On Fri, 17 Apr 1998 23:10:28 +0100 w7iuv@axtek.com (Larry Molitor)
writes:
>This thread about arcing tune caps comes at an opertune time for me.
>Progress on the latest amp project here has slowed while I root around
>in
>the very bottom of the barrel for a tune cap. I was going to use a
>nice vac
>variable but have been unable to find a drive unit for it. Now the
>problem
>is selecting a suitable air variable.
>
>In the last five amps I built, there has been no tune C arcs in spite
>of
>more than my share of "cockpit trouble". Wrong band, wrong antenna,
>wrong
>drive, wrong plate voltage, nothing can zap the switch or the cap.
>This is
>probably because I have in the past always selected the output
>components
>based on the design criteria (AM?) in the older Handbooks. (ARRL and
>Orr's)
>
>In this project I don't have the luxury of using up-sized components.
>Looking at the various commercial amps out there, the spacing on the
>tune C
>is usually about 1/4 what mine is for the same power level. My
>question is:
>How does everyone else out there determine the spacing and/or
>breakdown
>voltage rating of the tune C?
>
>How about some hard facts and formulas? I really don't care about
>statements like "Very high voltages can occur during unloaded
>conditions".
>I have tried to derive the requirements and each time I come up with a
>max
>possible peak to peak RF voltage that's a little over twice the DC
>plate
>voltage under worst possible case. This seems like it's too little
>based on
>the recent discussions about arcing.
>
>Thanks and 73,
>
>Larry - W7IUV
>w7iuv@axtek.com
>www.axtek.com/w7iuv/
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.
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.
GL Carl KM1H
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