> From: Harald Garberg Gule <hggule@online.no>
> The Communications Quarterly Winter 1995 issue have one article on using
> the 4CX 1600B.
"Power on a Budget" by W6FR was a very good article, but I'd suggest
a few changes.
First, the auto bias circuit has a slow rise time because the 1 mFd
capacitor at D1's cathode has to charge through a 1.5 k resistor in
series with the reactance of a 33 pF capacitor. On lower bands, the
time constant is very slow, and that will cause slow bias removal on
the leading edge of speech, and class C opertion for an instant of
time. The impedance at the output of D1 needs to be greatly
increased, meaning a much larger value of hang time resistor
and much smaller value than the 1 mFd cap should be used.
A better place for the hang time cap and resistor would be the base
of Q2, so the low source impedance of Q2 can drive the cap making it
fast attack and slow decay, instead of slow attack slow decay as in
the original circuit.
Second, be sure the screen is properly regulated and the control
grid supply source impedance is low at audio frequencies!
Finally, I'd use a L pad at the grid. The correct resistor values
are given by the following:
Find the peak current through 50 ohms with your expected maximum drive
power by:
Ipk = the square root of the drive power over 50, times 1.414.
Next find the value of the grid resistor by:
Rg= Bias voltage over Ipk
Finally find the series resistor between the grid and exciter by
Rs = 50-Rg
If you follow these steps, you will have a PA that allows you to run
your exciter at full output minimizing splatter and chances of
operator error.
> While on this subject, do anyone know if there is any problem related to
> using 2 each 4cx 800A in class AB1.
All you need to do is switch in a small parallel air-wound coil from
the grid to ground near the tubes on upper HF. It should be no
problem at all to do that with a small reed relay. I would expect the
Q will be so low if you set the compensating inductance for 15
meters, it will take care of ten meters.
If you run low on drive on ten meters, a small C can be placed across
Rs to increase drive at upper HF with the shunt inductance at the
grid used to null SWR.
73, Tom W8JI
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