Hi Jon & all..
Jon, no doubt you've got good, useful info. But, with all due respect, I
think you're a little off the deep end dissing "the so-called experts who
said it wouldn't work" without explaining why it DOES work. (My apologies
to all if someone has already made this point & I missed it!)
We know that a gg tube in class AB2 or B does in fact "look" pretty much
like its input C, in parallel FOR LITTLE MORE THAN HALF THE RF CYCLE with a
resistive load. On the other more-or-less half cycle the input signal
drives the tube into cutoff so that it looks pretty much like pure
capacitance. You might say the cathode input itself looks roughly like Cin
in parallel with a series resistor-diode combination.
So WHY does it work to adjust matching with more or less pure resistance
simulating the "half-cycle resistor" dynamic load actually presented by the
tube? It's because the value of "Rin" that you tack across the tube is
really an AVERAGE value of Rin. It's approximately equivalent to the result
of placing a resonant "flywheel" across the tube's real input R (which
loads the input during only about half of the drive cycle). If the input
tuned-circuit isn't used, or its "Q" is too low, results would be quite
different. My guess is that actual Rin presented by the tube during its
"on" half-cycle is roughly Rsim/sq rt 2 = ~Rsim/1.4. (Rsim is the fixed R
used to simulate the tube for input tuning purposes).
Anybody agree, confirm, disagree, or want to refine this for all of us?
73, Dick
-----Original Message-----
From: Jon Ogden [SMTP:jono@enteract.com]
Sent: Thursday, March 25, 1999 10:20 AM
To: amps@contesting.com
Subject: [AMPS] Cathode Impedance Measurements
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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