Most HV faults are caused by gas in the tube. It is doubtful
parasitics are behind any of them.
If the tube and other components have enough HV breakdown to
operate at the desired frequency, they almost certainly have
enough for any unwanted oscillation.
> 4 ms is like infinity for a power arc. This is shorter than one AC
> cycle, so if you can trip off the mains power this quickly you are
> doing good. But for the stored energy being released from the
> capacitors, it is a long time.
The best circuit is an electronic dumping device, or crowbar, that
clamps the supply rail to the supply return in the event of a fault,
and you let the current limiting resistor take the impact.
At the same time, you open the AC mains.
My homebrew amplifier has just such a system, except I omitted
the dump system. I have a 25 ohm HV 400 watt fault resistor (two
six-inch long 200 watt 50-ohm carbon resistors in parallel) and a
current sensing device that disables power mains triacs if triggered.
If I do something silly like hit the amp full drive while the antenna is
off, generally the grid current fault shuts off the antenna relays. If it
misses and the tube anode arcs over to the chassis or if the tube
outgasses and arcs internally (see no fictitious "parasitics"
required), the HV fault will turn off the mains while the resistor
limits discharge current. All you hear is a little "tic" and the amp
shuts off.
Recycle the switch, and it is as good as new.
Of course I have the negative rail properly clamped with a large
diode right from the capacitor to chassis.
73, Tom W8JI
W8JI@contesting.com
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