Achievement Unlocked: 75 Watt Class A Zero Feedback SIT Circlotron

Soft clipping at 75W

Because of the high input capacitance of the 2SK180’s and the gain of this circuit, we have to pull out all the stops to get our 75W beast to make audio bandwidth. Here you can see that each SIT is getting its own transformer and buffer.




8 thoughts on “V-FETRON 75 UPDATE #1

  1. admin Post author

    They don’t have to be tightly matched, but better matching will result is better distortion cancellation.

  2. Ravi Balasubramanian

    Do the 2SK180 have to be matched or tested to have a good triode characteristics on a tester?

  3. David

    Like Vincent, I’m interested in finding a circuit for my 2SJ28 VETs – 6 amps in total for a stereo 3-way. Any comments really welcome!

  4. admin Post author

    Each SIT in the smaller amp dissipates 33W (22V @ 1.5A) or 66W per channel. That sink looks a little small, one per SIT might work.

  5. Vincent

    I would really like to see and hear how my 2SJ28 VETS would work in this circuit. Do you have any to play with? If not, I could send you a couple. 🙂

    Would a Wakefield 435AAAA be sufficient to dissipate the heat for a pair of SJ28s?

  6. admin Post author

    Indeed, your math is correct. This is a hot one. It will operate in AB to an extent, so we could idle it a bit lower, but that’s no fun 🙂

  7. Tim

    I try to do my math here (as often it may be wrong), to get 75W on 8 ohm we need a minimum total bias of 4.33A, so with a supply voltage of 41V, is the heat dissipation per channel around 180W?

    This is likely to be a winter amp 🙂

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