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Re: [TCML] VTTC-tuning. John got me started ;-)
Bart
Nothing silly at all concerning this issue. John F. just explained
part of it. Here is the other part.
Every tube has a set of characteristics that are defined by the maker
usually referred as the "characteristic curves". Fortunately for our
common transmitting triodes this operating data is all readily
available.
Output impedance is an important issue for optimal performance. For
example, consider an audio amp that matches to 8 ohm speakers. If you
use 16 ohm speakers you will still hear sound but loose a lot of power.
Where did it go? In the output stage of the amplifier as heat loss.
It is not quite as dramatic with transmitting triodes, but this is how
you get as much energy as possible in the secondary. It is just old
ham radio technique. When you keep removing the energy from the tube
with a proper impedance match in the tank circuit then the secondary
removes it from the primary (resonance) and it then becomes corona or
a streamer. If all is balanced, all works as well as you can get. If
it is out of tune something typically gets hotter than it should,
usually the tube.
Of course, none of this has anything to do with over-volting the tube.
That can be quick death.
VTTCs can be portable, but they do behave different from surrounding
capacitive elements. Sometimes it actually helps.
A side note on secondary resonance. I use earth ground on the
secondaries ( an old water pipe feed no longer tied into the house
supply) on all my coils. I have too much cash in my computer gear
upstairs to risk. My DRSSTC also resonates at 250 KHz and is separated
by about 6 feet from the VTTC. If I put a 3/4 brass ball on the center
DRSSTC breakout point, the secondary of the VTTC starts to produce a
good 6" corona before breakout starts on the DRSSTC. Sometimes I just
feel like a kid watching E & M come to life.
Hope this helps.
John W. G.
John W. Gudenas, Ph.D.
Professor of Computer Science
On Mar 31, 2009, at 8:25 PM, bartb wrote:
Thanks Dr. John.
That is a fantastic list of tuning issues which should come in handy
for VTTC coilers. If the top terminal is making that much of a
difference, then it is certainly a finicky coil (it may be quite
difficult to make a tube coil portable). The tuning issues
themselves are fascinating to me from an LC aspect. I've noticed
this need to detune with VTTC's which is why I asked the question.
It appears there is an inductance or capacitance that is in series
or parallel with one of the coils (not related to a resistive
component). Is there an LC aspect regarding the tube itself? I'm not
a tube guy, so that question may seem silly.
Regards,
Bart
SNIP.................................SNIP
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