|Subject:||Re: [Amps] SB-220 parasitic suppressors|
|From:||"Ian White, G3SEK" <G3SEK@ifwtech.co.uk>|
|Reply-to:||"Ian White, G3SEK" <email@example.com>|
|Date:||Thu, 4 Nov 2004 19:47:46 +0000|
R. Measures wrote:
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
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.
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