[AMPS] Re: RF choke forms

[Steve Harrison]

At 05:26 PM 2000-01-15 -0600, Roy Koeppe wrote:
>Most modern "enameled style" magnet wire is rated at 2 kV insulation.
>But lets take just 1 kV, and use a solenoid plate RF choke example with
>100 turns. With 3 kV on the plate, divided by 100, that's only 30 volts
>per turn, and you have 1 kV of insulation facing another adjacent turn
>also rated at 1 kV. So it's unlikely that a properly functioning plate
>choke could ever arc between turns, with an effective 2 kV rating
>between them? Quite a safety factor, 30 to 2000.

3 kV DC across the plate choke? I think you'll have other, more expensive
worries before that became a problem.

Consider the peak anode RF voltage, particularly at odd-order harmonic
frequencies. Although the power at those harmonic frequencies may be 30 dB
lower than the fundamental, there's no matching and thus no loading.
Remember that the IMD of an amplifier is directly related to its harmonic
generation. You don't actually see the third harmonic at -30 dBc only
because of the relatively-narrowband resonant plate tank. But if you have
ever compared the two-tone IMD level to third harmonic level of a wideband
amplifier, you would have noticed that the third harmonic level could have
been predicted from the calculated third-order intercept point based on the
close-in IMD measurement alone, within several dB.

Eimac's "Care and Feeding of Power Grid Tubes" gives several examples of
calculating harmonic currents and voltages in the plate tank.

Worse yet is the situation where the plate choke or a portion actually
resonates at the third harmonic. Most of us have learned to grid-dip chokes
for resonances within the bands; but not many of us also look for them far
above the bands. That's why the staggered-design of some of the
more-effective and reliable plate chokes: to break up VHF resonances.

That enamel might not be as effective at VHF, either, particularly since
more RF current flows on the skin of the wire, thus warming the enamel.
Then there's the deformation factor from winding the choke tightly. And
nicks and scratches in the enamel from the winding operation.

No, doing a good, reliable plate choke really is an art form. And not all
plate chokes are suitable for all amplifiers.

73, Steve K0XP

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