At 09:37 5/19/98 +0000, Tom Rauch wrote:
>In fairness, here is how I do the test:
>
>1.) No load on input and output, and I use an external L/C circuit or
>stepped transmission line (at VHF) to rotate phase of input and
>output terminations while testing.
>
>2.) I bias the tube into and conduction with AC bias while testing.
>
>3.) I rotate all controls on all bands through all positions in
>small steps.
>
>4.) I apply about 25% more test voltage than operating voltage. (In
>solid state PA's, I test with less than operating voltage as
>well!)
>
A comprehensive suite of tests, to be sure!
But in 40 years of ham radio, I have never heard of a ham running a load
pull on a homebrew amplifier at home. Interesting to note how
professional/commercial experiences color our views as amateurs. And
sometimes, vice-versa.
>I have never found a solid state PA "unconditionally stable" in my
>test standards.
Or anybody else's either by my admittedly limited SS amp experiences. But
some are close.
>> How do you measure grid circuit resonances? It's easy in the anode circuit
>> (big parts, lotsa lead), but have never had much success with the grid. I'm
>> sure it's because I try to build them all like they were GHz range amps,
>> and there is virtually no lead length to couple my GDO to.
>
>Simple. Measure an effect that closely ties to the parameter you want
>to know but can't measure. Measure the coupling from anode to cathode
>using the grid(s) as a shield.
>
>Pull all the connections, and couple a low source impedance RF
>voltage to the cathode. Measure that voltage at the cathode and at
>the anode (nothing there but a resistor load) and plot
>transmission of power vs frequency.
>
>By playing with the grid connections, you can see the effects of grid
>IMPEDANCE.
Good suggestion and one that I sometimes use variations of in my lab at
work. With the virtually unlimited resources at my disposal on the job, I
could easily make repeatable measurements and judge the contribution each
component has by simple substitution. Unfortunately, I do not have access
to that equipment at home. I don't have a swept source or even a leveled
source or good detection device. Like most of the HAMS out there, I am
trying to do this stuff under barbaric conditions. So, as good as your
suggestion might be, it's unuseable to me.
>Resonance, as measured with a GDO, tells you nothing at all except
>where something external to the GDO couples to the GDO. You have no
>idea whether impedance is high, low or what.....or even if the
>resonance is what the grid actually "sees".
True, but it might just be all you have to work with.
I was hoping for a bit of witchcraft maybe, but thanks for the reply
anyway, Tom.
73,
Larry - W7IUV
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