On Friday 08 April 2005 12:07, you wrote:
> On Apr 7, 2005, at 11:57 PM, Steve Thompson wrote:
> > Steve Thompson wrote:
> >
> > Sorry, should have said this was on a 8877.
> >
> >> A had a few minutes before leaving the office yesterday, so I fired
> >> up the network analyser. I made some rough, not precision,
> >> measurements and, as these were done on a cold tube lying on the
> >> bench I'm uncertain as to how useful they are.
>
> The 8877 needs to be installed in a socket on a flat metal sheet - with
> the grid grounded via an Eimac grid-grounding collet. The needed
> measurement is to find the freq. where grid-Z is maximal.
If someone can provide the fixture, I'll happily measure the impedances. I can
report that attaching small and large sheets of metal to the grid ring while
measuring the cathode-grid impedance makes a difference that's smaller than
my hand wobbling as I hold the probe in place.
Mimicing poor grid grounding by adding .5" of 16swg (14awg) wire between the
grid ring and the measurement system ground shifts the low impedance point to
about 150MHz and the high impedance one to about 1500MHz while the
feedthrough increases to a fairly flat -40dBish from 200MHz to 1.5GHz. My
guts tell me that's not what I'd expect to see in something that's prone to
hooting at UHF.
Steve
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