[Amps] grounding grids

[Peter Chadwick]

Shielded ground leads can be very useful - but you must not connect the shield at both ends. They're used like that a lot in TEMPEST work.
But because the s parameters (or y or z) vary with frequency, especially in terms of phase, one expects the performance to change as the feedback changes. After all, this is why the input resistance of an amplifier tube drops with frequency - see Langford-Smith et al. If feedback changes, one expects linearity to change.
As for how myths start, look at the G2DAF....

73
Peter G3RZP