Don
Here is the process to figure this out. Take the tube data sheet for 3CX800A7,
available online from G8WRB's site. [http://www.g8wrb.org/triodes/index.php]/
The "P" tube is a pulsed version, usually tested for higher voltage holdoff.
The grid - cathode spacing and anode - grid gap are likely the same inside the
tube. Using the datasheet, you'll see that Eimac specified class AB2 typical
operation at 2200 VDC, with one tube and 8.2 volts of bias, is 750 watts
output. Zsac is given at 0.015 amps DC. Go to the tube curves at the back of
the datasheet, and get the grounded grid constant current curves. You'll see
that if you mark 2.2 kV for plate to grid V and +8.2 V of cathode to grid, you
get a spot just above 0.001 Amps of anode current. This is your operating
point, or Q point for this setup. The next line is 100 mA. So its down there in
the noise at 15 mA probably. Now extend the curve with a piece of paper,
drawing straight lines continuing the existing contant current li
nes.
When you reach your projected anode voltage, do the same thing. Note that the
100 mA line will drop further to the lower right, eventually crossing 8.2 volts
bias at some anode voltage. You don't want to be there, probably, so you'd have
to crank up the bias to make it less conductive, back to 15 mA or so. Make
sense?
I am not going to go through the trudgery of plotting this, but you get the
idea. If you don't raise the cathode positive bias, you will be taking a lot of
resting anode dissipation, 3200 VDC x 100 mA is 320 watts per tube. 15 mA is
only 48 watts. For the most linear operation, but still in AB2, you draw the
entire load line you plan to swing about that Q point. Allow the tube to have
positive grid current during the tip of the cycles where the plate voltage
drops low. This helps get more output in AB2. The Eimac Care and Feeding book
explains what to do here, maybe for AB1 or 2. AB2 gives more output at the
expense of some linearity. The more you raise the Q point for your resting
plate voltage without RF, the more you raise the load line vertically on the
plot. Note that eventually even the lower right portion of the line (off the
page on your second sheet of paper that is taped to the Eimac plot) will have a
large portion of the RF cycle that is drawing anode current.
I recommend experimenting with this graphical approach, then setting a bias
and anode voltage and seeing if it agrees. All of the work done so far is for a
single tube, multiply x 2 for a pair. You can check commercial amplifiers and
ARRL designs to see what they left the resting dissipation at for a particular
anode voltage, with a certain fan size. Then you might consider shooting for
similar dissipation at your newer anode voltage, by adjusting the cathode bias.
73
John
K5PRO
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