Re: [Amps] SB-220 parasitic suppressors

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

R. Measures wrote:
> 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.  .

Neither of those responses addresses my point: if VHF amplifiers using 
these tubes are stable under worst-case conditions, where the input and 
output tanks are unloaded and tuned to the same frequency, then it is 
unreasonable to expect HF amplifiers using the same tubes to be liable 
to VHF parasitic oscillations.

(Still provided that the grid ring is solidly grounded to the chassis, 
and all other feedback paths have been eliminated by shielding and RF 
decoupling.)


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

Nothing has literally zero inductance, but the GS35B is a planar triode 
with a shallow domed grid which is 360deg bonded to the grid ring. The 
grid ring is dropped onto a hole in the chassis, and then clamped down 
with 360deg contact. Grid inductance in a large power tube doesn't get 
much lower than that.

The resonant frequency where the grid becomes totally ineffective is 
probably some way above 1GHz... which is precisely why the tube is so 
reluctant to oscillate at lower frequencies.

Much the same applies to the 8877, but this is not such a high-frequency 
tube. It has coaxial construction with a tubular grid, and this will act 
as a tuned line, loaded by the anode capacitance. Quarter-wave resonance 
of the grid structure would be at a several hundred MHz (at a guess), 
but this is still high enough to ensure that - again, always, with solid 
grounding of the grid ring and no external feedback paths - VHF 
amplifiers using the 8877 are unconditionally stable.

If you want to know where the grid resonance is, you'd have to put the 
tube (cold) into a fixture with totally shielded cathode and anode 
compartments. Ground the grid in your chosen manner, and then make a 
frequency swept measurement of the RF leakage between the anode and the 
cathode. The grid resonance should show up very clearly as a peak in 
transmission.

Actually, that would be a very useful set of measurements for someone to 
make. Start with a 572B in a regular socket, and work upward to the 
VHF/UHF tubes with a solidly grounded grid ring.


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