Fw: Re: [Collins] KWM2A repair question

[Dr. Gerald N. Johnson, electrical engineer]

The first purpose of the screen is as a shield to reduce the feedback
capacitance from plate to control grid. In the process when its biased
to some voltage below the plate voltage, it acts as an electron
accelerator. The plate current for all plate voltages above the screen
voltage is directional proportional to the screen voltage (holding the
control grid voltage constant). The screen does intercept some of the
electron flow from the cathode at all times, though there have been a
few tubes created with the screen grid wires lined up with the grid
wires to minimize the screen current. When the plate voltage swings
below the screen voltage, the electron current switches to the screen
and heats it rapidly and excessively. That comes from operating unloaded
or too lightly loaded. Many good grid and plate materials will display
some electron emission when glowing red. Normally screens should be
running a lot cooler than that. Over the life of the tube (or due to
poor quality processing at manufacture) cathode material can splatter on
the grids, leading to emission at lower temperatures.

Grid emission, whether control or screen grid, is detrimental to tube
operation. In the case of control grids the current from grid emission
counteracts the negative bias (especially from AGC) because the grid
circuit resistances tend to be large. That's a standard problem in the
S-Line 6BA6 IF stages that are run hard to get enough gain. The main
symptom is the S-meter deflecting backwards without signal. Screen
emission effects, I've not analyzed. Since screen circuit impedances
tend to be much lower resistance than control grid circuits, moderate
screen emission shouldn't present a problem.

I suppose the KWM-2 feedback comes from the screen emission supplying
low level stages for a short time. The diode in the PA screen circuit
would prevent the reversed screen current from the emission supplying
voltage to those stages.

The classic Radiotron Designer's Handbook probably discusses these tube
characteristics in detail. The earlier Radio Physics Course by
Gheribaldi(?) should do even better. Its been a while since I read that
tome.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.