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Re: My VTTC fun begins...
Original poster: FutureT@xxxxxxx
In a message dated 1/24/07 11:32:52 PM Eastern Standard Time,
How about a large hv transmitting air
variable under oil?
I found the large air variable under oil to be very convenient
for final tuning when I set up my 36" spark VTTC. This
tuning cap is installed in a tank made of 6" dia PVC pipe.
The entire unit is about 20" tall. A plastic 1/2" pipe sticks
out of the top of the unit. This pipe is rotated by hand
while the coil is operating to tune the coil. Since this
variable cap is limited to about 600pF, I place it in
parallel with the existing tank caps for minor tuning changes.
To get a full range, I have to change the main fixed tank
caps in ranges. The air variable fills in the ranges. It is
fascinating to see the changes in vacuum tube plate redness
as this variable cap is tuned. A very minor change has
a significant effect on tube plate redness. When the tune was correct,
then I used a mix of fixed caps to obtain the proper value,
and disconnected the oil variable. I used the oil variable only
during the tuning procedure. I did this tuning under full power
while producing the 36" sparks. This way the coil was tuned
taking spark loading into account. This air variable has two
sections which can be wired in parallel or series or singly
which gives 3 voltage and capacitance ranges. All ranges
had sufficient voltage hold off for tuning the VTTC at 5.5kVAC
input. Although these suble tuning changes affected the
tube plate redness, they were too slight to affect the spark