At 12:29 4/20/98 EDT, you wrote:
>Larry, I suspect that many of the published spacing info is going back to
>the AM days as you surmised. Some of the ham companies use caps at their
>rated DC voltage and even mention in the manual that an occassional arc
>is to be expected.....and not from parasitics. Heath, Amp Supply, Dentron
>and maybe others all used the Johnson 154-9 or the OEP clone at voltages
>from roughly 2500 and up to 3200VDC or so under load.
>That cap has .075" spacing and is rated at 3KVDC "peak withstanding
>voltage".
>Heath went to .1" spacing in the SB-221 but then had a high rate of
>switch failures since the cap no longer acted as a spark gap.
>Unless you have a good junkbox it is becoming increasingly difficult to
>find some of those old National, Millen, Hammarlund 6KV breadslicers. The
>Cardwell/Johnson 154 series (and the OEP clones) is only available in
>.03", .045", .075" and .125" spacing. One source of the 240pf, 4500V
>(.125") is Ameritron since they use it in th ATR-15 tuner. MFJ uses the
>same basic cap but dont know if they are still using the phenolic
>insulators.
>
Thanks Carl, this is good stuff to know.
My ARRL handbooks, from 1959 on, all agree on spacing specs with my E. F.
Johnson catalog #964 (1937). My "Reference Data for Radio Engineers" (1977)
says something different; much higher voltage for a given gap. But if you
use the standoff voltage for a needle gap and derate for temperature and
altitude, the numbers are close. Those old timers really liked their safty
margins!
>One setup I have used that works well is two caps side by side, 100pf for
>10-20M and a 250pf or more for 40-160M. It requires an extra switch wafer
>but I can still find switches at fleamarkets. Small Parts Co. has some
>nice sprockets and bead chains for driving the caps.
>
Good idea. I always used a dual cap and took some plates off one section.
>GL Carl KM1H
73,
Larry - W7IUV
w7iuv@axtek.com
www.axtek.com/w7iuv/
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