Bill, I was thinking the nominal 50 ohm value was derived from the time
varying elements of the cathode voltage and current over the drive cycle.
It occurred to me that I can easily test this idea on an existing 70cm
amplifier that I already know has a matched input. The only effort required is
to leave the hv off, stay in standby to kill the screen voltage and disable
the grid bias with a clip lead. Then I can simply measure the return loss
with drive from a signal generator and observe the grid current.
I will report any findings.
73, Gerald K5GW
In a message dated 10/20/2010 11:07:30 A.M. Central Daylight Time,
dezrat1242@yahoo.com writes:
ORIGINAL MESSAGE:
On Wed, 20 Oct 2010 10:58:11 EDT, TexasRF@aol.com wrote:
>
>Any comments and criticism will be cheerfully accepted. I don't mind
>octagonal wheels so much, just don't want square ones!
REPLY:
I don't think you would be successful with only grid current flowing. In
a grounded grid circuit, a large part of the input impedance is created
by "modulating" the cathode current. Take that away and the impedance of
the grid alone is going to be way off.
I think a better approach would be to use a carbon comp 51 ohm 1/2 watt
resistor temporarily tacked from cathode to ground right at the tube
socket, and then (with the tube in place but no power applied) using a
SWR analyzer such as one of the MJF VHF types to rune the input circuit.
I realize at that frequency the reactance of the resistor's leads is
going to be slightly different from what the tube alone would be but I
think with very short leads you can come close enough.
Good luck and let us know how it comes out.
73, Bill W6WRT
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|