[Amps] RF Grid Current

[David G4FTC]

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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