On Feb 22, 2005, at 3:10 PM, Joe Subich, K4IK wrote:
>
> Kevin writes:
>
>> with respect to the burning on the plate choke: yup it's only
>> in certain areas. So the comments about vhf parasitics are
>> probably right on, if that's a key indicator.
>>
>> The 22 ohm suppressor R's physically looked okay, but I'll
>> be sure to measure them.
>
> These two observations point not to parasitics but an attempt
> to operate the amplifier on a band for which it was not designed
> perhaps 17, 12 or 11 meters. Burning in certain areas of the
> plate choke indicates high voltage at that point, probably due
> to a resonance. That often happens to older amplifiers not
> designed with "WARC" band operation in mind.
Joe: When, during WARC-band operation in a non-WARC band amplifier,
one burn mark appears at the top of the HV-RFC, it's probably due to a
WARC-band resonance, however, when multiple burn marks appear, the
problem is likely VHF resonances absorbing VHF energy. This problem is
easy to confirm with a dipmeter: Measure the choke resonances from 50
- 150MHz. Measure the anode resonances from 50 - 150MHz at the DC
blocker capacitor. If two coincide, odds are that it toasted the
choke.
>
> Some very good examples are the MLA-2500 which had a resonance
> near the 17 meter band and the Alpha 76 series of amplifiers
> that intentionally had a choke resonance around 25 MHz which,
> while it prevented operation on 11 meters, happens to prevent
> the amplifier from being used on 12 meters.
>
> 73,
>
> ... Joe, K4IK
>
>
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>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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