OK, I had promised to report back on this subject, so here goes:
Back a month or two ago, I said that I was going to simulate the
impedance of my 4-1K's cathode by using two 220 Ohm resistors from each
cathode pin to ground and by leaving the tube in socket (driving
impedance is 110 Ohms). I could then use my MFJ-259 to tune my input
pi-networks.
Several people here said they didn't think that would work and that you
couldn't simulate the complex impedance of a tube with a couple of
resistors.
Well, I have to tell these experts and the rest of us that they were not
correct. The simulation worked VERY well. After designing the input
circuits this way, and after using the amp for a while, I am pleased to
report that simulating the input impedance of a tube using a couple of
resistors DOES INDEED WORK!
My SWR on all bands with the exception of 10M is nearly 1:1 in at least
some portion of the band. On 10M, the SWR is about 1.3 or 1.4:1, which
is still fine (it's probably off because the inductance in the leads of
the resistors is probably significant at 28 MHz). 20M might be 1.2:1 or
so. On 40M and 15M, the SWR is absolutely flat and on 80M it is very
good as well (perfect in some spots, but the tuning bandwidth is
considerably less than the higher bands).
So folks, it does work. Don't let the so called experts fool you. Logic
and common sense told me it would and it does.
And by the way: all my caps are fixed. The only thing I can tune are my
inductors.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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