"New" (to me) Silverface Princeton Reverb

Discussion in 'Glowing Bottle Tube Amp Forum' started by ShawnRocker, Jan 18, 2016.

  1. Coop47

    Coop47 Poster Extraordinaire

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    Great score. It's worth the extra money to get it shipped properly rather than checking it or seeing if the TSA would allow it on. And with muchxs and Wally checking in, you're getting sound advice here. They're the real deal.
     
  2. BobbyZ

    BobbyZ Doctor of Teleocity

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    A PR will run fine on a real 5y3. Be fine on one of the new ones too but USA 5Y3s are dirt cheap so why bother?
    The 5y3 will drop voltage which isn't a bad thing in a PR. The 5v4 Muchxs recommend will get the voltage higher but not as high as a GZ34. You can season to taste there but bias changes with voltage so watch that.

    The hum could be tubes or something but also electrolytics I'd do electrolytics first. Just a no brainer on an old amp.
     
  3. muchxs

    muchxs Doctor of Teleocity

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    It's a "Catch 22" with these amps.

    The supply voltage is around 420v with the 02772 power transformer and a GZ34 rectifier. That's because the 022772 supplies 70 ma on the high voltage, adequate for a pair of 6V6s but just barely. It would be higher except the tubes drag it down. While we're at it, gosh, where else have we seen that 022772 transformer? Give up? You'll see them in Bronco, Champ and Vibro Champ amps through 1970. It's not as if Fender used "Princeton Reverb" transformers in Champs. It's the other way around.

    Let's look at 6V6 tube data sheets. Those tubes were designed for a 300v supply in old radios and phonographs. A 420v supply is literally "off the charts". The way Fender got away with it was excellent tube quality in the mid- 60s through the '70s. Those old tubes would actually survive the elevated supply voltage.

    After 1970 The 010020 power transformer got the nod. Hey, ho! Guess what? Fender had Schumacher whip up a big ol' batch and you'll find them in Bronco Amps, Champs, Vibro Champs, Princetons and Princeton Reverbs. On the one hand I'm lookin' at an AA1164 schematic that shows a 5U4G rectifier. On the other hand (literally) I have a 010020 PT yanked out of a '70s Princeton Reverb. There is no mention of the 010020 on the schematic I'm lookin' at and while we're at it no notes on the 022772. In other words they didn't document what they actually used. There are schematics with "022772" penciled in but no notes on the limitations of the 022772 power transformer.

    The supply voltage settles in at around 440v with the 010020 power transformer, a solid 100v more than 6V6 tubes were designed for. A 5AR4 in a '70s Princeton Reverb sends the high voltage even higher and it's too high start with.


    We're getting into the difference between technicians and engineers here. This is relatively lowbrow engineering but it's engineering just the same. A technician doesn't ask too many searching questions because searching questions make our heads hurt. A technician shoves a 5U4G tube in there because it works according to the available documentation. An engineer double checks because the available documentation is sometimes incomplete and / or incorrect.
    .

    And that's what happens. Like I said, your amp has the 022772 power transformer with its 2 amp rectifier filament supply. The 5U4G draws 3 amps, a full amp more than is available.

    The applicable rectifier tubes with your power transformer are

    5Y3

    5V4

    5AR4 / GZ34

    A 5U4G will eventually boil the guts out of your power transformer. Blowing fuses is a symptom the amp pulling more than is available. Don't let your tech tell you otherwise. Clearly, he hasn't gotten the memo.
     
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  4. BobbyZ

    BobbyZ Doctor of Teleocity

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    Muchxs knows his Princetons !

    Where I screwed up with my 64 PR. When I bought it I pulled the fuse and it looked "right" then I pulled the chassis and it looked untouched. No way of knowing for sure but I'd bet that was the time the chassis had been out.
    So I forked over 9 100 dallor bills and hauled it home. (real fast because that's PRRI money)

    Then I ordered electrolytics and played it till they got in. Did the service work and used it a couple of years no problem. Untill it started smoking !
    (suspect one of the tubes shorted but still haven't checked the tubes)
    When it happened I saw the fuse was 3 amps. I should've double check that! Might have saved the PT might not have.
    So somewhere along the line it blew the fuse and the fix was a bigger fuse. Probably from that bulging Astron cap can.

    Two lessons one check the fuse ! The other is those PTs are weak! I could see a 20 amp fuse not blowing while the PT releases it's magic smoke. But that 3 amp didn't.

    I'd rather not even talk about this because it proves I'm a dumbass. But if others learn fom my mistake that's a good thing.
     
  5. ShawnRocker

    ShawnRocker TDPRI Member

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    Muchsx, please check you pm. I need an address to ship this to you at in the next couple hrs or I'm gonna have to ship it home, then to you.
     
  6. jazzguitar

    jazzguitar Tele-Afflicted

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    Great find!

    Like it was said, you can run this amp with:
    5Y3,
    5V4
    5AR4 / GZ34

    The 5Y3 will lower the headroom and plate voltage some, but is safe to use. A 5V4 will work more or less the same as the GZ34.

    If the bias is not set to too much current (which will stress the power transformer as well, my aim would be 18..20mA) then a little more plate voltage will not do any damage, NOS power tubes (or JJ current production) will do fine (it is not overvoltage but overload - to much power dissipation that kills).

