John Becker wrote:
I want to put this amp on the WARC bands, but I've heard that the B&W
800 plate choke is self-resonant somewhere close to 12M, and is likely
to go up in smoke if I try to tune-up on that band. Is this true? If so,
what solutions have been found to be effective? Thanks!
Regardless of the type of choke, you should check it for series resonances
before using it. The following procedure is done with the choke in place, with
the amplifier disconnected from its power source: Short the choke with a wire
or strap across its terminals. Couple a GDO to the choke, and look for dips
from 1.8-30 MHz. The dips represent series resonances, which must be as far as
possible from ham bands. If you find one on 12 meters, for example, you can
move it up or down by removing or adding a few turns to the choke. Most chokes
seem to have one resonance somewhere between 14 and 30 MHz, sometimes more.
I have not ever used a B&W choke, but have had good results on 160-10 with the
Ameritron choke that is sold as a replacement for the AL80 series or AL82s.
Even if you replace the choke, make sure you check it as above (and don't forget
to remove the short before firing up).
You might be able to do this with an analyzer like the MFJ 259 series, but I
haven't tried it. In that case you do not short the choke, but you must
disconnect it from the circuit and connect the analyzer to it with as short
leads as possible. Look for low impedance points. I don't know how accurate
this method is, since it might be hard to do without removing the choke from the
amplifier, which might have an effect on the resonances -- maybe someone who has
done this can comment.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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