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.
>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.
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.
>>> 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.
>// 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.
--
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|