>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?
>
It seems to me that the actual piv across the Tune C is equal to the
anode supply V minus the minimum anode/to/chassis-gnd V during the peak
drive signal. For a 3-500Z or an 8877, this is about 250v. Thus, with a
4000v supply, the Tune C would see about 3750v-p. However, more
piv-capability in the bandswitch is not necessarily good Because the Tune
C needs to protect the bandswitch from arcing during glitches. In other
words, a Tune C should have a lower piv-capability than the bandswitch.
The TL-922 uses a (approx.) 6000v Tune C and a 6000v bandswitch.
TL-922s tend to eat bandswitches. {Under normal operating conditions,
the actual piv across the bandswitch is 3100v minus 250v = 2850v.}
>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.
During the grate debate on vhf parasitics between yours truly and Mr.
Rauch, someone measured the piv on a SB-220 Tune C during deliberate
mistuning. (the measurement was done with a voltage-divider and an
oscilloscope). . The result was 3600v -- which reportedly did not cause
an arc in the Tune C. I tried the experiment in my SB-220, and I could
not make the Tune C arc with the wrong antenna or wrong Tune C setting.
- In other words, the approx. mistune/misload piv on the Tune C is the
anode supply V plus 20%.
-+- The $64 dollar question is how is it possible to arc the Tune C in
a TL-922? The anode supply is 3100v. Adding 20% makes a bit under
3800v, yet the Tune C withstanding ability is 6000v, yet some TL-922
owners reportedly experience intermittent Tune C (and bandswitch) arcing
when the right antenna is in use and the amplifier is not being retuned.
. . The $128 dollar question is why do some folks get tight-jawed when
an old-fart in Somis suggests measuring the resistance of Rs in the vhf
suppressor of a Tune C-arcing amp.?
. . . "Everything is more complicated than it looks". - Murphy -
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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