At 06:23 4/18/98 -0800, Rich Measures wrote:
>
>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.
This should be the case for a class B amp. Class AB would be a little less
and class C a little more due to conduction angle and tank flywheel effect.
Correct me if my thinking is off here. In any case, this is the same rule
of thumb that I have often used.
>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%.
As I see it, the max worst case peak voltage could be twice the "normal"
peak voltage and would occur only at infinite SWR at exactly the right
phase angle to cause an additive function. I may be way off here again, but
since this condition is unlikely to happen with the "wrong antenna"
connected, it could explain a few things different people have experienced.
>-+- 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.
Who rated the capacitor? My reference data shows a spacing of 0.15 inches
is required to achieve a 6000 volt rating. I have yet to see a commercial
amp with a tune C with that wide of spacing. (I don't look in many of them
as they tend to annoy me.) A 300 pF air variable with that spacing is BIG.
BIG = $$$$. Perhaps the real problem here is creative specsmanship.
Please understand that I am not trying to dispute anyones opinions or
experiences. I am trying to figure out how to cut the safety margin down to
a bare minimum in this current project I'm working on. I really want to use
a cap with a spacing of 0.090 in. (about 4000 volts) but I am having
trouble convincing myself to do this even tho the numbers say it's OK.
>. . 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 -
Seems reasonable to me, don't cost anything, might prove benificial, and
you just never know.
73,
Larry - W7IUV
w7iuv@axtek.com
www.axtek.com/w7iuv/
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