[Amps] FW: 3-500z Silly question

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Sat, 5 Sep 2009 11:39:30 -0700, MIKE DURKIN <patriot121 at msn.com> wrote:

>
>New question .... im using a MFJ 259 analyzer to check the input C-L-C network's for each band ... and its not good.
>The question is - does not having plate voltage affect the input impedance and frequency that much ( alot !!!) - im thinking that the input caps have been overloaded and now cooked ( wich might mean that the tubes in this amp were over driven too )

REPLY:

This is a grounded grid amp, right? The input impedance of a single 3-500Z will
be on the order of 115 ohms according to the Eimac data sheet, but this is only
when cathode current is flowing. If there are two tubes, the value is about half
that, 57.5 ohms. About 90-95% of the input impedance comes from "modulating" the
cathode current with the drive signal, so when the amp is unpowered, any input
impedance measurement will be way off. 

To tune your input circuit, remove all power from the tube but leave the tube in
the socket. Connect a non-inductive 115 (or 57.5) ohm resistor from cathode to
ground and then use the MFJ-259 to tune for minimum SWR. 

If you don't have an exact value resistor, use something  close for initial tune
up. Either way, make the final adjustment with an SWR bridge and the tube
running at full power. 

73, Bill W6WRT