There is no screen in a 3-500Z. Probably a grid (control) to cathode short.
I personally would not mix tube types as I want the amplifier to balance, i.e.
equal power in all jugs. You no doubt could substitute with a graphite tube,
with unpredictable results. Are the other 2 full emission? etc.. Problem with
removing one of the 3 tubes is the plate impedance (and grid input) changes.
Not sure if the thing will tune properly.
That's only my opinion, I could be wrong (quite Dennis Miller)
Good Luck, Merry Christmas
ed K0KL
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