[Amps] Operational Input Impedance

[Steve London]

Several years ago, I modified my Alpha 76PA, replacing the 8874's with a 
pair of 3CX800A7LG's, purchased from Alpha. These were new, Chinese made 
tubes. Other than that, I know very little about them.

I have never been happy with the input SWR - it's in the 3:1 to 4:1 
range, with about 25 watts of drive (and 500 watts output). The input 
network is the standard Alpha 76 2:1 unun. The 95 ohm swamping resistor 
pack has been removed from the circuit. Despite the high input SWR, the 
amp seems to work satisfactorily (when using the transceiver tuner to 
"fool" the transceiver into putting out adequate power).

Today, just to make sure the input network wasn't fowled up, I pulled 
the tubes and put 22 ohms and 50 pf across the cathode pin to ground, to 
simulate the published specs for a pair of Eimac 3CX800's. Using an 
antenna analyzer, the SWR was close to the expected value - 1.2 at 3.5 
MHz, rising to 1.8 at 21 MHz. Nothing like the 3 to 4 SWR observed 
during the operation of the amp with the Chinese 3CX800's.

So, this begs the question: What is the actual input impedance during 
operation ? How would one go about measuring it with something like an 
antenna analyzer or VNA ?

73,
Steve, N2IC