W8JI@contesting.com said --
>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.
Good stuff to know and remember. While I'm still not wild about
"frequency-sensitive dynamite" I feel a lot more comfortable about this
feature being factory-installed.
I'm not sure how your legal boys feel about it, but I think it would be a
very nice addition to the user documentation to state that:
"<manufacturer> acknowledges that parasitic resonances can occur on certain
components at certain frequencies within the overall operating range of
this amplifier, but outside of amateur bands. This amplifier has been
designed to avoid these types of resonances inside existing amateur bands
as of the date of manufacture.
"Specifically, engineering design and manufacturing practice place
plate-choke resonances at or near 12 and 27 MHz, outside of existing
amateur bands, and may place resonances for other components at other
frequencies outside of amateur bands. Operation of this amplifier at these
or other out-of-band frequencies will result in serious damage to the
amplifier. This damage is specifically not covered under warranty."
"Do not attempt to operate this amplifier at 12 or 27 MHz."
If you use remarks from the peanut gallery, you can send me a t-shirt or a
signed "gimme cap" to my callbook address in exchange for all rights, etc.
Jim N6OTQ
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