Yes Tom might be closer with the 60 ohms, I've been working in YC-156's
lately, massive B+ hits, brain failures, cellular destruction, beer
consumption, standard ham stuff. I've got a couple 3 By 3 projects here but
its been a while since I've had to bring one in tune.
No matter what, from 1.6:1 you are close enough to tweak either way easily.
BOB DD
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Tom W8JI
Sent: Thursday, October 26, 2006 12:34 PM
To: Bob Maser; AMPS
Subject: Re: [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.
Don't let anyone tell you slapping a resistor from the
output of the network to ground simulates the load by the
tube. It doesn't.
You initially got lucky Bob. It fell apart on 160 because of
the filament choke. It will fall apart on the highest bands
because of stray capacitance, inductance, and transmission
line length to the tube that is not terminated perfectly in
50 ohms. In my experience the impedance is slightly higher
than 50 ohms, close to 60.
Part of the input impedance of the tube is modified by the
feedthrough capacitance. This means some phase shift is
involved in actual operation. Using the 50 ohm resistor will
only get you close on the higher bands, although there are
cases where dumb luck takes over. Once close you will just
have to trim it in.
73 Tom
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