[AMPS] parasitics

[Peter Chadwick]

The worst case of parasitics I ever encountered was in a 300 MHz tunnel
diode oscillator; we got non - harmonically related outputs on 5 frequencies
from 10Hz to over 3 GHz, and all the intermod products.

Refering to Terman, he says:-

'In fact, parasitic oscillations can be expected as a matter of course in
any new design of amplifier, either radio- or audio-frequency, or of power
oscillator, involving large tubes'.

I've had VHF parasitics in an 80m VFO using a  5763. I suspect that most
times, we didn't look for them, and they didn't get out. In RF heating,
maybe they don't always matter, and as someone said, the feedback is so
strong that it tends to make the thing oscillate correctly. Additionally,
you're looking for RF heating, so it's the tank circuit energy that gets
coupled to tye workpiece that matters.

 Back to Terman on the subject of parasitic suppressors:' At the same time,
they are not in the tank circuits for normal operation and so have little
effect on the desired oscillations'.

A further quote from Terman:' They are to be avoided because they absorb
power that would otherwise go to generating useful output and because they
also often produce excessive voltage stresses in portions of the circuit'. 

That last quote should bring a smile to Rich - it seems to be what he's
saying!!

I seem to remember that the 30Kw tx done 30 years ago at Marconi's didn't
need parasitic suppressors, but it used vacuum variables, which tend to be
physically smaller and have less inherent inductance.

And if the amp needs opening for servicing, it's worth going round with the
vacuum cleaner and a dust brush, (as carl and Dick suggest) and checking all
the resistors while you're at it. I know 'if it ain't bust....', but
sometimes preventative maintenance saves later problems. If you've got it
open, it takes less time to measure the resistors now than to have to open
the box to do it later. But I'm an idle so-and-so, so anything that saves
work.........


73

Peter G3RZP


73

Peter G3RZP

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