Perhaps some of the gurus can enlighten me. This is something which I've
never seen referenced in any tube manufacturer's publications, but I've
always wondered if it would ever be a potential problem and possibly
limiting factor in VHF/UHF amplifier design.
In a VHF or UHF tube amplifier, the tube's output capacitance normally forms
the dominant tuning element within the output tank circuit with a smaller
value external capacitor to provide fine tuning - usually in the form of a
flapper or disk.
At VHF or UHF, these capacitances are such that the output tank circuit will
invariably have a high Q. This Q will result in a high RF current
circulating within the tank circuit with most of this circulating current
flowing through the tube itself. Within a tetrode, it is the screen grid
which forms part of this circuit.
Whereas the screen grid has certain D.C. dissipation limits as per the tube
manufacturers datasheet, I have never seen any mention of the maximum A.C.
(RF) current ratings for the grids. I would have thought that an excessive
RF circulating current could damage (fuse) what is normally in most tubes a
fairly delicate structure.
Is this one of the factors that the tube manufacturers take into account
when deriving their "Maximum frequency for full ratings" figure?
Regards
David G4FTC
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