Upright Power Transformer for 5F2A Princeton

Discussion in 'Shock Brother's DIY Amps' started by dbaum, Aug 5, 2021.

  1. dbaum

    dbaum TDPRI Member

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    I'm working on an upright version of a 5F2A Princeton and need suggestions for an upright power transformer? Can anyone help?
     
  2. jimgchord

    jimgchord Tele-Afflicted

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    I used a hammond the only time I did a 5f2a build with an upright tx. Pretty sure I used a diode rectifier because most of their offerings in that voltage/current range only had 2A 5v windings.
     
  3. cometazzi

    cometazzi Tele-Afflicted Silver Supporter

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    EDIT: I think I'm wrong!
     
    Last edited: Aug 5, 2021
  4. jsnwhite619

    jsnwhite619 Friend of Leo's

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    How does this look for you? I forget if it's the Champ or Princeton that lists voltages, but the 275v PT's I've used in the past nailed the 340B+ and entire schematic levels using JJ tubes.

    upload_2021-8-5_22-41-8.png
     
  5. dbaum

    dbaum TDPRI Member

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    That should do it! Thanks!
     
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  6. cometazzi

    cometazzi Tele-Afflicted Silver Supporter

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    Is that 340B+ with a 5Y3 rectifier?

    Not to hijack this thread but I'm looking at doing a Tweed Princeton-based build myself. I was looking at somewhat cheaper transformers (269JX, for instance) thinking I could go the SS-rectifier route and save a few bucks. Problem there is it provides only 69ma on the HT. As far as I can calculate (and I may be wrong) I think I would run out of juice if I ever tried to do something stupid like add another 12A*7 or two. The 270DX provides 104ma, which should do it no problem.
     
  7. Drew617

    Drew617 Tele-Holic Silver Supporter

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    I like the Weber W022772 quite a bit for about $50, and you can get stand-up bells to convert it pretty easily for another $10. Pretty good sub for the 290AX at 2/3 price, although just a little different... I think you get 300-0-300 or 330-0-330.

    @cometazzi don't overlook the $28 WRVBPT if you're going to use diode rectification anyway. Those are crazy inexpensive considering how useful they are. I'm almost tempted to stockpile some...
     
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  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yes, with JJ 5y3. Their 290AX is my go-to for 5f1/5f2/5f10/5f11, etc., type builds. Anything with that same footprint. 100mA, handles a 5u4, and has a 275v/325v option. I got an email a few weeks ago that Hammond was raising their prices something like 8-12% across the board, so everything is getting more expensive these days.
     
  9. wallybob

    wallybob TDPRI Member

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    I used the Weber WRVBPT on a 6L6 5F2 with SS diodes and it's been working very well. That transformer is too cheap to resist!
     
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  10. cometazzi

    cometazzi Tele-Afflicted Silver Supporter

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    Thanks for the reply, and thanks for the tip!

    Oh yeah, I forgot about those. Thanks!

    I mean, SE amps don't benefit from tube rectifier sag anyway, right?
     
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  11. Drew617

    Drew617 Tele-Holic Silver Supporter

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    Uh, hang on, let me get my flameproof suit...

    *Ahem*

    No, it's a waste of time in SE for anything but reproductions.

    That doesn't mean it's exactly the same - in theory, preamps can still induce sag. Practically, IMO, doesn't make a bit of difference in something like a Champ, aside from the obvious B+ difference if all other factors remain equal.
     
  12. cometazzi

    cometazzi Tele-Afflicted Silver Supporter

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    Thanks again. Btw, I donned my Nomex underwear right after asking that (just in case). Meanwhile, those 8uf and 16uf capacitors in the 5F2A are pretty small, but isn't that because large filter caps kill rectifier tubes? I suppose I could use larger values there if I have an SS rectifier, but I could also just leave them as-is, too?

    *ducks*
     
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  13. Drew617

    Drew617 Tele-Holic Silver Supporter

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    So I think the RCA sheet specifies no more than 20uf at the first node, but thankfully years of practical evidence is available - Fender used 40-20-20 in later Champs and VCs with no apparent ill effects. I don't think I'd go bigger than ~40uf either way, so to my thinking filter caps really just aren't a factor in Champ design. :)

    My favorite configuration is ~30-20-10. Absolutely fine with a 5Y3.
     
