[AMPS] SB-220 Smoke Test

measures 2@vc.net
Wed, 27 Sep 2000 17:42:57 -0700


>
>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.
>
defending the ''expert''

cheers, Mauri
>
>----- 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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>


-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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