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/
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