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.
I've also some trouble to 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.
In any case, plate choke (expecially in a VHF/UHF amp) has very 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.
VHF/UHF half wave lines don't play any role than any other lead wire inside
the amplifier.
73,
Mauri I4JMY
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.com>
To: <amps@contesting.com>
Sent: Wednesday, September 27, 2000 4:48 PM
Subject: Re: [AMPS] SB-220 Smoke Test
>
> measures wrote:
> >>
> >>Did you actually mean to say VLF parasitics?
> >>
> >? Just how this is possible is somewhat less than obvious.
>
> VLF resonances are *the* main cause of parasitic oscillation in VHF/UHF
> power amps. They generally arise because half-wave tuned lines become
> open-circuits at low frequencies, so that both the input and output of
> the tube are unloaded. There are then possibilities for oscillation due
> to unwanted resonances between RF bypass capacitors and lead inductances
> or RFCs in the power supply and bias circuits. Another useful function
> of the "glitch resistor" is to damp any such resonances in the B+
> circuit.
>
> Returning to the SB-220 and similar amps, what is the parallel resonant
> frequency of the grid bypass cap and the RFC? Whatever it is, the grid
> is totally ungrounded to RF at that frequency.
>
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.com/g3sek
>
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