Re: [Amps] SB-220 parasitic suppressors

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

K7RDX wrote:
> My hf GS-35b AMP has a supressor only in the cathode and has been super 
> stable for the last four years.....

GS-35B amps on 432MHz, 144Mhz and 50MHz are unconditionally stable with 
NO parasitic suppressors at all.  That's with the anode and cathode 
deliberately tuned to the same frequencies, and any combination of input 
and output tuning and loading.

With the grid ring solidly clamped to the chassis, the GS-35B will not 
oscillate at VHF, regardless of *any* input or output resonances.

Exactly the same can be said for the 8877 - another tube that can be 
used in tuned amplifiers from HF though VHF. If its grid ring is 
grounded directly to the chassis, there will be no on-frequency 
oscillation in a VHF amp, and no VHF parasitic oscillation in an HF amp.

(The only exception would be if the feedback path is not through the 
tube, eg due to poor shielding and/or RF bypassing.)

My HF GS-35B provides a kind-of-backhanded proof of this. It has the 
usual parallel L&R in the anode circuit, but  by courtesy of Steve 
G8GSQ's network analyser, we discovered that doesn't provide any 
significant damping at the VHF parasitic resonance frequency! But there 
is no parasitic oscillation... because the grid ring is solidly 
grounded.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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