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Princeton reverb build

Discussion in 'Shock Brother's DIY Amps' started by Meteorman, Oct 2, 2020.

  1. Meteorman

    Meteorman Tele-Afflicted

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    Started a Princeton Reverb build, mostly following Hoffman's layout.
    My earlier Supro 6424 didn't make me sound like Jimmy Page, and frankly was harsh, so was hoping to re-purpose some of it to offset costs for the PR.
    The ClassicTone 40-18062 PT in the Supro puts out 610v (the stock PR calls for 650v); am hoping this is "close enough" for the PR. ???
    Chassis is a 1773 Hammond enclosure that i punched and drilled myself.
    Sourced passive parts from Mouser.
    Only surprise there was the 22uF/50v electrolytics - was thinking they'd be the normal Vishay aluminum-cased things; instead I got some molded with "flame proof" exterior... ?
    Maybe they knew something about my amp building skills.... o_O
    Anyone have any experience with them ?
    Anyway, posting build photos here.
    This is my 6th build, I'm slowly gaining some insight into tube amp operation, but I still have a long way to go.
    PLEASE feel free to point out any fatal flaws or smoking guns - I can still back up if required.
    thanks !!
    /mike

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    Due to space issues, I made my own cap board and mounted it vertically inside. these are all 22uf caps with the first stage x2 in parallel. the turrets face the board, of course, with a single central ground turret that will go to a transformer stud. anyone see an issue here ?
    the lamp is 110v job outta the Supro in case anyone was curious why it's wired direct to main hot.

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    I also added Robrob's switched Normal/Light/Heavy NFB mod.

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    I'm populating the board (garolite) now; you can see those gray molded cathode bypass caps.

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  2. theprofessor

    theprofessor Poster Extraordinaire

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    I don't think I've built any more amps than you have. I'm certainly no expert. But it's a conversation, not a competition, so...

    With your standup Supro-style Classictone transformer, I think the overall voltage reduction might be nice, but my comments/questions are these:

    (1) 630V on a common (modern) Princeton PT like a Classictone 18027 is 20V off of this Classictone 18062 you have, which is giving you 610V. I'd be interested to see others spec out what voltages that might produce at the end of the build. I suspect it'd be fine. I used a Hammond 290AX on a recent Princeton Reverb build, which is 650V instead of the Classictone's 18027's 630V -- so that's 20V in the opposite direction. And it worked out just fine.
    (2) What current does that 18062 PT provide? I don't see that listed on the spec sheet. The 18027 offers 100mA, as does the Hammond 290AX. Those are over-spec'd in relation to vintage PT's for Princeton Reverbs, but I think it's a welcome margin.
    (3) What difference would it make to the heaters that they are supplied with 1.75A on the Classictone 18062, rather than 2.25A, as on the Classictone 18027? I genuinely don't know, but it seems like something that one might consider.
    (4) I myself would like 3A to the rectifier so that I could use something like a 5U4 to bring the voltages down a bit, if necessary, rather than being "stuck" with a GZ34 with the 2A on the Classictone 18062. It may be that if this Supro-style PT works, you won't need to work about dropping voltages, since it is already coming it lower than the 18027.

    Also: the wiring on your board sure looks nice! Kudos!
     

    Attached Files:

    Last edited: Oct 2, 2020
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  3. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Boy, those look odd. Why'd you go with 50 VDC? I use TE series, they're rated for 105°C. I also use the good ol' TVAs when space is tight.

    You build neatness looks great.
     
  4. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Will the magnetic flux from the stand-up PT affect the OT in any way? Normally the PT is laying down. Would it pay to angle the OT in some way?
     
  5. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    You need, I think, 2.1A for heaters. So, right there, the transformer is under spec.
     
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  6. Meteorman

    Meteorman Tele-Afflicted

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    well dang. I was worried about the currents out of the PT. I also couldn't see the current on the 610V, and I saw the heater and rectifier supplies were lower, but don't know if "under spec" is a fatal flaw or if enough wiggle room exists in the configuration..
    I does appear two 6V6s and four 12A#7s add up to 2.1 for heaters.
    I have read there is some allowance for running tubes a little under spec, i guess the question is what is "a little".

