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

Output transformer question

Discussion in 'Glowing Bottle Tube Amp Forum' started by radiocaster, Nov 5, 2017.

  1. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    I was looking at Champ transformers, most had a 7K ohms primary, but I also saw one which was supposed to be vintage correct and had a 17K primary instead. Why such a big difference?

    Also, is there a way to calculate what primary an amp would need? I am aware of this and that trannie recommended for whatever set of tubes in the output, but is there some kind of mathematical formula?
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Ballpark formula is expected plate voltage divided by expected idle current.

    Plate voltage can be estimated from:

    HT winding voltage (i.e. a 330-0-330 would be 330) times either 1.1 (tube rectified) or 1.4 (solid state).

    From there, you can use your tube data or bias calculator equations to estimate idle current. 70% for fixed, 90 for cathode bias, 100 for single end class A.
     

  3. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    Well yeah, if you do a simple R=E/I calculation using the typical plate voltage and amperage from the manufacturers' charts, you get the most common output transformer primary resistances.

    But here's what bugs me, besides that vintage Champ transformer:

    If I calculate the value for a 12AU7 as output tube using the typical characteristics, I get about 25K ohms, which is the usual value for trannies for this type of amp.

    But if I use the values for a 12AX7, I get over 200K ohms, and the only amp I know that uses a 12AX7 (half of one in single-ended mode) is the Blackheart Killer Ant, but it uses only a 50K primary.
     

  4. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    Also, if say you have a single-ended amp with one tube that requires an 8K primary. If you make it parallel single-ended with the same type of output tube, would it then need a 4K primary?
     

  5. danlad

    danlad Tele-Meister

    177
    Mar 24, 2015
    Here and there
    It would. My gloriously feisty hot rod champ/ax84 P1ex will go either single 6L6/EL34 or 2x6V6 without change thanks to that.
     

  6. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    O.k., but can anyone explain those two stock transformers with weird primaries that I have mentioned?
     

  7. Wyatt

    Wyatt Tele-Holic

    782
    Nov 3, 2004
    Primary impedance is not an absolute. Tube data sheets give all sorts of examples depending on plate voltage and desired output and THD. Designers after start with an end point in mind and design backwards; since it's audio, they aren't necessarily maxing out sheer performance. And sometimes its a matter of what parts they can source easily.

    But I don't see how you came up with 200K for one triode of a 12AX7. Calculating with the maximum plate voltage, I only got 110K.

    Assuming 250VDC, SE Class A, 1/2 of a 12AX7 (1.1W max diss.)...
    • Using R=E/I, I get ~82K
    • Using R=E^2/P, I get ~56K
    Both these formulas are only for ballparking only, they ignore a lot of other variables. I can't find a schematic or any OT specs on a Killer Ant anyway.
     
    Last edited: Nov 14, 2017

  8. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    Impedance varies with frequency, too.

    Tweed Champ (5F1) is 17k

    I'm guessing the 17k in 1958 is just what Leo was able to get in quantity at reasonable price. I would think the 5F1 OT would be stable with 6V6 at 2 ohms or 4ohms, but would hesitate to run 8 ohms (34k reflected).
     

  9. radiocaster

    radiocaster Friend of Leo's

    Aug 18, 2015
    europe
    They're both at 4 ohms, there's one that also has a 5K ohm primary for 8 ohm. Look at the list below the transformer description:
    http://www.classictone.net/40-18110.html
     

  10. danlad

    danlad Tele-Meister

    177
    Mar 24, 2015
    Here and there
    17k into 4 ohms sounds almost like a reverb transformer! Match it to an 8 ohm tank giving 34k, sounds perfect for that.
     

  11. T Prior

    T Prior Poster Extraordinaire

    Mar 17, 2003
    Charlotte NC
    Correct, as stated above, don't overlook cost, and bulk purchase by Fender. The two OT's although different, at the end of the day either will work just fine and the user will notice no differences in output. 3 watts, 4 watts, 5 watts, it's all the same within relativity. But the bulk costs may not be the same.

    I recently acquired an early Blues Deluxe, 1996 I believe, it's marketed as 40 watts. The schematic clearly states 38 ! I'm missing 2 watts. I'm gonna sue.
     
    Last edited: Nov 17, 2017 at 5:27 AM

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