> Designing a cylindrical power amplifier cavity at 100 MHz, using the
> 4CX3500A tetrode series (also tried the 3000A and the 5000A tubes), I
> 'picked' a geometry which supported a mode near the 2nd harmonic. Computer
> models didn't show this, as they were using pure TEM mode calculations,
> such as ladder analysis using ABCD matrix transmission line
> approximations. There was apparently enough feedback through the tube and
> gain at 210 MHz that it could come back in phase to make an oscillator.
> This essentially made a parasite which would be excited when driven by the
> 2nd harmonic. The amplifer was class C.
Class C amplifiers biased amplifiers, like class C biased
oscillators, are cut off when there is no drive. Since the tube is cut
off, it won't oscillate.
Drive the tube, and it will turn the oscillator on.
The proper way to test a PA for stability is to bias it into
conduction, and load it with all possible phase angles and
impedances on the input and output that it will see in operation.
I'd bet money your PA would have oscillated like a champ if it was
tested that way.
There is no such thing as an oscillator that requires excitation on a
harmonic related frequency to start it, that won't start on it's on
when the amplifying device isn't cut off.
73, Tom W8JI
w8ji@contesting.com
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