[Amps] LK-550

[Edwin Karl]

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