> characteristic remains the same. There is a single series resonance in the
> low/mid vhf region, and the impedance stays low either side of it. When I
> next get the bench clear (might be a while), I'll run some plots. Please
> say if there are any particular configurations you (or anyone else) would
> like me to try and simulate to measure.
>
> Steve
Hi Steve,
What I found useful and educational is the measure the
impedances along the path from anode to the switch contacts, and
calculate the amount of current the tube must sink in order to
develop the voltages necessary to cause the switch to arc at
say...100 to 200 MHz.
The 3-500Z, in the fiberglass socket with groundplane on it, has
minimum S21 (or S12) loss at about 180-200MHz, indicating the
grid is "ungrounded" at that frequency. This would be near the
frequency where the tube oscillates in a parasitic mode.
As a matter of fact if you remove the suppression you can measure
the oscillation in some cases where the tube takes off, and it is up
there in that area.
What I found was a steady stepping down of VHF voltage through
the distributed impedances in the tank ans associated wiring at
VHF. Measurements indicated it is IMPOSSIBLE for the tube to
drive the tank at VHF and cause a switch arc.
Of course you nee a real amp to measure. What amplifier do you
have available? If you have one that I have, we can go through the
measurements and compare results on here.
It should be very simple, with the right test equipment, to prove the
possibility of switch contact failures from VHF parasitics.
I maintain contact failures are caused by problems at or near the
resonant frequency of the tank, where driving impedance matches
or nearly matches the tube's optimum operating loadline. It is easy
to get several times the operating voltage of the tube in the tank if
the tank is improperly loaded.
Do you agree or disagree with the above statement?
73, Tom W8JI
w8ji@contesting.com
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