>
>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.
** Grids are usually rated in watts, not mA.
>I would have thought that an excessive
>RF circulating current could damage (fuse) what is normally in most tubes a
>fairly delicate structure.
** Grids consist of many conductors in parallel - which results in a low
RF-R. Even though the anode-to-screen - or anode-to-grid circulating may
be quite high at Fmax with max DC anode potential, the I^2 x R heating in
the grid is quite likely to be within the mfg's grid dissipation rating.
For example, the 8938 is rated at 500MHz/4000VDC.
since the anode-grid C is 10pF, (32-ohms@500MHz) and the anode potential
is c. 2600V.rms, the RF circulating current through the grid to ground is
c. 82A.rms. Sure, 82A sounds looneytunes/bonkers, but is it ? The
8938 has over 100 parallel gold-plated grid bars, so the current in each
conductor is under 800mA.
>
>Is this one of the factors that the tube manufacturers take into account
>when deriving their "Maximum frequency for full ratings" figure?
>
** undoubtedly
>
>Regards
>
>David G4FTC
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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