I have an old TMC amplifier with a pair of 3-500 tubes. It originally had a
toroid tank that had turns shorted for the higher bands. The core was quite
large. The tank covered 160 - 15 meters. Efficiency got poor on 40 meters
and worse as you went higher. I added an air wound copper coil for 20 and 15
meters and efficiency improved. I think the only place I short turns on the
toriod now is for 40 and 80 meters. Yes it got hot too.
W8JI says that shorting turns on a toroid is not a good thing to do. He said
that shorted turns on an air coil cause much less of a problem because there
is considerable flux leakage on the air coil.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Larry Benko
> Sent: Saturday, December 22, 2012 11:25 AM
> To: amps@contesting.com
> Subject: Re: [Amps] hf-2500
>
> I used to think that shorted turns on powdered iron toroids was bad
> until I did some tests. My tests were not exhaustive but winding some
> #12 wire on a T200-2 core and shorting out some of the turns to reduce
> the inductance to ~1/2 of the inductance with the turns not shorted
> showed a moderate reduction in unloaded Q and a slight increase in
> temperature rise but nothing hugely significant.
>
> I concluded that if shorting turns on an air core (u=1) was fine and
> shorting turns on a 60Hz transformer (u=10000 or more) was really bad
> then shorting turns on a powdered iron core (u=6 to 10) was much closer
> to the air core case than to the u=10000 case.
>
> Of more importance is that toroidal cores are not easy to cool since
> they usually get connected to the chassis with an insulating washer
> beneath them and air flow not that good. Perhaps this leads to some of
> the reported failures.
>
> 73,
> Larry, W0QE
>
>
> On 12/19/2012 6:16 PM, Jim W7RY wrote:
> > Bill
> >
> > We already had this discussion on here about a year ago. But I 'm
> > going to repeat it.
> >
> > Alpha, Ameritron, Jerry K8RA, myself W7RY, and many others all use
> > toroids with shorted turns on our "L" coil in a PI-L circuit. It
> > works just fine.
> >
> > Do you mean that a shorted turn toroid for the PI network is not
> > good? Or are you making a general statement that they don't work at
> > all in an amplifier?
> >
> >
> > 73
> > Jim W7RY
> >
> >
> > -----Original Message----- From: Bill Turner
> > Sent: Wednesday, December 19, 2012 12:26 PM
> > To: Amps
> > Subject: Re: [Amps] hf-2500
> >
> > ORIGINAL MESSAGE:
> > On Wed, 19 Dec 2012 17:15:03 +0000, K1TTT wrote:
> >
> >> Agreed, that is why I have modified one to do only 40-10m... removing
> >> the
> >> 80m and 160m coils from the toroid removes the shorted turns problem
> and
> >> makes it run very nicely doing rtty on 40m.
> >
> > REPLY:
> > As David found out, shorted turns on a toroid or any iron core coil
> > are a no-no unless you do some very careful design and testing, which
> > Command obviously did not do. Shorted turns on an air core coil are
> > usually no problem. A word to the wise.
> >
> > 73, Bill W6WRT
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