    I would have the can capacitor and bias cap renewed, check and keep all others if fine, and with a good matched pair of power tubes the hum should be negligible unless there is some other problem, maybe in the reverb circuit.
     
  7. ShawnRocker

    ShawnRocker TDPRI Member

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    Thanks for all the feedback on this amp. I was going to to send the chassis to muchsx for repair, but once I got it home I decided to put my seldom used electronics training to use and dig into this thing as a learning experience, and so far I've learned a ton.

    1st thing I did was ring out the reverb tank and,sure enough, found an open on the input side. Opened the tank, soldered the wire back together and found it worked fine using my Vibrolux Reverb to run it.

    Installed a 3 prong cord next.

    Replaced the filter cap can the other day. The clean headroom improved a ton; where it used to break up around 3 or 4, it now breaks up around 8. The hum didn't go away.

    Rather than buy new 6v6's right away I ordered a bias probe; I figure I'll need it going forward- a buddy has a blackface super that breaks up at 3 he wants me to look at next- I suspect it's the caps, he's had it since the early 70's and says no work has ever been done on it.

    The probe arrived tonight. With the 5y3 in I measured 25.12 mA on the left tube and 15.66 mA on the other. I measured plate voltage at 390VDC. Curious, I replaced the 5y3 with the GZ34 from my Vibrolux and measured again and got 410V.

    I'm suspecting the tubes for the hum more than ever. I'd swap in the 6v6's from my PRRI but it's at a friends house now and said friend is out of town. I think I'll just buy a matched pair or borrow the pair out of another buddies PRRI 1st.

    I also ordered an adjustable bypass kit online I'm waiting for to show up. Once I get the kit and go to set the bias per the guidelines I found online, am I better off with the 5y3 or replacing it with a GZ34 or 5v4?

    If a matched pair of 6v6's doesn't silence the hum, what's the next likely suspect?
     
  8. muchxs

    muchxs Doctor of Teleocity

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    If the hum doesn't go away the source may be elsewhere. The reverb circuit is a common source of hum. For example if the reverb pan isn't correctly oriented you'll get big hum.

    Your 6V6 tubes are woefully mismatched, something that was likely just by looking at them. One is drawing way hot, the other one is drawing way cold. What's happening essentially is one tube is doing all of the work while the other one is just along for the ride.

    I often buy allegedly "matched" pairs of NOS 6V6s. I save the closely matched ones for fixed bias amps like your Princeton Reverb. Anything else goes in Champ style amps that use single 6V6 tubes so matching doesn't matter. If I see a pair of hidden gems on eBay I'll try to point them out.

    If you're good with spending the dough for an NOS 5AR4 then go for it! I don't endorse new imported tubes in general and new imported GZ34s in particular.

    A cheap and cheerful 5V4G will do the trick for far less dough.

    While we're at it I was rummaging around in my back room the other night. I tend to buy parts toss them in there and then forget where I put them. My current scavenger hunt is for the two 8-32 stainless steel lock nuts I know I bought the other day.

    I kept kicking into a plastic bag on the floor. I figured, "Best check it out!" Turns out it was full of 6SL7s, 6SC7s, 6SQ7s with a couple British GZ34s mixed in for good measure. :cool: :D :cool:
     
  9. ShawnRocker

    ShawnRocker TDPRI Member

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    Thank you, please do point out if you see anything.
     
  10. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Sorry to go a little off topic here but, the very best mod for those amps is to make a new baffle board and use a 12" speaker. They sound absolutely heavenly with an alnico 12" speaker. The 10" unfortunately has such a snappy attack and makes an otherwise excellent amp sound a little boxy.

    Hope you get it sorted out.
     
  11. TeleFunk Man

    TeleFunk Man Tele-Afflicted

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    Very nice amp. I own a 70s Fender Vrobrolux Reverb. The Princeton is on my GAS list.

    You can find plenty of info and mods for this amp (and other Fender tube amps) at these sites:

    http://fenderguru.com/amps/princeton-reverb

    http://thevintagesound.com/ffg/
     
  12. muchxs

    muchxs Doctor of Teleocity

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    .
    Performance starts @ :42.

    Lou sez,

    "You can't depend on any wise men. Can't find 'em 'cuz they're not there."


    They don't mention that a 5U4G is unsuitable for many if not most pre- 1970 Princeton Reverbs. Then there's the silicon diode trick for "more headroom". Sure, increasing the plate voltage results in a modest increase in power and "headroom". Bear in mind the supply voltage in post- 1970 Princeton Reverbs is typically around 440v. The filter caps are rated at 450v. What do you suppose happens when you exceed 450v? :rolleyes:

    .


    .
    D. Wayne sez,

    "I don't need no freaky deeky fractal geometry crystal silicon chip.
    I ain't walking on lay lines, reading no High Times put me on another bad trip.
    Timothy Leary just check out this theory,
    He sold acid for the F.B.I.
    Well, he ain't no website wonder, the guru just went under,
    You can keep your California Sunshine."

    :D:D:D
     
  13. krrraaannnggg

    krrraaannnggg NEW MEMBER!

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    Hey! How goes the PR adventure? I have one that was built 7 weeks before yours...thot I'd show you the gutshot. I know my PR really well...muchxs was right on. Any questions give me a shout.
     

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  14. RedRock

    RedRock Tele-Holic

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    Plus excessive heat.
     
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