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Registered Member #478
Joined: Tue Jun 20 2006, 11:57PM
Location: Shrivenham, United Kingdom
Posts: 8
Hi, I was just wondering how the the staccato circuits operate, with regard to tube cathode switching. First I imagine the switching device has to be able to handle the the current through the tube, i.e. the plate current, and so has to be rated as such.
In the case of a thyristor, I assume the current from the supply is phase controlled. That is to say that if the supply is AC, the thyristor will half-wave rectify this and with the correct triggering offers phase control. So the supply is AC, but the thyristor results in a pulsed unidirectional plate current.
What about the RF, does this effect the thyristor, or perhaps it's high effective anode-to-cathode capacity just allows free passage to RF current?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Hi Gavin
Tubes only conduct in one direction too, so the thyristor sees a unidirectional current, both from the point of view of the line current and the RF. Thyristors turn off quite slowly, so it stays on in between half-cycles of RF, and only turns off at the line voltage zero crossing.
The staccato timer introduced by John Freau isn't a phase controller: it lets through entire half-cycles at a time. It might pass about one half-cycle out of every 100 to produce a spark once per second, say.
The thyristor has to be rated to pass the average cathode current (an amp or so in a MOT driven coil running flat out with staccato disabled) and hold off a voltage equal to the cutoff voltage of the tube, that is to say the amount of bias between grid and cathode needed to make the cathode current (almost) zero.
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Gavin Dingley wrote ...
Hi, I was just wondering how the the staccato circuits operate, with regard to tube cathode switching. First I imagine the switching device has to be able to handle the the current through the tube, i.e. the plate current, and so has to be rated as such.
In the case of a thyristor, I assume the current from the supply is phase controlled. That is to say that if the supply is AC, the thyristor will half-wave rectify this and with the correct triggering offers phase control. So the supply is AC, but the thyristor results in a pulsed unidirectional plate current.
What about the RF, does this effect the thyristor, or perhaps it's high effective anode-to-cathode capacity just allows free passage to RF current?
Thanks in advance,
Gavin
Gavin,
I have a VTTC stacatto board / circuit i designed with some of the theory / waveforms on my website here:
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staccato operation
