Discussed...
On Apr 23, 2004, at 3:20 PM, Jeff Weinberg W8CQ wrote:
Hi All,
I am trying to find a plate choke for the SB-220 for a friend in
Australia. Anyone got a spare lying around?
ð No, but they are a piece of cake to make. See
http://www.somis.org/D-amplifiers4.html
- cheers, Jeff.
Thanks,
Jeff Weinberg W8CQ
Lots in that URL I don't agree with. First he seems not to distinguish
between series and parallel resonances at all. According to Ohmmite
solenoid plate choke engineering data sheets I have here (some where),
the ideal situation is to be operating intentionally at parallel
resonance frequency. Actually there are many parallel resonances above
the fundamental one. At these points reactance is nill and effective
choke resistance is extremely high, in order of megohms. The choke will
appear 'invisible' to the plate tank circuit at these points, which is
optimum efficiency. I built a single bander 80M amp from an old
LK-500ZB, and designed the plate choke to be parallel resonant when in
situ (it has many turns). Connecting and disconnecting the choke from
the tank makes no noticeable change to tank resonance. This fellow in
the URL speaks of fires--that is characteristic of series resonant modes
only, which are the great enemy. Concern about winding insulation is
un-called for, because when you add up the total insulation rating of
all in-between turns, it is great indeed. As for high voltages somehow
building up around a choke, I do not know of such a mechanism for it,
nor have I observed it ever. With linear operation, we have resting
current flowing at all times through the choke. Such current prevents
any voltage build up higher than the power supply voltage on the plates.
Roy K6XK
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