[Amps] Does ANYONE know what the zero signal anode current should be for a pair of 3CPX800A7 tubes? (Don Kessler)

[John Lyles]

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 lines.
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