Hello all
This reflector is getting busy ! Difficult to get a word in on
the frequency !
I have a homebrew 4CX1000A amplifier (passive grid,
grounded cathode) and over the past few months the
grid current meter (control grid) has started to read off
the scale in a negative direction, about 1mA ?
The reading slowly appears over 3 mins. as the cathode
heats up.
I think this may be the beginning of the end for this
particular tube ? I'm biasing the tube 'off' on receive
with about -150V DC, and if I disconnect this supply
(with the filaments on but no HV or screen volts) the
current drops to zero i.e the current seems to be flowing
between the control grid and the earthed (hot) cathode.
(on receive I'm also earthing the screen grid)
On TX the bias volts drop to about -55V, but the current
remains approx. the same. I've checked for leaky
bypass caps., etc.
This problem does not seem to affect the operation
of the amp., other than to maybe increase the drive level
needed slightly, which may degrade o/p signal quality ?
Maybe this is a common problem with this series of
tubes, but I've not seen it before. Any ideas please ?
73
Chris GM3WOJ
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