We use switching or linear regulated supplies for screen power,
charging a large capacitor. A crowbar is nice, but small series R is
often adequate. The foil hole test should be done. By the way, our
main HV power supplies, which have about 100 kilojoules of stored
energy (30 kV, 25 Amp DC capacity), will not break AWG30 magnet wire
when shorted across the output terminals. Crowbar and mains shutoff
occur VERY fast. Crowbar is in a few microseconds, mains within 1/2
cycle, or 8 milliseconds max. We test these crowbars often, by
dropping a Ross HV relay across the output remotely, with the test
wire. The output resistor, a few Ohms, takes about a 3500 Ampere
spike during a crowbar. If the wire blows, an interlock opens to
prevent further operation until the crowbar is repaired. We use
Pearson Current transformers, and sometimes low value shunts to
measure or view the current in the power supply. Pulsed plate current
makes this easy with a curretn transformer.
AG6K wrote:
>
>Shunt regulators are not suitable for tubes that use over 800v on the
>screen.?
>
>
>
>Tetrodes with handles usually do not have negative screen current.
>Negative screen current does not cause overdissipation. The problem is
>voltage runaway.
I wish this were always true, but alas, even tubes such as the
4CW100,000 can operate with - screen current at times. Depends on the
load line and other factors. The 4616 tetrode (RCA) is a 150 kW peak
output tube, which is guaranteed to do it. Usually a big 1 or 2
Kilohm shunt resistor works to stabilize the regulator, but it has to
waste a lot of power.
With pyrolytic graphite grids, newer tetrodes do not have this
problem. Again, I am speaking for big tubes with handles, don't
expect to find them in the < 10 Kw department. FYI.
John
K5PRO
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