[AMPS] parasitics

[Peter Chadwick]

I guess the nice part of Monday morning is catching up with what's happened
on the reflector over the weekend!

Guess this is where I stick my neck out....................

I believe that under the heading 'parasitics' we talk about 2 different
things.

1. 3-500's and the like in g-g with bent filaments etc
2. devices with boiled gold on the grids.

Case 2 I am quite prepared to accept as 'classical' VHF/UHF parasitics.

Case 1 I am not so sure about. Just suppose that we have  several amps of
VHF. How does it bend the grid/filament? By definition, it's AC, so the net
mechanical force just makes it vibrate. But because of the inertia of the
metalwork, it's unlikely (!) to vibrate at 100MHz. On this basis, the
'flash-arc' or Rocky Point effect seems more likely.

Resonances.

Does it matter how many of these resonances there are? Provided:

1. they shouldn't be at a hamonic of the operating frequency. If they are,
then because they are almost certainly unloaded, you can get pretty massive
voltages built up.

2. just because there's a VHF resonance doesn't mean that there will be a
parasitic oscillation. There has to be feedback and gain.

Obviously, in a practical case, it's desirable to minimise the number of
resonances, because of the possible problems, but even so, one resonance may
not cause parasitics per se.

I've never understood why calculations on parasitic suppressing resistors
always seem to give lower dissipations from the fundamental current than
practice does. Maybe it's me........

Solid state v Tube amps.

The biggest problem solid state is the matching, i.e the need for a low SWR.
Have this go wrong, and the voltage breakdown can be exceeded. At HF, that's
probably good bye. {There are lots of arguments about how long you can
exceed the voltage breakdown for before it breaks down, and there are two
semiconductor physicist here with opposing views} Plus the fact that filters
to meet spurious (in this case, harmonic) specs are by no stretch of the
imagination negligible affairs. A push pull AB PA has a third harmonic 13dB
down at full bore, so the amps in the capacitors is a major factor. Just to
complicate matters, the internal feedback can cause oscillations if the
filter shows the wrong impedance at a harmonic frequency - sometimes, you
need a filter that goes high impedance at the harmonics, not low. Another
point is static discharge. I learnt the hard way to arrange my amplifier so
that the ant relay is energised for receive - lightning induced emp kills
receiver FETs much more easily than it kills big tubes.

Add the costs, the PSU problems, the cooling problems.........I don't
believe that the KW amp is that cost effective for home brew. Commercially,
provided you cover all the above problems, you've a winner - if it's cheap
enough.

>>So I guess I am a schlock designer!  

Jon, what's a schlock, and what's the design spec for it??  Hi,hi.

73,

Peter G3RZP


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