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5f2A Power Transformer....

Discussion in 'Shock Brother's DIY Amps' started by MPCNYC, Dec 3, 2018.

  1. MPCNYC

    MPCNYC TDPRI Member

    39
    Sep 4, 2013
    Brooklyn, NY
    Just checking on what you guys think is the best choice currently for a 5f2a power transformer in order to get vintage correct voltages? Planning on using a nos 5Y3.

    Thanks in advance.
     
  2. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Last edited: Dec 4, 2018
  3. MPCNYC

    MPCNYC TDPRI Member

    39
    Sep 4, 2013
    Brooklyn, NY
    Cool link..interesting. I wasn’t particularly calculating things. I was just noticing that on various 5f2a build threads that there didn’t seem to be a consensus about what power transformer
    would give you a B+ that was in the proper range to have a 6V6 running at its optimum without modifying the stock circuit. But I might have missed something.
     
  4. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    I looked at a schematic for the 5f2a and its showing ... derp, I read that wrong. Thats voltage rating for filter caps.

    My first amp I just took the PT mojotone recommended, and my voltages were way high. You should study where you want to end up at for B+1 and work backwards from there, calculating the voltage drop acorss a 5Y3 ( 30 volts, IIRC).
    You know you need 6.3v on the heaters, right.
    As for current, I can't help there. 6v6 needs 35ma, not sure about 12ax7, but 45ma would probably suffice.

    So maybe the question to ask is what were the vintage voltages for this. Or pay attention to what voltages they are getting with specific PTs and 5y3.

    Edit: Ok, from Hoffman, I'm seeing 320v B+1 for 5f2 (but thats with slightly lower wall voltages from the 50s.) In any case you are aiming for +/- 20%.
     

    Attached Files:

    Last edited: Dec 5, 2018
  5. MPCNYC

    MPCNYC TDPRI Member

    39
    Sep 4, 2013
    Brooklyn, NY
    Thanks again....I’ll do some research. Mercury Magnetics sells a lower B+ version for that amp, 320v, albeit it’s quite expensive. The “correct”/suggested transformers from various kits is invariably 360v and seems to yield higher than desired voltages in almost all threads I’ve checked....
     
    JuneauMike likes this.
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    I used the ClassicTone 40-18019 and it worked perfectly. 325-0-325 @ 70mA, plenty of heater current, and as a nice bonus a center tap on the 6.3V heaters.

    The 5f2a is 'self-biasing' so within reason B+ is less an issue. But what range do we want? For 5f2a voltage targets it's ideal to use the later schematic for the fantasmic 6G10 -- though called a Harvard, it was really just the last model of tweed Princeton. So B+ 360V is fine, you don't need to get to the earlier 5f2's 320, and IMHO MM iron would be expensive overkill.

    Harvard schematic with voltages.jpg
     
    JuneauMike likes this.
  7. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    @King Fan, how much current does the circuit require? My back of the envelope was 35ma for the 6V6 and 2 each for both sides of the 12ax7. Is that about right? (although more is better, there's a minimum you need to hit).
     
  8. King Fan

    King Fan Friend of Leo's Ad Free Member

    Jan 1, 2013
    Salt Lake City
    You're better at that than I am, Mike. My one-synapse brain says it's about like a 5f1, and IIRC Fender just used a 5F1 PT, which actually had enough mA (nominally) to drive a PR, so I never thought about minimum current rating.
     
    Last edited: Dec 5, 2018
  9. jimgchord

    jimgchord Tele-Holic

    Age:
    45
    580
    Apr 3, 2018
    victor,ny
    Classictone
     
    Last edited: Dec 5, 2018
  10. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    @MPCNYC, I should have added that this website is a very handy estimating tool but in my limited experience with it it always estimated my B+ voltage as higher than what I actually got.

    So you might apply your own "Kentucky windage" to the results and subtract 25 or so volts to whatever answer it gives you for B+1.
     
  11. MPCNYC

    MPCNYC TDPRI Member

    39
    Sep 4, 2013
    Brooklyn, NY
    Thanks everyone for the replies....that Classictone sounds Like it’ll be what I want. Just got to look at the wiring up of it! So you guys may hear from me again....lol
     
  12. mgwhit

    mgwhit Tele-Holic

    866
    Aug 1, 2008
    Louisville, KY
    I just stumbled across the original voltage chart I made for my Weber 5F2A. I was getting 351V at the A node filter cap and 341V on the plate of the 6V6 with the W022772 PT and the WY3GT copper cap rectifier. Based on an early post of mine I think I was getting 123V at the wall and used the 125V primary. Good luck!
     
  13. MPCNYC

    MPCNYC TDPRI Member

    39
    Sep 4, 2013
    Brooklyn, NY
    Thanks for the info!
     
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