Sounds like a neat trick, Tom, but I am not quite clear on the connection
between the light bulb and the T connector. Is the lightbulb connected
between one leg of the T connector and chassis ground so that the bulb
is lightly coupled to the choke via mutual coupling, or do you actually
connect the top of choke in series between one leg of the lightbulb
and ground?
Thanks,
Mike, W4EF....................
----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: <amps@contesting.com>; "Jim Strohm" <jstrohm@texas.net>
Sent: Friday, May 10, 2002 4:34 PM
Subject: Re: [Amps] Ameritron AL82 @ 17 meters
> > > I was tuning my AL82 amplifier on 17 meter band and the plate choke
> > > blew. I haven't used the amp much before on 17m.
> > >
> > > Has anybody else had such problems? This was the first problem ever
> > > with this amp during 6 years.
> >
> > Spurious resonances on plate chokes are not uncommon. There was a
> > very famous mod to Alphas some years back that involved moving the
> > plate choke resonance to the center of 11 meters (27.185 MHz).
>
> Ameritron triess to park resonances at 12MHz and 27 MHz.
>
> The problem is the exact frequency varies with stray capacitance from
> sheet metal around the choke, the choke materials, and the winding
> tolerances.
>
> Each choke batch is supposed to be tested for resonances in a fixture
> for the application they are going in.
>
> In the amplifier, you can test the choke by setting the tank to 160
> meters, disconnecting everything from the top of the choke, and
> measuring the choke impedance from the top of the choke to the
> chassis as you sweep frequency. You can use something as simple as a
> small incandescent light bulb from a T connector grounded to the
> chassis top rail near the choke. The T connector should also feed a
> dummy load, and you can sweep frequency with a radio to see if the
> light glows brightest in or near any ham bands.
>
> Or you can use something else, many things will work.73, Tom W8JI
> W8JI@contesting.com
>
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