[Amps] 3CX3000 Tuned Input Calibration

[Tom W8JI]

>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.

Don't let anyone tell you slapping a resistor from the 
output of the network to ground simulates the load by the 
tube. It doesn't.

You initially got lucky Bob. It fell apart on 160 because of 
the filament choke. It will fall apart on the highest bands 
because of stray capacitance, inductance, and transmission 
line length to the tube that is not terminated perfectly in 
50 ohms. In my experience the impedance is slightly higher 
than 50 ohms, close to 60.

Part of the input impedance of the tube is modified by the 
feedthrough capacitance. This means some phase shift is 
involved in actual operation. Using the 50 ohm resistor will 
only get you close on the higher bands, although there are 
cases where dumb luck takes over. Once close you will just 
have to trim it in.

73 Tom