Hi Phil,
I've had great luck with the following circuits in my 4cx250 amplifier.
Screen grid regulated (zener shunt method) to +340 Vdc
The control grid supply is variable by using a 500 ohm 2 watt pot in
series with two paralleled 3300 ohm 2 watt resistors across a negative 82
volt (zener shunt regulated) source. The grid is voltage fed through a
choke, properly bypassed.
I cut the tube off by taking the cathode to ground through a pair of
paralleled 15K 2 watt resistors. The cathode is well bypassed to rf by 6
caps. The paralleled resistors are removed (shunted toward ground) from
the circuit by a relay contact to ground wired in series with two
paralleled 270 ohm 2 watt resistors. The 270 ohm resistors are a great
place to meter the current.
I "lift the cathode" rather than play with the grid. This single tube
4cx250 amplifier is fed with a 9:1 input transformer which is terminated
into the combination of two paralleled 1500 ohm 2 watt resistors, a
parallel tuned circuit and the tube grid through a small parasitic
suppressor.
In linear operation (51MHz SSB) the performance is as follows.
4.2 watts input (Icom 551D)
310 watts output. (full tilt boogie)
no stability problems, no parasitics, no problems
cheers
skipp
[snip]
From: psmeaton@globalnet.co.uk
I'm building an amplifier for 70cms, using the GS23b tetrode tube. I've
built a regulated supply for the Screen (520v @75ma) and a supply for the
anode (3500v @1.5 amp), but I'm having trouble getting my head around the
grid supply.
As the amp will be used for SSB as well as CW, I think that a variable
supply will be required - rather the same as that for a 4CX250B. This is
so
that the standing current can be altered etc. Of course, the actual
voltages involvd may be different, but the principle sounds the same to
me.
I am presuming that the switching for the voltage will be similar also -
such as a negative voltage applied during Tx and a even more negative
voltage applied during Rx to shut the tube off.
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