Mostly correct as general statement but also some tubes mechanical
construction (internal electrodes or wires, socket dimensions and
shape) lead to inherent stability problems and the best RF designer
can't do anything else than trying to lower the amplifier gain at
critical frequencies.
On the other hand I've seen a number of professional grade HF tube
amplifiers totally without any VHF suppressors personally verifying
they never had any single problem, including those that might be caused
by VHF parasitic.
73,
Mauri I4JMY
> For myself, once I began designing with valves whose dimensions were
less
> than 1/8 wavelength at my operating frequency, I determined that all
> instances of parasitic oscillation were due to poor component
layout. Pay
> particular attention to where ground currents flow. That tuning
capacitor
> has a grounded end. Where does that ground current need to flow? To
the
> grid of the grounded-grid valve! Is that distance short? What about
the
> load capacitor? Do you know where that ground current must flow?
Yes, to
> the output connector! You see, star grounding is not necessarily
correct
> all the time. Just because a component has a short connection to the
> chassis does not mean it's well grounded.
>
> Whew. Enough for me for now, I'm at risk of being called the
Deuteronomist.
>
> FF
>
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