[Amps] Bleeder resistor across screen grid terminal
jtml at losalamos.com
Thu Nov 28 20:08:19 EST 2013
Weeks ago here there was some discussion on the merit of having a shunt
resistor to ground from the screen grid of a 4CX1000A. The reason that
was most posted was to prevent disconnection of the screen grid from the
source, resulting in momentary elevation of the screen potential.
Three years ago when I first began firing up an expensive new tetrode
from France (Thales Electron Tubes) I was instructed by the manufacturer
to be sure to have a shunt R across the screen grid. I seem to recall
the wording used was something like 'for depolarization'. Mind you, this
was a large tube with pyrolytic graphite grids, known for not being
prone to primary or secondary electron emission during normal operation.
As we all have experienced, wire grids can exhibit reverse current at
some operating points,as evidenced by the G2 'islands' on the
characteristic constant current curves on tubes like the 4XC1000 and
1500 series. With PG, all current in G2 is in the normal direction, not
reversing. I had a 1800 VDC spark gap there, and it kept popping during
tune up. Eventually I discovered that it was reacting to modes that were
getting excited in the tube/cavity, up in L band. That is another LONG
story, that got solved.
I added the big R, a series of 225 Watt resistors, as screen voltage is
1500. A fan and a series stack of diodes between this and the power
supply complete the arrangement. There is also a high value shunt R
across each diode. The diodes are there to prevent damage to the power
supply in case of a plate to G2 arc in the tube. Couldn't use them if it
was a wire-grid tube with reverse current potential. Screen power supply
is a 10 kW Glassman switcher with very low stored energy (< 10 J).
At some point years ago, we used 4CX1500B as a series tube switch to a
particular RF tetrode, 4616 from RCA. It was a pulsed modulator, no RF.
The smaller tetrode would run into reverse current sometimes, and this
led to loss of control of the big tube, rising screen voltage. A 1K ohm
bleeder was added to shunt the circuit. This helped a lot. Eventually we
eliminated the smaller tetrode and used a MOSFET switch.
My point being, a stiff shunt R across screen grid is a good idea, not
only from the standpoint of keeping the screen always terminated during
switching, but also for circuit stability. I have done it both with and
without the R, but these days I prefer having the shunt R there, and
burn up some screen power from the supply.
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Ian White
> The bleed resistor is not part of the power supply. It is installed
> close to the tube itself, connected directly between the screen grid and
> ground (cathode). Its purpose is to prevent the screen grid from
> floating unconnected during the few milliseconds while the screen relay
> contacts are in motion between the RX and TX positions.
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