[AMPS] SB-220 Smoke Test

[Ian White, G3SEK]

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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