Eric the Magnicicent:
^^ To measure the resonance of a 3-500Z grid, I used a dipmeter and
grounded one grid pin with a Cu strap to make sure I was not measuring a
pin-to-pin resonance.
-- What was the resonance of the 833A's grid?
tnx
>...
>I have some unfinished business here,
^^ agreed. Do you know W8JI?
>regarding g-g amplfier input circuits.
>When I attempt to read the postings I am somewhat frustrated by a certain
>vagueness to the language. I do not believe in or participate in arguments
>which are sematic only - if there is a REAL issue, the language problems can
>always be eliminated. So in the present case I will simply post my own
>comments, some of which are facts and some of which are opinions. I will be
>very careful to distinguish bwteen the two, and to make the intended meaning
>of my statements as precise as possible.
>
> First, there are no resonances in the grid structure of a 3-500Z at
>anywhere near 90MHZ. This is a FACT which I obtained by measuring two
>different tubes, using three different measurment techniques. I would like
>to
>dwell for a moment on this technique(s), since it is of prime importance
>here.
>
> When you look at the base of the tube, it is clear that the shortest
>possible lead from grid-to-cathode (i.e. the "grounded grid" lead itself)
>will be about 1" long. This immediately implies a lower limit on the amount
>of external circuit inductance to the range of 25nH. or so. In other words,
>any conceivable circuit will have at least that amount of L. in series.That
>situation also applies to the measuring fixtures which I used. I was able to
>fabricate interconnect leads to my instruments which were in the 1" range
>(total length.
>
> The three different instruments I used were: 1. An HP4815A Vector
>Impedance Meter, 2. An HP 8407A/8601A Network analyzer, and 3. An HP 8920A
>Communocations analyzer.
>
> On the direct Z measurment (4815) the Grid-Cathode circuit, as a
>terminal pair with a driving point impedance, read -j120 (negligible R) at
>100MHZ. This translates to about 10pF opf capacitance. I was able to
>sweep F
>from 40-125MHZ (by means of some of my own proprietary enhancements to the
>4815). As I did that sweep manually, there were virtually NO signs of any
>resonant behavior. The thing just acted like a perfect capacitor!. The slope
>of the dX/dF curve, which gives some insight into the inductance of the
>circuit, also was consistent with an extremely low value of Linternal.
>
> To cross check with the NA's, I simply fed the terminal-pair thru a
>50
>ohm source resistor and looked at the output amplitude/phase response. Since
>the impedance happens to be not very from 50 ohms (although it is fully
>reactive) this is an acceptable way of doing quick and approximate searches
>for resonances. For example, connecting a short (5-20') piece of coax in
>place of the tube shows the very market resonances at 1//4-lambda lengths.
>Well, once again, no hint of resonance in the tube. With the 8920A, in the
>Analyzer/Tracking Generator mode, I was able to sweep up to 1GHZ.
>
> Conclusions: 1. The tube does not have any resonances visible up to
>over 200 MHZ. I don't really know what happens at that frequency and beyond,
>except that the conditions for a lumped-circuit analysis are probably not
>valid any more. Trying to establish a meaningful terminal-pair to which an
>impedance can be assigned becomes progressively meaningless.
>
> 2. The tube does act every bit like a 10pF capacitor. When you use it
>in a GG amplifier, your "grounding" lead inductance had better be less than
>100nH or so. This is not too hard to achieve. Recall that the entire L
>required for a 6Meter PInet tank circuit may be 300nH .
>
> 3. If you are not careful to do this you almost certainly will have
>an
>unstable amplifier.
>
>Eric von Valtier K8LV
>
>P.S. I also tried 811's, SV572's and an 833A. The latter has a minimum G-K
>terminal path of almost a foot, and would resonate exactly as per simple LC
>theory....
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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