At 11:40 AM 5/13/98 -0800, Rich Measures wrote:
...
>In a SB-220, a max. mistune potential of 3600pk-v was measured.
>>
>>The peak anode voltage can swing below the zero rail of the
>>chassis (negative), as well as greatly exceed twice the dc HV.
>
>not according to the above measurement
>>
>>It is possible, by unloading a tank in a conventional PA, to generate
>>voltages of many times normal operating voltages. Underloading of a
>>tank is the single largest cause of tank circuit arcs, and is a
>>repeatable and measurable cause source of damage.
>
>Underloading will not cause a Tune-C arc in my SB-220 or TL-922. The
>breakdown potential of a SB-220 Tune-C is c. 3600v, and for the TL-922,
>c. 5900v.
I have repeatably (not repeatedly, or at least not very often) demonstrated
tune-C arcing in my SB-220 in the SSB position. It's an earlier unit, with
the narrower-spaced tune-C. In all cases, the arcing happened as a result
of underloading, or antenna selection error. I have never experienced
arcing when the right antenna was connected and the loading was set just
past maximum output. The amplifier is well-constructed and stock except
for the Harbach metering/bias board and KM1H's parasitic suppressors.
The same errors also produced arcing in a TL-922 I used years ago at 4U1WB,
but I can't swear that it was the Tune cap, because you can't see into a
TL-922 as well as an SB-220. The amp ran fine throughout an entire WPX CW
once I worked out the procedural issues, with never a wiggle.
For what it's worth...
73, Pete Smith N4ZR
In wild, wonderful, fairly rare WEST Virginia
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