>
>
>> 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 TL-922 bandswitch seemingly arcs at roughly 120MHz. The contacts
that are the most subject to arcing can be easily spotted on page 15 in
the October. 1990 *QST*. The most severely burned contact is the 10m L
tap.
>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
? NB
>for the tube to
>drive the tank at VHF and cause a switch arc.
>
>Of course you need 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.
In a SB-220, max. mistune test, it was not possible to push the Tune-C
potential above 3600v peak.
>
>Do you agree or disagree with the above statement?
>
Your statement is at odds with the performed measurement on the 220.
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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