Ray di Tutto" wrote:

One point you mentioned is very interesting. You wrote that when a tube
gets gassy, plate current goes up to the point the plate can turn red hot.
I'm no specialist and i would have thought that the gas molecules in the
tube would be an obstacle for the electrons so plate current would
decrease.

Vy 73 de HB9SLV

Sorry to be so late getting back to you on this. I had
forgotten I left a thread open over here on .boatanchors...

What really happens is the gas molecules (atoms) become
ionized due to the high plate voltage accelerating the plate-
current electrons to energies above their ionization
potential (typically 10 - 15 electron volts). When
this happens the now-ionized gas becomes a very good
conductor (almost a dead short) and the control grid
is totally ineffective in limiting the cathode-to-plate
current flow. The only thing that can halt the process
is the removal of the plate voltage. Unfortunately,
there is usually enough energy left in the conduction
electrons striking the plate to increase its power
dissipation disastrously.

This process is actually used to good effect in mercury-
vapor rectifiers and thyratrons because that "dead-short"
effect means the losses through the conducting tube are
low. Both of these devices "fire" (ionize) at some
forward voltage and then stay in conduction until the
AC driving them reverses polarity.

Jim Bromley, K7JEB
Glendale, AZ