BOB,
Sorry I didn't respond to this post earlier, the input impedance of a 3 BY 3
is closer to 35 OHMS if I remember correctly. While this resistor procedure
will get you very close it takes tweaking to make it right.
The sources say an 8877 is 51-54 Ohms. I've seen both listed. I just
brought one of my industrial heater 8877 conversion units on line on 80
meters the other night and without a tuned input yet the SWR is 3:1. I know
the impedance is not 3X off 50 ohms it is that the tubes are reactive in
actual operation. You need to tune out this reactive component and there's
only one way to do that. Grab a knob. You don't have one? Then it will
take a lot of solder and tune & try.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Bob Maser
Sent: Thursday, October 26, 2006 10:10 AM
To: AMPS
Subject: [Amps] 3CX3000 Tuned Input Calibration
I have completed the alignment of the tuned input of my 3CX3000A7 using a 50
ohm non-inductive resistor connected between the cathode and ground. I then
drove the input with my FT1000 and was able to adjust all bands to 1.1:1.
When I put it all back together and drove the amp on 20M lo and behold, the
SWR is 1.6:1. I believe that the reason for the discrepancy is that the
input impedance for a 3CX3000 is not 50 ohms but probably lower. I am
running 5200VDC on the plate of the tube and into a dummy load I am driving
it with 200W. The grid of the tube is connected directly to ground. Anyone
have any info on what the actual input impedance is for my conditions?
Thanks
Bob W6TR
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