>Someone was telling me about a way to test tubes for gas by running them
>in "ion-gauge mode". The control grid is biased positive so that it
>collects electron current from the filament, while the anode/screen is
>biased negative and collects any current due to positive ions. Do you
>know anything about that technique, John or anybody else?
>
>73 from Ian G3SEK
I learned this from the people at Econco (tube rebuilders) for the
4CW250,000B tetrode a few years ago:
Connect a variac to the filament transformer.
Connect a 75 volt (can be variable) power supply, Positive to grid, neg to
cathode DC connection (maybe center tap on filament transformer for TT
direct heated tube). A milliammeter should be between the power supply and
grid.
Connect a 45 volt 1 mA or so, power supply, NEGATIVE to the plate, and pos
to the cathode connection.Install a microammeter in the plate lead. Connect
screen directly to plate. For triodes, this obviously isn't needed.
So both power supplies are backwards to normal operation.
The tube should be disconnected from the normal DC connections in an
amplifier for this. A fan or blower may be required for the filament heat.
(water had to be on for the 4CW250,000B).
Raise the filament heat, or the 75 volt supply, until 25 mA of pos grid I
flows. In smaller tube, this should be lower, as this is pure DC
dissipation on the grid, since it is acting like a plate. For the 7835
super power triode, the number is 50 AMPERES of grid I. Be careful doing
this at home and don't hold me responsible for warped grids!
The 4CW250,000B procedure ran the filament at 3.5 Volts ac, normally a 12
VAC filament. The grid was left with +75 on it.
The Iz is read from the microammeter, should be 1uA or so for a good
vacuum, after a few minutes. Not mA!
73
John
K5PRO
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