[Amps] 3CX3000 input impedence

[Tony King - W4ZT]

Barrie,

The first rule of amplifier building after keeping your hands in your 
pockets is to know what your plate load resistance is.  Without it you 
can't know the proper values for C1, C2 and L to maintain the proper Q.

Your friend must first decide what OUTPUT power his amp is going to be 
designed for.  Then what the input power is going to be with some 
reasonable efficiency (usually around 55%) (Pin=Pout/.55).  Then, 
knowing what his INPUT power will be, use the plate voltage (under load) 
to determine the plate current at that voltage (I=P/E).  Then use the 
formula (RL=Ep/(1.6xIp)) to approximate his plate load resistance. That 
will be the value of non inductive resistance to place between the plate 
and ground for testing.

Don't fudge on the value of C1 and L. There are several on line tools to 
calculate the values of the tank circuit.  Stay within reasonable limits 
to keep the Q of the circuit between about 12 and 15.  Get the Q too 
high and tuning will be extremely sharp and things will begin to melt 
down because of the high circulating currents.  Too low and tuning will 
be too broad and you'll have practically no harmonic suppression.

Good luck with your project!

73, Tony W4ZT


Barrie Smith wrote:
> A friend is building a 3CX3000 amp for 6M.
> 
> He wishes to ballpark the tuned input circuit by installing 
a resistor between the tube input and ground.
> 
> What would be the proper value for this resistor?
> 
> TIA, Barrie, W7ALW
> _______________________________________________