> > 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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