One further clarification, in grid driven configuration: yes, the screen is
grounded for r.f. but so is the cathode (less so if a degeneration resistor is
used), so the drive is still applied between the grid and cathode.
If the peak drive voltage exceeds the bias voltage, grid current will flow.
If not, then grid current will not flow. This happens in grid driven or cathode
driven service. The grid current curves for a TH347 (the tube I have most
tested) , in cathode driven service, seem to be independent of actual screen
voltage and plate voltage.
Interestingly, as the peak drive voltage just reaches the bias voltage the
grid current begins showing reverse current flow in many tubes. Does anyone
know
why this happens?
73, Gerald/K5GW
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