[Amps] 3CX3000 Tuned Input Calibration

[Bob Maser]

I have completed the alignment of the tuned input of my 3CX3000A7 using a 50 
ohm non-inductive resistor connected between the cathode and ground.  I then 
drove the input with my FT1000 and was able to adjust all bands to 1.1:1. 
When I put it all back together and drove the amp on 20M  lo and behold, the 
SWR is 1.6:1.  I believe that the reason for the discrepancy is that the 
input impedance for a 3CX3000 is not 50 ohms but probably lower.  I am 
running 5200VDC on the plate of the tube and into a dummy load I am driving 
it with 200W.  The grid of the tube is connected directly to ground.  Anyone 
have any info on what the actual input impedance is for my conditions?

Thanks
Bob  W6TR