[Amps] SB-220 parasitic suppressors

[R.Measures]

On Nov 4, 2004, at 11:47 AM, Ian White, G3SEK wrote:

> 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,

Ian --  I know of VHF amplifiers that oscillated, but there were no 
fireworks.

> 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.

I have autopsied too many tubes that were removed from HF amplifiers to 
agree

I had a 4cx250R 70cm amplifier oscillate, and a friend had a 8938 70cm 
amplifier oscillate. However, in each case there was no damage because 
the oscillation's energy simply passed into the load.  - note - In each 
amplifier, the problem was solved by using fewer pF in the HV bypass-C.

  -  HF amplifiers suffer damage from parasites because there is no way 
for the VHF energy produced thereby to reach the load.
>
> (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,

Agreed.

> 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.

Yea, verily.

>
> 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.
>
makes sense

> 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.
>
Excellent idea.
-  Since the grid-resonance of a 3-500Z is c. 88MHz, has anyone tried 
lengthening the anode-lead to move the anode-resonance below 88MHz?
>
> -- 
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org