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Choosing a Princeton Reverb Transformer

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

  1. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    Hi there,

    I'm in the process of building a Fender Princeton Reverb (AA1164) clone and am interested in Mercury Magnetics transformers. I also want to have a universal voltage option as I may be using this overseas.

    Their standard blackface PT is called #125P1B; one of the 'universal' options is called the FBFPP-M/U, 50V bias tap, lower B+ 325-0-325.

    Does anyone know the voltage of the former or of a standard Princeton PT? I'd love to hear thoughts on the pros/cons of having a transformer with lower B+.

    Another universal voltage option is the FBFPP-S/DP, 50V bias tap, 360-0-360 unloaded B+. They mentioned that this one has 'unloaded AC open circuit' with 'more current capability' than the #125P1B. Can someone please briefly explain what 'unloaded B+' and 'open circuit' means in practical terms? Higher voltages and ...? More clean headroom? Better suited for different tubes? etc.?

    I've been successful at building a few amps from directions, layouts and schematics, however my understanding of electronics is relatively basic. Thankfully these forums have been a great help so far! Any insights would be greatly appreciated.
     

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Paul at Mercury Magnetics is always helpful with tech questions like this.
     
    King Fan likes this.

  3. AlabamaOutlaw

    AlabamaOutlaw Tele-Meister

    423
    Feb 20, 2011
    Alabama
    I really like the Classic Tone trannys
     
    Telenut62 and 6stringcowboy like this.

  4. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I looked at some of the other companies, but I thought the 50v bias tap was a good idea since I plan on adding a bias pot. I've also been exploring individual bias pots for each tube since I've had a few mismatched sets of old tubes.
     

  5. CapnCrunch

    CapnCrunch Friend of Leo's

    Jan 7, 2011
    Washington, USA
    I've used Allen amps transformers in a couple amps, and they've been great. They're Heyboer trannies built to David Allen's specs, so they're US made and high quality. Don't remember if he offers universal voltage options. It's worth a look at his website though.
     

  6. Phrygian77

    Phrygian77 Tele-Holic

    639
    Apr 30, 2016
    Crawfordville, FL
    Just out of curiosity, what is your reasoning behind this?
     

  7. Wyatt

    Wyatt Tele-Holic

    782
    Nov 3, 2004
    Aren't Allen's PTs by Marvel Electronics?

    Heyboer make his OTs.
     

  8. red57strat

    red57strat Friend of Leo's Silver Supporter

    Oct 4, 2003
    Massachusetts
    The bias in a PR does not need a separate 50v tap, even if you add an adjustment pot.

    On my clone, I used the first one shown on this page. A tech installed the second one in my '68 many years ago. They both work(ed) great.

    http://el34world.com/charts/Biascircuits.htm
     
    Last edited: Jan 23, 2017
    King Fan likes this.

  9. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I built an 'Accomplice' kit from David Allen over the summer ('16) and he did not offer universal voltage options as of then.

    From what I can tell, the AA1164 circuit powers the bias supply from the transformer via pin 4 of the rectifier. I just assumed that having a dedicated bias tap would be more efficient.

    I also wondered if having individual bias pots without a dedicated bias tap would effect the tremolo effect. I'm still learning about how the 'bias wiggle' actually works.
     

  10. red57strat

    red57strat Friend of Leo's Silver Supporter

    Oct 4, 2003
    Massachusetts
    The tremolo works fine with the bias schemes that I linked to. It's one of my favorite parts of the amp!
     

  11. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    So it sounds like the 50v bias tap is gratuitous. How about their options for 'extra iron' - same thing?
     

  12. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    O.k. Looks like the Allen TO20 is a comparable output transformer to the one by MM that I first listed. For half the price.
    It looks like many of the other manufacturers (classic tone, heyboer, hammond etc) do make a power transformer that can accommodate both 120v and 240v, depending on how you wire them. David Allen offers one with the 50v bias tap (which doesn't sound critical), but no 240v option.
    I'm still not clear how much extra iron benefits the PT, though a lot of people do claim it's an improvement (bass response in particular) for the OT.
     

  13. tele_player

    tele_player Tele-Meister

    234
    Aug 9, 2007
    Sacramento, CA
    Extra iron in the PT usually means it can deliver higher maximum current. But if you're building a 6V6 amp, the power is ultimately limited by the tubes.

    If you want to build a PR that sounds like a PR, resist the urge to get a beefier PT and OT.

    I have a 67 PR, and an Allen Sweet Spot. The circuits are very similar - the Sweet Spot is a bit louder and cleaner, but the PR just sounds better. The Sweet Spot has larger PT and OT.
     
    Last edited: Feb 3, 2017
    Sandhill69 likes this.