  14. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I'll be the guy who doesn't understand how rectifiers and single ended amps operate and say that if you can't tell a difference in a rectifier swap in a Champ or Tweed Princeton, just grab yourself a used Mustang and save the trouble. But, I really am that guy. ;-)
     
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  15. Drew617

    Drew617 Tele-Holic Silver Supporter

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    My honest to god electrical theory is pretty weak and I know that you know what you’re doing generally, so neither mean to insult or assert too strongly. I’m an engineer by trade, but in a different realm entirely, though I suppose I’m competent with complex systems in general. Otherwise, I’ve been dicking around with homebrew amps for about 18 months and years ago ready the excellent Basic Electricity Navy manual, and that’s it. :D

    I agree that one can usually hear a recto swap, because two recto tubes will generally produce different B+. If that’s what we’re talking about, it’s a difference that could be achieved other ways - not inherently a tube thing.

    Ref dudes in forums: “This GZ34 really made my amp (stiff, juicy, lively.)” Well, yeah, +40 VDC did, anyway.

    I just don’t perceive sag in a healthy SE Champ one way or the other.
     
  16. sudogeek

    sudogeek Tele-Meister

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    On my Champ-like Objects (switches between 5F1 and AA763) I’ve used a Classic Tone 40-18085 which gives nominal 315-0-315 or 275-0-275. Specs here.
     
  17. jsnwhite619

    jsnwhite619 Friend of Leo's

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    And the usual response is "Well, it's the difference voltage and B+ that you hear, not sag; and it also changes the bias level when you swap rectifiers, but it's not sag." Well...yeah. That's the point - it makes a difference. And, I'm sure you could set up switches or go back and solder in new bias resistors every time you wanted a different sound, but it's a lot easier to just swap the rectifier and hear the difference.

    Last few I've built, the 5y3 resulted in a colder bias and nearly zero clean headroom; GZ34 was a hair over 100% and the tightest and highest headroom; wouldn't you know that a 5u4 was in between and closer to the GZ34 sound. From my post in that thread:

    So, after running it through the website with the correct numbers, it turns out the 300R bias resistor places the 5u4 at 98%, GZ34 at 103%, and the 5y3 at 87%.
    The coldest bias had the lowest headroom/output and dirtiest sound. I accidentally installed the GZ34 instead of the 5y3 one time and could not for the life of me figure out why the amp was so clean all of a sudden - and that was only 1/3-1/2 of the way up the dial, not anywhere near maxed out. So, attribute it to whatever reason, and describe the scientific function of how it operates, blame it on the shift in bias and voltage, but saying that a 5y3 is going to make no difference in the amp compared to a SS diode is nowhere near the actual experience I've had with SE amps.
     
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  18. jsnwhite619

    jsnwhite619 Friend of Leo's

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    And I forgot to mention the difference the supply voltage is going to make downstream in the preamp. Change 40v, 50v, 60v in the power section and lose that supply voltage to V1 and see where that goes. That last 5881 SE I built, I ended up lowering the plate resistor to get more voltage on V1 - 12ay7 - and going to 82k there from 100k made more difference than the dozen of other things I'd tried before that. Get used to a SS rectifier B+ voltage and swap to a 5y3, and when you starve that V1 level of 50v or so, you're going to hear it for sure.

    So, whether "sag" is an issue between rectifiers in SE amp operation - the engineers can decide. Does it make a difference in the sound and feel of the amp and affect more than just the B+ voltage supply and sag? I'm voting a firm YES.
     
  19. Drew617

    Drew617 Tele-Holic Silver Supporter

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    Sure, and it's probably just different of perspective, what factor we view as the denominator in the equation. None of what you say is wrong, some of us probably just approach from the other side: Do whatever you want with the voltage, however you want to do it. Hell, a variac would do it. :)

    I might argue for my perspective just because a lot of the usual chatter about rectifier tubes is about "sag." We can quantify and measure that, though it might mean anything to the average forum member I guess.
     
  20. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Knock 50v off of your preamp voltage and the average forum member is probably going to notice. Again, not arguing HOW it works, but just that it doesn't all happen in a vacuum. "Rectifier changes make no difference in SE operation" and "supply voltages make no difference in preamp operation" just don't work in the same statement. Everyone talks about sag and SE operation and that it makes no difference. OK, run with that. Operate whatever SE amp you want at a 125v V1 plate voltage that is a direct effect of the rectifier choice and supply voltage, and you're going to have a completely different amp than one that is running a 180v-200v V1 preamp voltage. If you build a stock Champ and go from 5y3 to SS and do nothing else, to hell with with the sag and power section, the preamp is going to change drastically due to supply voltage. So, again, rectifier choice can make a huge difference in the tone and feel of a SE amp. But not in the way that gets copied and pasted on a daily basis as common knowledge...
     
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