[AMPS] gas test for tubes

[Peter Chadwick]

Back in the mid 1960's, I worked on a tx that had a TWT in it - 10kW at
about 5.6GHz. That had a vac ion pump, and we had trip points of 2, 25 and
100 microamps. In those days, the sensing circuits were interesting - no FET
op amps! Had to build a DC amplifier, with a MOSFET input because of input
bias currents. The vac ion current had to be below 100 microamps to get the
filament on, and below 2 microamps to get the HV on, then it could rise to
25 microamps before the HV went off. Switching the heater on produced a
burst of current that fell back over a minute or so, and powering up the HV
on the tube produced the same thing.(Helix was 22kV at 1Amp, collector was
18kV at 3 amps) As I remember it, the vac ion supply was about 3kV, negative
wrt cathode. Used about a 10k current sensing resistor, and various series
resistors/shunt diodes to protect the MOSFET against shorting the vac ion
supply - very similar to the comments we had here about glitch resistors and
things flying around in potential when an arc occurs.

It would be interesting to know if anyone has cleaned up an amateur size
tube this way.

73

Peter G3RZP



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