> Tom you said:
>
> "If the tube has enough voltage breakdown to handle the
> fundamental RF voltage without problem, we can be sure it will
> handle almost any spurious oscillation."
>
>
> Is this really true? The unloaded anode rf voltage swings down in an amp
> that oscillates with a parasitic and may hurt the cathode.
Well, anything that can be done at 200 MHz can be done at 1 MHz.
The tank Q is higher at HF than it is at VHF in a typical HF PA,
and the HF stuff dominates the anode current. Not only that, anode
load impedance is generally higher at HF than anyplace in VHF.
You can bet if the tube insulation fails at VHF when the load Z is
nowhere near as high as at HF, it will also fail at HF where peak
anode voltages can regularly exceed 1.8 times the dc supply
voltage. Under conditions of mistuning, it can be several times
supply at HF because tank Q goes into the dozens or higher.
This parasitic stuff is almost a fetish!
73, Tom W8JI
w8ji@contesting.com
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