>
>
>
>
>
>
>> At higher frequencies, where given
>>a typical tube the plate C
>> represents a significant part of the
>>plate tune cap, the effect is to
>> significantly raise the plate
>>impedance (as seen by the Pi-Net input)
>> and the balance of the pi-net would
>>need to be adjusted to maintain
>> the desired Q & impedance ratio.
>
>Not true. It sounds "nice", but it is
>not factual. In truth, the
>impedance barely changes.
>
// Yea, verily.
>Let's assume the blocking cap, at the
>lowest frequency, would be
>set a reasonable value of 10% of the
>impedance looking into the
>tank, or approximately equal to the
>value of the tuning cap with a Q
>of ten (using the simple but not
>absolutely correct value of Rp/Xc.
>
>Using an Xc equal to 10% of the plate
>operating impedance, and
>assuming a choke somewhat larger than
>the plate impedance, the
>change in impedance looking into the
>tank is:
>
>300pF blocking (~300ohms 1.8 MHz)
>2878ohms
>
>1200pF blocking (~75 ohms 1.8MHz)
>2964ohms
>
>The tank input impedance, with no other
>changes except a
>readjustment of the tuning cap by 2 pF
>to compensate for the
>reactance change, changes less than 100
>ohms out of 3000
>ohms....a totally insignificant change.
>
>As I said, I learned my lesson many
>years ago when I laughed at
>someone for reading a blocking cap wrong
>by a factor of ten in a
>160 meter amp. After poking fun at him,
>I was amazed when we
>changed the capacitor to a "traditional"
>value of 1000pF and
>nothing we could see changed.
>
>People waste far too much time worrying
>about enough blocking
>capacitance, and spend far too little
>time considering current
>ratings of the capacitors ...
// Excellent points. The popular ITT/Jenning/Centralab 1000pF /5kV 50
- 58 (smaller-series) doorknob cap is rated to carry 3.7A-rms at 30 MHz.
The 100pF version is good for 9.2A-rms. With a pair of 3-500Zs at 30MHz,
the circulating current through the DC blocker can easily be more than
4A-rms.
>and the plate
>choke design.....which has a
>MUCH larger effect than the blocking
>capacitor on system Q and
>other problems.
>
// However, if the Tune-C has enough C at the lowest freq. of operation,
it can be adjusted to cancel the XL of the HV-RFC. Provided that the RFC
can carry the RF circulating current, all is well.
>And the small effect above is on 160
>meters, where the problem is
>at its worse point. On higher bands, the
>effect would be less.
// True
"I know that most men, including those who are at ease with problems of
the greatest complexity, can seldom accept even the simplest and most
obvious truth if it be such as would oblige them to admit the falsity of
conclusions which they had delighted in explaining to colleagues, which
they had proudly taught to others, and which they had woven, thread by
thread, into the fabric of their lives." - L. Tolstoy
> ...
congrats
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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