>
>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.
>
? well put.
>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.
? amen, Dick.
>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?
>
? The actual Rin appears to vary somewhat during the on/half-cycle,
probably due to the nonlinearity of the diode component?. // The great
puzzlement to me is why should one try to simulate the average cathode Z
when one has a perfect model at hand - a real cathode. With a resistor
substitute, one can have the Q wrong and there is no clue. With a real
cathode load, one can not take the wrong road. .
?? Dick: Your e-mailer does not do attributions correctly.
>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
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|