[Amps] SB-220 parasitic suppressors

[Tony King]

At 02:47 PM 11/4/2004, Ian White, G3SEK, wrote:
><snip>
>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.

I'm curious about what kind of mounting fixture you are using.  There are 
two basic fixtures seen in the many HF amps on the web:
1) Directly clamped to the chassis
2) Clamped to a plate which is fastened below the chassis with threaded 
standoffs.

Both of them are popular if the pictures are any indication. The plate 
below chassis using threaded standoffs appears that it would have a much 
larger stray inductance than the other.


>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.

Looking forward to seeing the results of that.

Watching/reading with much interest... enjoying it a lot!

73, Tony W4ZT