    I can make a lay-down PT fit if I have to.

    Hoffman spec'd the 22uF/50V bypass caps in his BOM.
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I would assume the PT HT secondary mA rating for the Supro 6424 would be adequate for the Princeton Reverb. The 12W 6973 power tubes look to be fairly equivalent to the 12-14W 6V6 and the Supro 6424 is cathode biased which would have much more constant current draw than the PR.
    The 6.3 heater mA rating is 20% shy. Can it take it? Is this a candidate for a small filament transformer? Should this PT be used for a smaller amp?
    If you decide to *make it take it* try to monitor the temp of the PT so as not to burn that winding.
     
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  8. D'tar

    D'tar Friend of Leo's

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    What's the draw on 6.3vac pilot light? You can make up a couple mA by using a 120/240v pilot. If your 6.3 supply sags more yhan you would like then youre asking too much of it.
     
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  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    He mentions in post #1 the lamp is powered from 110v.
     
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  10. Meteorman

    Meteorman Tele-Afflicted

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    think I'll try a rectifier (5Y3) that needs 2A and see what resulting voltages are. I read the 5Y3 drops about 50V, and the 5U4 drops about 40, and with a bias pot to tweak, maybe I can get away with it...
     
  11. Nickfl

    Nickfl Friend of Leo's

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    What voltage are you aiming for? I can't imagine you're going to need to drop it down with a 5y3 when your HT voltage is already so much lower than spec. I'd guess you'd get about 400v B+ with a 5ar4. I used a 600v transformer on a princeton reverb based amp and got 375v with a 5v4 rectifier.
     
  12. Meteorman

    Meteorman Tele-Afflicted

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    Thank you Nickfl.
    I think 375v is a nice target.
    I will try a few different 2A rectifiers.
    I really need to learn how the properties of each impact B+.
     
  13. Nickfl

    Nickfl Friend of Leo's

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    Heres a rough guide: https://300guitars.com/articles/rectifier-tube-voltage-drop-chart/

    That's a rough guide relative to a known starting point, so if you know the voltage you got from that power transformer in your first build you can kind of go from there. With a power transformer you haven't used before that you just know the unloaded voltage for you also have to factor in sag under load, which can be calculated but it's a huge pain in the ass so most people usually just make an educated guess.
     
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  14. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    I've got this Halldorson here, if you want it, that I pulled out of a '67 Princeton Reverb that someone hacked in there, probably back in the day when it was hard to get a proper replacement. I'll probably never use it. It's roughly 3.75" x 3.75" x 3.125". Specs are 300-0-300VAC at 120mA, 5VAC at 3A, and 6.3VAC at 5A. I'm guessing with a GZ34 you'd be right around 400VDC on the plates, at least with the 8k OT.

    PXL_20201003_171939699.jpg
     
  15. Meteorman

    Meteorman Tele-Afflicted

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    Thats darn gracious of you Phrygian. I’ll PM ya
     
  16. schmee

    schmee Doctor of Teleocity

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    It should be fine. I've built a few Princeton variations with trannys not turned 180, trannys close together etc, never seemed to matter.
    BTW, you may love the lower voltage actually.
     
  17. Meteorman

    Meteorman Tele-Afflicted

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    Board 98% populated and installed... still waiting on delivery of a few .022 caps and a 5V4
    Backside leads hooked up.
    Starting the cabinet soon. 3/4 curly maple.

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  18. Jeru

    Jeru Tele-Holic

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    Mike,

    VERY nice work. I have (I think) 5 builds done, most of them conversions of some kind.
    My one scratch build has nowhere near the tidiness and order that your build does.

    I'm gearing up for a BF-era Deluxe build this winter, and I am inspired by your attention
    to detail. I'll let others chime in re: any circuit-related tips and such, -- I just wanted
    to shout out, cheer you on and thank you for the detailed post/pictures -- exceedingly
    helpful to us wannabees.

    JR
     
  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Interesting filter cap board attachment.

    I like to see the brave techs show radial caps in the axial world of TDPRI.

    Carry on.
     
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  20. mjcyates

    mjcyates Tele-Meister

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    Nice looking work!
     
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