  14. hsmaxim

    hsmaxim TDPRI Member

    34
    Jan 28, 2016
    nashville
    Ive used mercury, classic tone and mojo transformers in tweed, marshall, and blackface builds.
    My experience with them in my area of the country with 126 volt power in the winter, 127 in the spring and fall, and 125 in the summer is as follows.
    They all sound good. Having built multiple copies of tweed (5f1, 5f2a, 5e3, 5e7, 5e8a, 5f6a, 5f10, 5f11 and some modified tweed circuits) as well as deluxe reverbs and princetons, and a couple of 18watt and jtm45 marshalls, the classic tones are closest to the voltages they are designed to be with stock value components, with the exception of the heaters which are always kissing 6.9 plus volts.
    The mm voltages are always higher than they are supposed to be, but the heater voltages are a little lower at the same wall voltage than the classic tone and mojo transformers. (This is using the lower voltage version of the mercury transformers. The cloned ones are so hot i just keep that amp plugged into a variac. I think it would burst into flame at my normal wall voltage. It had well over 7 volts on the heaters and the b+ was ridiculous as well.). Strangely it still has 6.6 volts on the heaters with the variac putting out 109 volts so im not sure what exactly they "cloned"
    The mojo transformers are even higher voltage than the voltage adjusted mm (but not nearly as high as their clones) and the heaters are higher than the classic tone.
    One thing I have discovered is how similar they all sound when you use enough diodes, or resistors, or whatever combination to get the voltages to line up for an apples to apples comparison.
    At the end of the day I use classic tone and do what is needed to get the heaters down to 6.3. Why, because they are the easiest to source and the easiest to find wiring diagrams for without having to call and deal with someone.
    For the life of me i dont get why mercury doesnt put their schematics online. I honestly feel they want you to have to call so they can try to sell you some more cool aid. I dont buy it, ive tried it and at the end of the day i dont have time for the extra amount of effort it takes to use a product that is as much as twice the price of the other brands. Additionally, I dont have time to have to call and try to figure out which transformer is correct for my application, only to find out when it comes in and i get to see the schematic for the first time, that it is not at all what the guy on the phone said its specs were.
    Bottom line, if you know how to read the transformer specs and would like to before you buy, then classic tone is your company. If you dont care, then use whatever transformer you want. With any of them though, you better be prepared to do some voltage adjustments, at least on the heaters, if your wall voltage is over 120.
    Good luck with your build.
     
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  15. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    Thanks for the input. Initially I wanted to just make a blackface era PR clone that I could take overseas. I'm tempted to 'upgrade', but I don't really want a significantly 'different' amp from a Princeton Reverb. I've just been enticed by all these posts from folks who say they improved the bass response by upgrading the OT e.g. TO20.
    As for the wall power, I hope to use it at multiple locations and not just at home or as a dedicated studio amp. As a result, I hadn't really considered how different wall voltages will affect the amp. I just imagined that everything would more or less be in the ball park for optimal performance.
    I've seen how setting the bias and using different rectifiers can affect the plate voltage, but I haven't learned how to adjust the heater voltage yet. How necessary is this?
     

  16. hsmaxim

    hsmaxim TDPRI Member

    34
    Jan 28, 2016
    nashville
    If your heaters are over 6.9 vac they are too hot in my opinion. Specs call for 6.3. I adjust anything over 6.6 but im probably a little anal and most would argue that its not nessecary until your really close to 7v. I cant say that logic is wrong, im just telling you where i like mine to stay.
     

  17. RyanV

    RyanV TDPRI Member

    6
    Oct 4, 2014
    austin texas

    I happen to be in this unique situation described above with a 68 princeton reverb. The amps sounds killer and its loud enough to keep up with my loud funk band(believe it or not) but I noticed that if I hit it with the boost pedal and overdrive that the low end E string especially tends to flub out(bass knob on 6) and it can sound like the speaker is blown. Boggles my mind, especially since I have installed a 75watt Eminence Copperhead which is one hell of great speaker

    Last night I took it to my amp repair guy and we spent about 3 hours going over this amp. We first checked all of the tubes and replaced everything with new tubes while looking at the peak curves on a oscilloscope which produced hardly any change in the curve on the scope. The peak was pretty much the same. He determined that the amp was just plain down on power. He then determined it was only outputting roughly 6.5 watts instead of stock wattage

    We then took the amp chassis out. Low and behold somebody had swapped out the power transformer with what appears to be an old Triad power transformer that matches the FBFPP-M/U specs above.

    So back to your questions above based off my experience and the situation I'm in.

    1. stock Princeton power transformer output is at 360, not 325
    2. my amp is still VERY loud with the 325 power transformer installed
    3. my amp tech says that the 325 output PT is making my princeton breakup sooner, and when I have the volume/gain knob at 4 and kick on my boost and overdrive that I'm probably overloading the input on the amp which is the undesirable flubbed out tone I hear sometimes.

    I'm fairly certain a stock 360 mercury magnetics PT is in my near future.

    I should take a video with the amp dimed as it sits, and post it on youtube for future reference...
     

  18. tubeswell

    tubeswell Friend of Leo's

    Jul 1, 2008
    NZ
    The PT on my one puts out 350-0-350VAC from memory, for a B+ of about 420 (with a 5U4G and JJ6V6S).
     

  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    I have a Couple of Rivera era Super Champs, which are hot-rodded Princeton Reverbs. I like to run the amp hot. I have EV’s in them. IF I run the amp with Gain, I understand that I cannot have the tones up....especially, the bass control has to be no higher than 4. I like the amp with all volume and gain controls on 10. (8^O At those settings, I have great cleans with firm low end in the clean mode. When I pull the midboost, all is still good. When I pull the Lead boost for high gain, the signal still holds together....screaming high gain.
    The need to roll those tones...especially the bass....is well known. Mesa has written this in there MK amp manuals for decades....and those preamps are based on the AB763 thing. If you run the bass above five and push gain...whether in the amp or with a pedal...the signal goes to pieces in the preamp.
     
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  20. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    To add to my post above....I am spoiled by having a bass cut control in addition to the usual treble cut control on my guitar. When going into high gain mode, it is sometimes advantageous to be able to trim a bit of low end off of the guitar signal to maintain clarity and note definition in the low end.
     

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