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Was: Grid R/C VTTC Now: DC Staccato
Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-uswest-dot-net>" <sccr4us-at-erols-dot-com>
David, Carl, All
A HV bipolar transistor (horizontal fly back), IGBT, or FET will be needed
to turn off cathode current with DC powering. With no '0' crossing if you get
a tube "burp" your cathode switch may be permanently on... :^C
A rotary cathode switch / modulator like John Freau (Hi John) was working with
might be a better choice with DC. Remember with DC you have a lot of
stored energy, and if the tube burps, you may take the tube and switch with the
The 555 circuit should drive a horizontal fly back transistor fine, I would
two in a darlington configuration to increase gain, and add a reverse bias
diode between base and emitter of driver transistor. Even with a beta of 5 on
each transistor, would result in darlington gain of ~25. If you limit 555
100mA (bipolar device only please), fly back will handle better then 2A cathode
current. That's a serious DC VTTC folks (at least 3-5kW input).
Dave Sharpe, TCBOR
Chesterfield, VA. USA
> I think there may be some problems with using the 556 timer SCR cathode
> lifter with DC mode, as there is no 0 crossing, maybe some other problems?
> Sputter mode with DC to the plates was pretty stable, but did show some
> tendency to drift (as seen on the scope). I really did not spend that much
> time on this. It was just something I had thought about some time ago, and
> with the DC setup for the Audio mod work, I thought I would give it a go.