Comments on Profile Post by leland331

  1. dcgillespie
    dcgillespie
    That depends on how you tread the screens, and the load that the tubes operate into. What is the P-P impedance of the ST-40 OPTs, and what plate and screen voltages are you intending to use?
    Dave
    Nov 1, 2015
  2. leland331
    leland331
    I believe the opts are 8250 ohms and I will run the plates around 400v. The screens and bias, I will run wherever they are optimal according to the plate voltage
    Nov 2, 2015
  3. dcgillespie
    dcgillespie
    With that high of a load impedance, cathode bias would most likely produce better results than any form of fixed bias would. The screens should operate at ~ 365 vdc. Expect about 17-18 watts from that configuration, with very low distortion.

    Dave
    Nov 2, 2015
  4. leland331
    leland331
    That sounds good. I see I still have a little bit to learn. Should I regulate the screens? Or only if I have the entire supply regulated?

    Thanks
    Nov 2, 2015
  5. dcgillespie
    dcgillespie
    The change in current draw from 0 to full power doesn't change as much with cathode vrs fixed bias, but it does still change.

    Use cathode bias, but use the EFB screen grid regulator to power the screens. That will keep the screen grids locked into place relative to the plate supply regardless of the current they draw.

    You'll find examples of the EFB screen grid regulator in my Fisher 400 and X-101C threads.

    Dave
    Nov 2, 2015
  6. leland331
    leland331
    I reviewed your schematics and it seems to make overall sense to me. Couple quick questions though.
    I'll use your sa-100 as an example. the screen regulator portion, the source of FQP9n50c is attatched to the bias supply which i dont have. Do i omit that part of the connection?
    Nov 3, 2015
  7. dcgillespie
    dcgillespie
    Correct. Then you can tailor the resistor supplying the Gate to produce the screen voltage you want.

    Dave
    Nov 3, 2015