[Amps] Tuned input design / measurement questions

[Gary Schafer]

Hi Chris,

You may not be able to measure the impedance as you are trying to do as the
tube impedance changes quite a bit with plate current. At full power as
listed in the spec sheet the average impedance should be close to what the
spec sheet says.

Placing the 50 ohm resistor across the output of the input network should
get you close provided that you have the tube in the socket as the pi
network output capacitance uses in part the grid to plate capacitance of the
tube to tune it. On 27 MHz this will be a large part of the output
capacitance of the network.

I am not familiar with the AIM analyzer but what I see on the chart with the
50 ohm resistor and tube in circuit looks like it is doing what you would
expect.

Just remember that your input SWR to the input network will vary as drive
power is applied as the tube input impedance changes with power. So you
would probably see the best SWR for the original circuit when the amp is
operated at its intended power level with intended drive power, plate tuned
and loaded properly to deliver the power to the load. 
Anything else and the input SWR will not be as intended.

Note that when using a pi network to match 50 ohms (input) to 50 ohms
(output) the input and output capacitors will be nearly the same value. The
output capacitor will be minus the amount of tube capacitance.

73
Gary  K4FMX


> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Chris
> Wilson
> Sent: Tuesday, April 16, 2013 6:41 PM
> To: Amps
> Subject: Re: [Amps] Tuned input design / measurement questions
> 
> 
> 
> 
> 16/04/2013 23:32
> 
> 
> Thanks for the replies so far. I have taken a backward step and
> decided to use the AIM 4170 to scan the stock factory 27.12 MHz Pi
> input circuit. As I did not get the exciter with the RF Generator, I
> cannot say for sure if it expects a 50 Ohm match, but assume it did.
> 
> I took some screen shots of various scans. The long file names are
> pretty self explanatory. One scan is the the stock 27.12 MHz input Pi
> network disconnected totally from the de-coupling caps on the valve
> base. One is with a 50 ohm carbon composite resistor across the output
> cap of the stock Pi network, with the network disconnected from the
> rest of the RF deck. One is with this resistor still connected *AND*
> the output of the Pi network connected back up to the de-coupling caps
> on the valve base, with the valve in place. To be frank, I don't
> understand what I am seeing. The cursor is around about 21.12 MHz in
> each screen shot. The only one that seems to make any sense to me is
> the one with the Pi network open at the output side....
> 
> I have added a screen shot of a page from the Eimac spec sheet to the
> zip
> file. This makes me assume I should be aiming for a 50 Ohm match.
> 
> I feel I am doing something wrong, or expecting something
> I shouldn't be! Thanks. Files are at:
> 
> 
> http://www.chriswilson.tv/pi/Pi_scans.zip
> 
> 
> Thanks!
> 
> 
> 
> --
>        Best Regards,
>                    Chris Wilson.
> 
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps