Peter Chadwick g3rzp at g3rzp.wanadoo.co.uk
Wed May 31 12:30:29 EDT 2006

Provided that the grid parasitic tuned circuit is resonant at a higher frequency than the anode parasitic tuned circuit, the amplifier will be stable. Unless Terman got it wrong........
This suggests that well grounded grid through say a collet ring fixture may well not need an anode parasitic suppressor if the rest of the design is right - and that may need all sorts of niceties taking into account, such as the amount of shielding adding capacity to the anode circuit.. It also suggests why 572Bs are such wotsits to keep free of parasitics, they having a lengthy grid structure.
Very often, parasitic suppressors are fitted on a 'belt and braces' ('belt and suspenders' in American English!) basis, and are determined experimentally - even on big transmitters. I suppose one of the EM modelling programs could predict the result if you had good enough models of everything in the box, but the usual approach may well provide suppressors where they aren't needed.
Peter G3RZP

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