[Top] [All Lists]

Re: [Amps] SB-220 parasitic suppressors

Subject: Re: [Amps] SB-220 parasitic suppressors
From: "Ian White, G3SEK" <>
Reply-to: "Ian White, G3SEK" <>
Date: Thu, 4 Nov 2004 19:47:46 +0000
List-post: <>
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) _______________________________________________ Amps mailing list

<Prev in Thread] Current Thread [Next in Thread>