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Fender Champ 5f1 power transformer question.

Discussion in 'Amp Tech Center' started by GratefulDean, Nov 25, 2020.

  1. GratefulDean

    GratefulDean TDPRI Member

    Dec 28, 2019
    Rocky Mountains
    I am looking at building a Fender Champ 5f1 from MojoTone. They were out of their power transformer, 760EX, when I was putting the kit together so I picked up a Classic Tone 40-18112, which both Stew Mac and Classic Tone said would work in the amp.

    Now that I am at the power transformer install part of the build I see that the values are off on the red leads by quite a bit. 650v @ 70mA vs 330v @ 75mA. Is this a problem? With the Amperage similar I think I will be OK but have no experience in such things and would like to know more before I wire her up, plug her in and electrocute myself.

    So, will this work? If so, why? and what will it mean for my tone?

    Thanks everyone!

    Attached Files:

  2. Jon Snell

    Jon Snell Tele-Holic

    Aug 31, 2015
    Jurassic Coast. UK.
    The first shot stating 650v CT means 325 - 0 - 325 so not a problem. Similar voltages, just a different way of expressing it.
    Pick_n_Strum, King Fan and Nickfl like this.
  3. Pick_n_Strum

    Pick_n_Strum Tele-Meister

    Sep 28, 2016
    Lino Lakes, MN
    I believe the Classic Tone transformer is actually closer to "standard," for the 5F1.

    Also, the current rating should be plenty as well and, again, I believe is standard:

    1.2mA / 12AX7 triode x 2 = 2.4mA.
    45mA / Anode Current 6V6
    5mA / Screen Current 6V6

    Total Current Needed = 52.4mA

    Taken from JJ datasheets with typical operating point of 250v - I assume this to be a bit higher at champ voltages?

    I could be missing something here so someone please chime in and correct my math as I'm still learning. Is there a current draw on the rectifier tube from the B+ winding? I assume not (not to hijack GratefulDean's thread but to clarify).
    Last edited: Nov 25, 2020
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Dec 30, 2019
    Since you are new to this, I suggest following Rob Robinette's diagrams on his 5F1 site. Mojotone just follows the 50's wiring diagrams. Rob's site has updated the wiring to more modern standards.

    The HT Center Tap (red/yellow) can make for a quieter amp if it is connected directly to the negative side of the first filter capacitor. Mojotone does not ground it there.

    I suggest elevating the Heater Center Tap to reduce hum. To do this the green/yellow CT is terminated at the cathode of the 6V6. (Pin8 of the 6V6 or the eyelet on the board.)

    In other words, don't cut those leads short.
  5. JuneauMike

    JuneauMike Poster Extraordinaire

    May 5, 2015
    This is a great video to help you understand and maybe answer your own question.

  6. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Yes - there's internal resistance on the diodes, but it's minimal. The load is the plates, mostly power tube but the preamps dissipate a little.

    Don't freak about 5-10 volts, it's not that critical. Old Fenders were made when wall voltage was lower. Now it's higher they're still working OK.

    On two SF Champs I worked on B+ at first cap sectiin was 40 volts higher than nominal but still within tolerance according to schematic. The plate voltage was 15 volts high.
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