Hi Ian
Alpha has been doing this in
the "L" section toroid of the
87,89,91b, and 99 with no
apparent problems. They use a
pick-up and hold type switch,
(9 wiper contacts) with the
wipers connected to the L1, L2
junction and short toward the
10 meter tap, (output end).
The L1 wafer (with wipers
connected at the same
junction) shorts the air-wound
inductors toward the 10 meter
tap, (tube end). This layout
keeps the wipers at the lower
image impedance at all times,
allowing a smaller, (read
cheaper) switch.
Cheers, Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://wd7s.home.att.net
> -----Original Message-----
> From:
> amps-bounces@contest
> ing.com
> [mailto:amps-bounces
> @contesting.com]On
> Behalf Of Ian White, G3SEK
> Sent: Tuesday,
> August 05, 2003 10:59 PM
> To: amps@contesting.com
> Subject: Re: [Amps]
> Shorted PI-NET turns
>
>
> Stan Wigh wrote:
> >A year or two ago
> there was a
> discussion on the
> wildly accepted practice of
> >shorting out
> unused sections of
> a PI or PI-L output
> tank when not in use.
> >One commentator
> gave an exception
> that when toroids
> were used, shorting out
> >part of the
> inductor could be
> problematic.
>
> You can short out
> whole chunks of a
> normal air-wound
> solenoid coil
> because the
> magnetic coupling
> between the turns
> is not overly tight. You
> don't see high
> circulating
> currents (the
> "shorted turn
> effect") in the
> shorted part. The
> reason for shorting
> the coil rather
> than leaving
> unused parts
> open-circuit is
> that it prevents
> high voltages appearing at
> the open end of the
> coil, and also
> helps avoid stray
> resonances.
>
> The turns of a
> toroid are very
> tightly-coupled
> magnetically, so any
> shorted turns will
> almost short the whole coil.
>
>
> >I do notice that
> several of the
> >latest ARRL
> Handbooks do
> exactly that in
> their "Saint
> Petersburg" amplifier
> >as well as Command
> Technologies and
> QRO amplifiers,
> judging from their ad
> >photos. So what
> determines whether
> shorting out
> toroidal turns in the PI-Net
> >will work or not?
>
> What works is to
> short the whole
> toroid. That's why
> you usually see
> multiple toroids in
> these amps (I
> believe Command and
> QRO also use a
> toroid transformer
> at the output, but
> that's a separate thing).
>
> It can also OK to
> make a non-shorting
> tap on the winding,
> but only if
> that doesn't cause
> high voltages at
> the open end.
>
>
> --
> 73 from Ian G3SEK
> 'In Practice'
> columnist for RadCom (RSGB)
>
> Editor,
> 'The VHF/UHF DX Book'
> http://www.ifwtech.c
o.uk/g3sek
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