>
>Looking at the 1946 ARRL handbook last night (when I got it at Dayton this
>year,
>someone asked if I was modernising the station!) A number of their tx's have
>parasitic suppressors consisting of air wound VHF type coils ( 5 turns, #18,
1/2
>inch dia, 3/4 inch long) parallel tuned by 30pF mica trimmers in series with
>plate or grid - not both. No resistors.
>
>Now these circuits will have a reasonable Q. First thoughts are that they
>could be a good way to cause parasitics, but I guess that if they add
sufficient
>phase shift at the parasitic frequency, they will stop them. Seems a very hit
and miss
>approach to me.
>
/\ so it seems, Peter
>Of course, these tx's also used plug in coils, and judging by the construction
>rapid band changing wasn't part of the approach. So you could 'tweak' the
>parasitic trimmers and the neutralising caps every time you changed
>band.......also, I suspect many of the tubes didn't have the gain at VHF
>we have in tubes now.
>
/\ good point. 813s won't osc. with no parasitic suppressor whatsoever.
>Has anyone here ever tried the parasitic 'trap' approach in a modern
>multiband amp? I know Rich won't be impressed by these high Q traps,
/\ Before I wrote the October, 1988 *QST* article on parasitics, I
brainstormed on 40m with a group of amplifier builders looking for an
alternative to the conventional L/R parasitic suppressor. Every
possibility we came up with had a flaw. In 1926, Amateur Radio Handbook
editor, Mr. F. E. Handy was having good success with L/R suppressors.
The advantage of the L/R suppressor is that it does not need to be
adjusted for different settings of C-Tune. With a L/R suppressor, once a
low VHF Rp is in-hand, one can safely operate on any band.
>... and I can't say that I am, particularly. But one assumes they can (or
could) be made
>to work in the limited application.
/\ ... perhaps on one frequency.
>...
cheers, Peter
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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