I haven't used a GU-74b before, but I have used a 4CX1000A and a
4CX1500B and I think they would be similar in operation.
It is normal for a tetrode's screen current to vary a lot with
different drive levels. It can go either negative or positive
depending on drive level and plate loading. The important thing is
that screen VOLTAGE should be constant. Do you have a regulated screen
supply?
Second, what plate impedance are you designing for? I figure with 2100
VDC on the plate, current will have to be about 1.2 amps for 1500
watts output. Is that what you are planning for? Using the factor of
1.8, plate impedance would be about 970 ohms. Given that, my G4AXX
spreadsheet gives 80 meter values of C1 = 467 pF, C2 = 1856 pF and L1
= 5.28 uH for a Q of 12. Are those values about what you are figuring?
73, Bill W6WRT
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