Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups

Choosing a Princeton Reverb Transformer

Discussion in 'Shock Brother's DIY Amps' started by bryansbizaar, Jan 20, 2017.

  1. muchxs

    muchxs Doctor of Teleocity

    Aug 14, 2004
    New England
    Can't leave you people alone for a moment.

    Definition of terms: "POC" is Piece of Crap, abbreviated.

    A tale of two (or more) transformers:

    I have a couple paperweights right next to my computer from back when I used to post on this topic nearly every day. One is a barbecued Schumacher 022772, the pre- '71 stock power transformer for Champ, Vibro Champ, Princeton and Princeton Reverb. Don't care what it puts out for voltage. Doesn't really matter. What matters is the available current which BTW is around 70 ma on the high voltage and 2 amps on the rectifier filament.

    (cue best boxing announcer voice)

    In this corner weighing in at a fraction of a pound or a pound and some fraction we have the Mercury Magnetics FBFPP shiny new and an exact copy of the stock POC. That makes it a POC as well in my book. Judging by the height of its core and the fact that it's supposed to be a replica of the stock PT I'd guess it performs exactly the same. It's all shiny new with wires dangling everywhere. I could do a comparo to the stock PT if I were so inclined. I'm not so inclined.

    The stock POC PT is barely adequate for a pair of 6V6s.

    Before repeating what I've said before, we'll take a peek at the tube data sheet.

    First thing is says is max plate voltage 315v, max screen voltage 285v. We're exceeding that by a good 100v in a Princeton Reverb.

    Then we scroll down the page some. It says zero signal plate current 70ma, max signal plate current 92ma disregarding screen current. The stock PT makes around 70ma. That means when you whack it with a good hot signal the plate voltage is going to drop substantially. It has to make the current. There's ain't no such thing as a free lunch here. Clip an old analog VTVM into the B+ rail and watch it some time. Say it ain't so.

    Being as they copied the same old tube data sheet over and over there is no such thing as published ratings at the 420v- 450v supply seen in Princeton Reverbs.

    Doesn't really matter what the supply voltage is as long as it's somewhere between 300v and 400v as long as the available current is adequate.

    Next shiny paperweight we have is a Mercury Magnetics FBFFP-S, presumably fat stack, reduced voltage. This one is similar to the post- '71 Schumacher 10020, presumably around 100ma on the high voltage. I could tell you more if I could be bothered to hook it up.

    I'll state this as my opinion because I don't care to present a white paper to prove it:

    Dropping your supply voltage approximately 50v under the nominal stock and IMO way too high 420v will dramatically extend your tube life. If you want new reissue tubes to live reduced supply voltage is mandatory.

    "Better" is subjective. Dave is using a modified PI and admittedly his TP25 is one hefty chunk of iron. Dave also has Nashville leanings even though he's located way up on the northern Kentucky border rather than the southern border, closer to Nashville.

    To me a Sweet Spot looks a whole lot like a Princeton Reverb and it can easily be tweaked to sound and act a whole lot like a Princeton Reverb. If we assume the difference is just the transformers... it ain't.

    Short answer for those of you who like short answers: IMO we want 100ma minimum on out PR PT for cool operation and good tone. Anything over 325v-0-325v is too damn high and is going to cause problems with new production tubes. Ideally we want to see around 300v-0-300v and remember, 100ma minimum.

    We'll multiply that by our 1.15 fudge factor to estimate our supply voltage:

    1.15 x 300 = 345. We might want to fudge our fudge factor to reflect actual reality. Multiply by 1.2 or 1.21. Starting with 300v-0-300v 100ma GZ34 rectifier we're probably looking at 365v.

    Don't make me come down here.
    Last edited: Nov 5, 2017
  2. RyanV

    RyanV TDPRI Member

    Oct 4, 2014
    austin texas
    I don't really understand everything you posted, but I'm trying to make sense of this.

    Somebody installed an old Triad cloth wires direct fit Power transformer with a 50v bias tap(unused) into my 1968 PR. The amp tech it was putting out 325 instead of 360

    I'm trying to figure out if it's even worth replacing this Triad with a stock replacement.

    Last edited: Nov 10, 2017
  3. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    If I had a PT that was working, I'd just use it.
    King Fan likes this.
  4. RyanV

    RyanV TDPRI Member

    Oct 4, 2014
    austin texas
    Good point, and the amp sounds fantastic.
  5. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    Big grin to see that avatar up there.... AFAIK, no one knows more about the PR than MuchXS. An upgraded (less B+, more mA) PT is clearly a great idea for all the reasons he lists.

    True confessions: I'm stubborn. Even knowing Much's preferences, I once pressed him about my plan to build a 'true-to-1965' PR. Thinking: Everyone agrees the stock PT is overmatched and may eventually die, but I figured that for the vintage amps, 'eventually' was often 50+ years. Plus, I don't push my amps hard, I had a mellow vintage speaker, I use robust NOS 6V6s that were old when the PR was young, and I run NOS rectifiers. If I recall (apologies, Much, if I got this wrong), he said although it wasn't his #1 or even #2 approach, he would occasionally build or re-build a 'stock PT' PR for someone who wanted it. So my compromise was to try the Mojo (? Heyboer) 760 (330-0-330, 75mA) that came with my Mojo kit. Using an NOS rectifier, I got B+ under 400, and the amp sounds great to me, with whatever virtues the small PT might offer -- sag, bloom, absence of stiff-nosity -- and so far it isn't eating NOS power tubes. I really love it, and believe it's about as 1965 as possible.

    *But* for my recent 6G2, I tried to go the other way, the MuchXS way, finding a PT that gave me more current and less HT. Enter the ClassicTone 40-18027. It has your dual 120/240 wiring. Dual HT options (630v and 550v) to definitely get your B+ under 400. A nice 100 mA to bring to the party. And as a bonus I know Much favors, a heater center tap. Result: Awesome.

    For me, going both roads worked great and got me two nicely different amps. But knowing what I know now, if I were only going to own a PR, I would listen to Much... :)
    FRANKENJAG likes this.
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