Re: [Amps] SB-220 parasitic suppressors

[R.Measures <r@somis.org>]

On Nov 4, 2004, at 12:23 AM, Ian White, G3SEK wrote:

> 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.

--  But of course --  the only amplifiers that have VHF parasitics are 
those that have two resonances in the anode circuit.   In a VHF or UHF 
amplifier, there is but one resonance in the anode circuit because the 
anode-C itself is all or most of the Tune-C.  HF and MF amplifiers are 
unique in that a separate Tune-C connects to the anode by a conductor 
with a DC-blocker cap. in the middle.   The L in this conductor plus 
the L in the DC blocker -- in conjunction with the anode-C in series 
(via the chassis) with the Tune-C forms a VHF resonant circuit that is  
Not  on the schematic diagram.  
-  note - The lowest anode circuit resonance that I've measured was 
42MHz in a 40kW pep, 12MHz broadcast amplifier and the highest was 
160MHz in a 1, 8873 HF amplifier.
> 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.

During the testing phase, the Eimac engineering team that developed the 
8877 discovered that it was capable of sustaining an "oscillation 
condition" that caused thin layers of the gold plating on the grid to 
boil off and condense into meltballs.  An 8877 has 0.1 pF of 
anode-cathode C.  A GS-35b has 0.12pF of anode-cathode C.
-  note - The presence of loose gold can be non-destructively confirmed 
with a high-pot tester by measuring leakage-I, reversing polarity and 
re-measuring leakage-I.  If the leakage current is higher with pos. on 
the anode than with neg. on the anode, the leakage is quite likely from 
loose gold.  If the leakage is = for both pos. and for neg. the leakage 
is from a tube flatulent or from a bad metal-ceramic seal solder job.  
.
> (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.

Ian -- What is the resonant frequency of the grid in this fixture?  
Does the grid-grounding fixture have zero inductance?
>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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