EF80 µChamp OT swap question

Discussion in 'Shock Brother's DIY Amps' started by Huddy, May 16, 2021.

  1. Huddy

    Huddy Tele-Holic

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    So I think most of Robs single ended micro designs with the EF80 call for a the Hammond 125ASE using the 10 kΩ Primary or something similar. I've got a handful of the Hammond 1750A 25k/8Ω reverb transformer.

    How awful would it be if I used a 4Ω speaker to reflect a 12.5 kΩ primary. Doable? Would love to complete one of these for $80-90+/-. I'm not quite up to snuff enough for drawing/reading load lines.
     
  2. 2L man

    2L man Tele-Meister

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    Edit: In my notes 125ASE is a single ended and 1750A is a push pull OPT!

    Bigger impedance changes loadline more horizontal and there can come two issues for pentodes but I can't say are they critical for low power pentode? I don't know EF80 at all.

    If bias point current is kept the same the anode voltage sweep on right end of loadline can get too high and tube might spark. Also when overdriven hard and anode current can't rise anymore on left end of loadline where this point meets g1=0V drive point "below the knee" the screen current tends to rise.

    Bias point is easy to lower using bigger value cathode resistor and screen current can be limited using bigger screen resistor but they effect the sound. But again I can't say does it effect too much.

    I think you can try!
     
    Last edited: May 17, 2021
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  3. Huddy

    Huddy Tele-Holic

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    Thanks for the input. I think i'm going a different route though. I had a long drive this morning and go to brainstorming. I've got a MOD102 EL84 SE amp kit that I built a couple of years ago. Already has the 269EX 190-0-190 PT and it's got a 5k Primary / 8 Ω 5w transformer. So I think I'm just going to convert that to a µPrinceton with a master volume and pair it to a 16 Ω speaker to reflect the 10 kΩ.

    Then I was thinking the 1750A was 25 kΩ primary but it's only 22800 so that's a smidge closer to the 10kΩ primary than before. Maybe I'll try both. ;-) Wish me luck on getting started. That's the hardest part.
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Ime 10K to 12k5 primary impedance is not a deal breaker. Both will work.
     
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