>
>2 wrote:
>>
>
>>>> As Rich says, all the components on the cathode/G1 side have to be
>>>> insulated to withstand a large negative voltage in the event of an arc
>>>> or other current surge - including the anode and G1 meters and all the
>>>> bypass caps. A varistor between cathode and screen (chassis) will limit
>>>> the negative voltage surge, but it has to be rated to handle the full
>>>> surge current... which is why you still need a limiting resistor in the
>>>> B+, to limit the maximum current that the Varistor has to handle.
>>>>
>>// I tried the metal-oxide varistor approach. Even though the varistor
>>was substantial, it would destruct during a glitch,
>
>That's how you find out that the Varistor wasn't substantial enough.
// Indeed
>
>>The slam-dunk fix
>>was a spark gap set to 500v more than the screen V (with a high-pot
>>tester).
>>
>The choice between a Varistor and a spark-gap protector isn't always so
>clear-cut. The Varistor can start to conduct and provide protection only
>a little above the normal operating voltage, but it might be destroyed
>in a big hit. On the other hand a spark gap is much more robust and has
>a low-voltage clamping action when triggered, but it triggers less
>reliably.
// The anode supply was c. 9kV. The screen supply was 1300V. The spark
gap fired at c. 1800v. It worked ok. The cathode bypass caps were
rated at 3kV.
>
>500V above the normal screen voltage is leaving a *lot* of headroom
>before any protection kicks in. Also an open-air spark gap may not
>create a reliable low-voltage arc which effectively clamps the screen to
>the cathode, and holds it there. A sealed spark-gap protector does that,
>but it requires a specific gas (argon? xenon? mixture?) at a specific
>pressure - and they wouldn't do that if they didn't have to. Also the
>breakdown voltage is more reliable than an open-air gap, so you don't
>have to leave as much headroom above the normal operating voltage.
>
>Another alternative is a thyristor crowbar, like they use in low-voltage
>power supplies but with a higher voltage zener chain. When fired by
>excessive voltage across the zeners, this will very firmly nail the
>screen to the cathode.
>
// The filter C stored >1kJ.
>>>> Another problem with grounded-screen is that all your RF bypass
>>>> capacitors are grounded to the chassis, not the cathode, and when you
>>>> switch to TX there is a surge of current through the screen connection
>>>> to charge these bypass caps.
>>
>>// Normally, RX cutoff is achieved in a grounded-screen tetrode
>>amplifier by increasing negative grid V, not by switching screen
>>potential off and on. Thus, screen potential is unchanged between RX and
>>TX.
>
>That is probably a better way.
>
// For VOX - surely. Diagram at Figure 4 on my Web site.
>>// The above scenario was for a g-g triode, not a tetrode. It is my
>>opinion that the heater should always be connected to the cathode and the
>>heater should float -- unless one has a freebee supply of new tubes.
>>
>You're probably right, for both triodes and tetrodes, because in both
>cases an arc condition can pull the cathode potential a long way from
>the (grounded) filament potential.
>
cheers, Ian
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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