I had some experience of "shorted turn" tuning in the W2GN 6m amplifier
which was used a flat rotating ring of copper. You're correct, Alex,
that the tuning range is very narrow. It required some careful squeezing
and stretching of the coil to make it tune at all... and that was only
for a narrow segment of a single band.
The surprising good news was that the heat losses in the tuning ring
were very low, even though the circulating currents must have been huge.
I think this was specifically because the ring was made like a large
flat washer, sawn and punched out of sheet material so there were no
seams. (W2GN's constructional article is archived on the web - google
for the call.)
A rotating ring almost certainly would not give sufficient tuning range
for a multi-band HF amplifier. The other configuration that drives a
longer length of copper tube into the centre of the coil should give a
little more tuning range, maybe enough to cover the lower part of the
10m band.
Also there is no point in using a solid metal slug, because the induced
RF currents will flow only on the outside surface. And definitely make
it from seamless copper - not brass, probably not even aluminium, and no
joins of any kind - because the whole idea depends on inducing very
large (and opposing) circulating currents in the shorted turn.
73 from Ian GM3SEK
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Saandy
>Sent: 16 January 2014 09:50
>To: 'Chris Wilson'; amps@contesting.com
>Subject: Re: [Amps] Permeability tuned tuning coils, with taps?
>
>Somewhere in the seventies, in the ARRL handbook, there appeared a 6
>meter
>amplifier tuned variometer style with a one turn shorted coil, rotating
in
>the stationary tank coil. It wasn't very wideband as I can recall, but
it
>worked.
>It seems to me, Chris, that you won't have any qualms about plagiarism:
it
>was definitely more than 20 years back!
>Alex 4Z5KS
>
>-----Original Message-----
>From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Chris
>Wilson
>Sent: Wednesday, January 15, 2014 8:25 PM
>To: amps@contesting.com
>Subject: Re: [Amps] Permeability tuned tuning coils, with taps?
>
>
>
>Wednesday, January 15, 2014, 10:22:05 AM, you wrote:
>
>
>> I vaguely remember seeing a 4 channel, 2-10 MHz, 100 watt output
>> commercial amplifier using three 'instant heat' 6146s (actually
>> QZ06-20)which used permeability tuning on the PA coils. I think the
>> coil had three taps to cover the frequency range: it was driven from
a
>> nominal 10 watt input SSB packset, and ran from a 12 volt vehicle
>> supply, with a germanium transistor DC-DC inverter. The packset was
>> mainly germanium transistors, too....That was 50 years ago.....
>
>> 73
>
>> Peter G3RZP
>
>Maybe people wonder why I ask? I have a Henry 2000D RF generator a
>friend
>and I are converting to a 10n meter linear. It's near as damn it done
now, a
>simple thing for many on here, a challenge for us beginners, as I had
to
>make the PSU and control stuff from scratch.
>
>I bought two big vac caps off Ebay a while back, new old stock. One
seems
>fine, the other arcs on test at 50% below its rated DC volatge.
>These things are *EXPENSIVE* new. Used ones are a lottery, as are old
>stock,
>as there's no way to test the depression inside them without doing an
HV
>test, after you've bought the thing. So, I was looking at how Henry
used a
>brass slug permeability tuned tuning coil of heavy copper tube, instead
of a
>variable tune capacitor, and a sheet metal flapper cap for loading. I
was
>wondering if at a later stage I could plagiarise the design to make a
20
>meter or even a 20 / 40 meter version and make a longer coil, with taps
for
>other frequencies, and just have to use one vac cap for load? I am in
the
>race car engineering business, so have good fabrication facilities, the
>hardware for a bigger brass slug tuned circuit would be pretty
>straightforward. Photo of the Henry 2000D deck is at
>http://www.gatesgarth.com/henry6.jpg
>
>Thanks everyone.
>
>
>
>
>--
>Best regards,
> Chris mailto:chris@chriswilson.tv
>
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