> With some socket, also the actual method to ground grid is prone to
> have inductance, and inductive reactance yes increases with frequency.
> With proper bypassing capacitors and short leads, like with mica
> multistrate caps, one can aspect that loss (if any) and not reactance
> increase with frequency.
It is a popular myth that adding a series C will decrease reactance
in HF amplifiers as frequency is increased and improve stability.
The normal capacitance in the tube parallel resonating with the grid
inductance is a few pF. Adding a few hundred pF of capacitance
outside the tube barely changes the VHF anti-resonance, the
capacitance would have to be only a few pF to move things at VHF
significantly.
What the capacitance like use in a SB-220 actually does is add
series inductance at VHF up near 150 MHz and higher (where the
tube tends to oscillate) because of the long thin leads and
excessive value of C. This brings the tendency for oscillation down
in frequency, where it is harder to suppress.
If the capacitors were low inductance chips mounted directly
between the socket and chassis, it would be another story. But
adding any long thin leads to the grid is generally a bad idea,
whether in series with a capacitor or not.
The capacitors are just a poor idea that made it into many
amplifiers because no one checked the effects thoroughly.
73, Tom W8JI
w8ji@contesting.com
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