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.
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.
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, and I can't say that
I am, particularly. But one assumes they can (or could) be made to work in the
limited application.
Incidentally, the pictures of the Hallicrafters HT4E (otherwise known as a
BC610) don't appear to show a parasitic suppressor at all on that PA. Is that a
fact, or is just a smallish photograph doesn't show it?
73
Peter G3RZP
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