>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.
>
Only for linear amplifier service. For a. tetrode, the minimum anode
voltage is close to the screen potential. For a pentode, roughly half of
the screen potential.
>
>>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.
How did you arrive at the 2x figure?
>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?
I measured it with a high-pot.
>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 = $$$$.
The 922 Tune C is c. 150pF. They run padders on 80m and 160m.
>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.
>
In the last two cases of arcing Tune caps reported here, the measurement
was not made.
- later, Larry
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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