Steve N2IC London wrote:
As I have previously mentioned, I have two Alpha 3CX800A7LG's that
appear to have grid-cathode shorts that develop within a few minutes of
applying filament voltage. After seeing VE5RA's posting about shorts
clearing themselves after 6 minutes, I did some experiments with the two
tubes.
/* snip */
After a 20 minute warmup, I have these voltages:
+ filament to grid 9.5
- filament to grid 0.5
+ filament to cathode 8.9
- filament to cathode -2.9
The grid ring is extremely hot.
What's going on in the tube ?
I know I answered this before, but I can't locate it in my reply's to
amps. (must-a gone to another reflector ???)
Modern transmitting tubes, mostly of the coaxial style have a
cylindrical heater (cage or mesh construction) or cathode (pipe)
surrounded by a cage type control grid which is surrounded by a
cylindrical cage type tetrode screen grid or by a (triode) cylindrical
anode. (plate)
In this construction the electrons travel to the plate in "vertical
sheets" in line with the "slots" in the control grid, the "slots" in the
screen grid (if any) are aligned with the electron sheets.
Keeping the electrons in sheets has several benefits.
Electrons moving in a vacuum are repulsed by each other, only a strong
magnetic or electrostatic field can keep them moving in a organized
manor instead of re-ordering into a cloud.
With extended use (15,000 to 30,000 hours of normal use) these control
grid wires will slump (inwardly and outwardly at their bottoms) until
eventually some touch the cathode or filament creating that problem.
If oriented horizontally they would sage in the center in a very
short time.
If these tubes are operated in excess of their specification for
control grid dissipation the overheated control grid will sag
prematurely causing this problem at an early age.
--
Ron KA4INM - Understanding is much better than
knowing how.
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