Fellow amp addicts,
I have been building amps for years but I am on somewhat new ground
I am trying to estimate the drive impedance of a 4CX1500B in order
to design a tuned input with minimal guess work. I understand the
impedance varies throughout the input cycle, but there must be some
value which can be used to at least get close on the tuned input
network. How does one arrive at that value? I have read that it can
be derived from tube data sheets, but I fail to understand how. I am
sure I could experimentally derive a reasonable input network but I
would rather learn something in the process, if possible.
The tube is cathode driven with grid at RF ground and screen at RF
and DC ground. While the Eimac data sheet only gives operating
conditions for grid driven service, under direct inter-electrode
capacitances it does specify 38pf Cin "with grounded grid and
screen". That would seem to apply to my cathode driven arrangement.
I still don't know how to arrive at a figure for the real part of
I have come up with a somewhat wild guess that it is about 100 ohms
shunted by 38pf. This is derived from SWR measurements directly at
the amplifier input with real operating conditions at frequencies
between 1.8 and 50 MHz, using 1) no input swamping resistor; 2) 50
ohm resistor across the input; 2) 100 ohms across the input. From
those measurements I can seemingly justify my 100 ohms shunted by
38pf hypothesis, but I would be lying if I said I had much
confidence in this.
From this work it is apparent that having a 50 ohm resistor across
the input is desirable in that 100W is perfect to drive it with that
in place. Is there any reason why I shouldn't use a pi network tuned
input with the 50 ohm shunt resistor on the tube side of the
network? I don't recall ever seeing a "swamping" resistor in
combination with tuned input.
I will appreciate any guidance and education I can get here.
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