Tom Rauch wrote:
>
>You need enough capacitance near the screen to absorb the initial spike.
>Neither a MOV or gas tube will work well by itself. Even a modest
>overvoltage is too much for regulators. Slow the transient down.
>
>Actually, the MOV is a last resort just to protect the socket
>capacitor...as would be a gas tube.
The MOV or gas tube at the screen terminal is actually the *first* line
of defense because that is where the arc hits. Tom correctly identifies
that the screen bypass capacitor in the socket is at least as valuable
and important as the tube. The tube may survive, but if you blow that
capacitor, you need a whole new socket.
However, slowing the transient down - stretching and lowering it -
requires some kind of series impedance. This is where it gets tricky,
because in most designs the series impedance will tend to spoil the
voltage regulation in normal operation.
>Neither will do much for protecting
>a screen supply...since most series regulators won't like voltage
>applied to the output and shunt regulators will hard clamp and protect
>the MOV and gas tube.
>
I agree about the series regulators (though they can be protected by
reverse diodes that dump the transient back into the output capacitor of
the screen supply). Passive shunt regulators will tend to hard clamp,
and pass destructive amounts of current instead, but active regulators
offer the opportunity to design around that problem.
The circuit I published in QEX (downloadable from my web site) has a
series impedance that is *inside* the normal feedback loop. Thus it has
no effect on normal voltage regulation. However, when a voltage
transient appears on the output, the series impedance helps to protect
the vulnerable semiconductors in the regulator. Equally important, the
series impedance contains a current sensor which disconnects the supply
and grounds the screen in a few milliseconds, and also turns off the
mains to the HV transformer. By a combination of clamping at the tube
socket, lowering and stretching the transient, and quickly grounding the
screen, it seems to provide as much protection as is necessary.
>> Returning to strings of zeners, one useful feature is a large
>> electrolytic capacitor (say 50uF) in parallel with the zeners. This
>> will help to absorb the rising edge of any voltage surge. In the last
>> amp I had that used zeners, I had several arcs from B+ but never lost
>> any more zeners after fitting the capacitor.
>
>That's a good idea with almost any regulator. I can't imagine NOT
>using one!
One word of caution. Don't connect the electrolytic directly in parallel
with the screen bypass capacitor. The self-inductance of the
electrolytic will parallel-resonate with the bypass capacitance,
somewhere in the HF region - and at that frequency you will have *no*
screen bypass at all! The solution is to separate the electrolytic from
the screen cap by a 100 ohm low-inductance resistor with an RF choke
wound over it (20-100 turns over a large-bodied resistor, not critical).
The resistor kills the resonance but the RFC provides DC continuity.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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