On Tue, 6 Feb 2001 17:55:35 -0500 "K4MO" <K4MO@carolina.rr.com> writes:
Hi Theo,
>I have just about collected all the parts to build my next project and I
>will soon be faced with the decision of how to switch bands with the
final
>tank circuit. I see three options:
>
>1. Use the EF Johnson 222-101 variable inductor that I've been saving
for
>the last few years. I used a roller in my last large amp, chiefly do to
the
>fact that I had it and it saved all the trouble of fiddling with taps
and
>such.
Rollers work great. And they do seem to be the military HF
amplifier's preferred component in this application.
>2. Use a big strap wound inductor and large ceramic switch. I have the
coil,
>but will need to locate a switch.
That's a good approach, too. Perhaps, better in the old days with
only 5 bands... a mite crowded with today's assortment.
>3. Use the aforementioned coil but switch it with relays, which I would
>still need to procure. This offers the advantage of being able to remote
>control the band change at some point in the future.
I've looked at relays, in lieu of option #1 or #2, on several projects
and, whether for a tuned input or output, they end up being just so much
more... stuff... that they become ... inelegant.
You didn't mention what power level you're considering but, just
guessing from the 222 series coil, for an output tank you would need
largish vacuum relays -- pricey and space consuming, too.
Of course, relays are the only way to go if you need to frequency hop.
:-)
>I am hoping some of the more experienced builders can offer some
opinions
>and suggestions regarding the technical merits or shortcomings of these
>three choices. Is one method or the other more subject to random
parasitic
>oscillations?
Nope. The plate circuit up to the tune C and it's return to the
cathode are the only tank item's concerned here.
Is tank circuit heating more of an issue with one or the
>others?
Just depends on the wire size / cross section. You'll need a lot of
strap to go to the switch(es) if you are going to do option #2 right --
score another point for the rollers.
What other considerations do I need to ponder in this regard?
To meet modern harmonic specs you'll need to use a Pi-L network, which
implies a second roller coil or switch deck. That can become
mechanically limiting with a switch as the two coils should be mounted at
right angles.
Depending on your output impedance, it may be more practical to switch
(or relay) in an extra fixed coil for 160 rather than have a roller big
enough to cover it. There is a lot of "space" between 80 and 160 where
no ham bands will be created in our license times.
If you use an SPDT arrangement to short out the 160 coil, then the
switch/relay can also connect in some extra load C for 160.
>I have looked through most of the archives and could find no previous
>discussion on Rollers verses Switches so any and all thoughts and
>ideas are welcome.
I vaguely recall some... discussion... of the relative merits back in
the late 2nd millennium, though I'm afraid I can't peg it closer than
that for you.
73,
Marv WC6W
*
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