> In the meantime, I've been thinking about just what
> specific application(s) such capacitors might have
> available for them, since they're potentially so
> meddlesome in the way I had used them here: are they, in
> essence, good only for BYPASSING purposes...? And if so,
> then why would manufacturers even bother to make variants
> of these things, like the "N750" units that I tried
> here...?
The reason ceramic transmitting doorknobs have such high
negative temperture coefficient is the expansion of the
ceramic, not the design. They would like to have a NP0 or
temperature stable type available, they just can't. The
higher the capacitance and voltage the worse the drift
becomes.
Many times people blame drift on exceeeding the current
rating but that isn't always true. If you look carefully at
the real ratings you will see they are higher than is often
claimed. The real problem is temperature stability in tuned
applications.
For example
http://www.highenergycorp.com/ceramic/ht50.html
shows the operational current (when not limited by voltage)
can be operated at 75 degrees C and still have a 10 degree C
margin. The working current can be higher than the maximum
current rating if the capacitor is cooled and doesn't above
75 C.
They're obviously over-rated for the likes of padding in
low-powered VFO circuits --- so what good are they,
specifically...?>
Tank circuits. If you have a drift problem you just use a
lower capacitance part that has a lower temperature
coefficient. A pair of 170pF doorknobs in parallel is much
more stable than a 250 and 100 doorknob in parallel.
> Now, here's the question that REALLY begs an answer: if
> capacitors of this ilk are SO inappropriate for use in
> QRO-type applications due to internal heating, their ity
> to handle excessive RF current, etc. etc., then just how
> appropriate are they for use as output coupling
> capacitors --- i.e. the blocking cap that's placed between
> the final tube(s) and the pi-network tuning circuitry...?
The nice thing about that is the current through the
capacitor is lowest on the lowest bands where the capacitor
might handle the lowest current. It only occasionally
becomes a problem on the upper end... near ten meters.
>I know that components placed here must handle quite a
>load: years ago, when I built my first amplifier (4 x 811A)
>I naively inserted a simple 2.5 kv (if I recall correctly)
>0.001-ufd. disk ceramic capacitor in that position ---
>whilst undergoing tests with the linear, the little cap
>literally exploded like a small grenade, shooting ceramic
>"shrapnel" all over the place!>
The current rating of that .001uF disc could have been less
than one ampere. It also could have been defective.
> Is the door knob coupling capacitor in my amplifier yet
> another candidate for some future reoplacement with a
> vacuum cap...?
Not unless you run 10-20kW.
Don't confuse drift with temperature change with excessive
current or potential to fail. Odds are you simply picked
very unstable values and only had a little bit of
temperature rise.
Lots of amplifiers running a lot more voltage and current
than you have live for years and years with small
transmitting doorknobs shunting the tank capacitors.
73 Tom
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