[Amps] 4-400A Input impedance

[k7fm]

>Anybody know what the input impedance is for a pair of 4-400A's operating
>plate modulated class Connected in parallel?  Is 50 ohms close?

Since you are talking about 50 ohms, you must be considering Cathode drive. 
Since the input is part of the output with cathode drive, it cannot be used 
with plate modulation - unless the driver is plate modulated also.  It is 
rarely done.

Perhaps you are plate modulating the driver and not the final amp.  If the 
4-400A is used with all the grids grounded, I would guess about 70 ohms 
impedance.

If you are plate modulating the final amp, it is easiest to drive the grid 
and hang a resistor on the grid to determine the drive impedance.

Colin  K7FM