Rich says:
>/\ good point. 813s won't osc. with no parasitic suppressor whatsoever.
Dunno about that (I've never tried, but I've never had parasitics in an 813
stage, even without stoppers), but most of these designs were for triodes. The
odd pentode - HK257 and the like - but mainly things like T55, 100TH, 812 (for
the 'new' stuff) and so on. I think there's one 813 design in there.
>/\ ... perhaps on one frequency.
My feeling, too. Some of the older books have shown suppressors where there's a
circuit tuned to the parasitic frequency, resistively loaded, and link coupled
to the plate circuit. That just seems a complicated way of doing it, but perhaps
if the tube is one where the self neutralisation frequency is so low that a
proper suppressor adds too much L, it could be useful.
All in all, I suppose it's just one of those interesting by ways of
technological history, rather than anything particularly useful these days. One
of my books (Wireless Valve Transmitters, W. James) doesn't mention parasitics
at all - I suppose that they hadn't got tubes capable of them when that was
published in 1923! (Ultra short waves were about 50 metres)
Rich, have you ever tried ferrite beads? I've seen tx's with them used on 6146s,
but could never get them to work satisfactorily myself. In gate 2 of dual gate
MOSFETs, I've gnerally found changing to a 22 ohm resistor more satisfactory.
73
Peter G3RZP
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