuit.
    The resultant high current to ground would blow the 100 ohm resistors.
    Such an event may then damage other tubes, due to the heater circuit being pulled up towards VB+, above the limiting value for the insulation in the tube between the heater and cathode.
    This would cause line frequency hum, resolvable only by replacing the tube/s with damaged heater-cathode insulation (as well as the bad power tube).

    With those resistors burnt out, the heater circuit would have no ground reference and I would expect the amp to have a buzzy hum at line frequency.
    Also I suspect that at least one of the power tubes isn't original.
     
  12. Laurent Brondel

    Laurent Brondel Supporting Member

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    I'll remember that… Until my meter totally died I was wondering what was going on!

    Now I am getting normal readings, a bit on the hot side perhaps.
    With the original tubes, at 17ma I get 429v on the plates.
    With JJ's 6V6S's, at 19ma I get 423v, with the 5U4GB rectifier.
    With JJ's 6V6S's and a GZ34 rectifier I get 435v with 24ma.
    I have it now with JJ 6V6's and a 5U4GB.
    BTW is it safe to use a GZ34 rectifier in those?

    Oddly enough, even before I changed all the elecrolytics, it was the quietest Princeton reverb I've heard, albeit not sounding so great.
    The 1k last resistor on the board going to the rectifier had burn marks on it. Although it tested fine, I replaced it.

    Speaking of tubes, that amp was as 100% original as any I've seen, I think nobody opened the chassis before I did.
    Here are the original tubes that were in it, GE's maybe?

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    And the added adjustable bias pot, I used an existing hole in the chassis:

    [​IMG]

    [​IMG]
     
  13. 335guy

    335guy Member

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    When I bought my '78 PR, it came with the original Fender labeled tubes as well. I've wondered who the manufacturer was of these small, skinny base 6V6's. They look identical to yours. Mine work, but the tone is less than inspiring. I've gone to properly biased ( with an added bias pot ) and matched JJ's. But I kept the Fender labeled 5U4GB in there
     
  14. PushedGlass

    PushedGlass Member

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    I have a '78 PR. I believe the "wafer base" 6V6es were made by GE. Unfortunately "bear trap" style tube retainers want to push those out; I have a couple extra retainers on hold for the next power tubes.

    When I was testing some on-hand 6V6es with a dual bias probe/ammeter combo, one Silvertone-branded one I was checking out started out with a cathode current of about 30mA and it started to climb up to the 70s and red-plated (~21-23mA is where the original GEs sit).
     
  15. Laurent Brondel

    Laurent Brondel Supporting Member

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    I have a 5U4GB in mine as well, but I put a JJ.
    The original tubes sound new, I think my amp was barely played. The wafer base 6V6 GTA's break up earlier than the JJ's and sound a bit harsh when they do, but sound great "clean" with a nice shimmer.
    Anybody knows what the preamp tubes are? The lettering is identical to the 6V6's, are they GE too?
     
  16. PushedGlass

    PushedGlass Member

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    Same here with my '78's wafer-base 6V6es, which, if not original, had to have been in there a very long time. I'm not hearing a problem with them (not that my ear is super-experienced with amps like this). If they start popping out of their sockets (I've retensioned them but they just don't seem all that secure) I may go ahead and replace them just because I've got a couple of new bear-trap-style tube keepers on hand that I can't actually use with these tubes. I just hope that when I do get replacement 6V6es that I don't have to mess with the bias; the bias probes now read 21-23mA (which I understand to be plenty OK) and I opted to not add a pot or anything like that.
     
  17. swiveltung

    swiveltung Member

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    If it originally had the 5U4GB then you are safe with the 5AR4... but not the other way around!
     

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