[Amps] FW: FW: 3-500z Silly question

[MIKE DURKIN]

> 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? 
yes
>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. 
The tubes are powered (2X 3-500Z) .... just no plate voltage. And the MFJ uses RF in its measurements ... Are you saying that the signal level is too low?
> 
> 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. 

Nope .. wish i had some thing close .. even semi close. Mabey a pair of 100ohm ..mabey.Full power ?? is the drive that critical for input measurements?? at what level does the accuracy drop by 10% ??Still wondring if the plate voltage is critical for input adjustments.The input Interelectrode Capacitance1 (Grounded Grid Connection) is listed as 10.5pf. That dosent seem to be a huge amount compared to the value given for the C-L-C = 100pf/unknown henry's/150pf-Variable for 10m for instance.
KC7NOA
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