> The resonance of SB220/TL922 grid-ground circuit is given by the parallel
> of few mH and few hundred pF, actually that circuit is never really "open"
> at any frequency because of the low Q caused by the consistent Ohmic
> resistance of the long inductor (litz) wire.
The chokes were various types through the life of the amplifier, but
none I'm aware of were ever Litz wire. Even if they were, the grid
impedance would still have been very high.
> understand why a Gg mounted triode with some series resistance between
> grid and ground should have a bigger tendency to oscillate at VLF (or any
> other frequency) than when the grid is straightly grounded and gain
> increased.
1.) The Pi network is a lowpass that looks almost transparent at
low frequencies.
2.) The blocking capacitor can be made to present a wide range of
impedances at the anode with an inductive load (which is
independent of the SWR on the operating frequency) at the output.
> low reactance at VLF (bypass capacitors have neglectable reactance) so,
> unless you have a PS with high internal resistance, the tube plate is
> dynamically closed on a very low load expecially at DC and VLF.
Not when parallelled by a low reactance Xc caused by series
tuning the blocking cap with external inductance (high side of
resonance).
Putting a choke in a grid, especially without a low resistance
shunting the choke, is poor engineering.
It commonly creates problems. Look at the history of mods to the
30L1 Collins, to see how long it takes people to figure that out.
73, Tom W8JI
w8ji@contesting.com
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