Tips for building tube TC's


I like to build my tube tc's as research test beds so I can easily install
various tubes, transformers, grid networks, primaries, secondaries, etc.
This all makes it easy for comparing various components, etc.

As a base, I just use a wooden board, but I often install a raised wooden
platform at one side.  This platform can be about 12" to 14" square, and
the primary and secondary coils are mounted onto this platform.  The 
platform should be about 8" to 10" above the main board so there's room
below the platform to mount other components.  I usually mount the 
tank caps, and the grid leak network and bypass caps below the platform
on the main board.  The main board can be about 14" by 26".  This leaves
room at the side of the platform to mount the tubes and sockets.  In any
case, you want to have plenty of room to work under the platform, etc.

It's often helpful to mount the tube socket onto a small piece of wood,
then mount that wood onto the main board.  This way if you want to
change the tube and socket, you just remove the whole modular little
wood base with socket, and replace it with another.  THis same modular
approach can be used for the grid leak network, and can even be used
for the primary and secondary.  Mount everything onto it's own base, so
it can be easily removed and replaced.  This saves a lot of time in the long
run, and keeps the project neat.  The transformer which tends to be heavy
can be placed on the table near the TC, or another level can be built, and
the transformer placed below the main board, etc.

One thing which is particularly helpful is to wind the grid coil onto a
separate form or sleeve which then slides over the primary coil form on
a part of the primary form that has no windings.  In other words, the
primary form just has windings at the bottom, and the top area is
reserved for the grid coil which slides over that part.  The grid feedback
can then be easily adjusted by sliding the grid coil nearer or farther 
from the primary coil.  This is important because some tube coils are
extremely sensitive to the amount of grid feedback, etc.

Another important area is the wiring of the coil.  The connections can
use screw terminals such as barrier strips, or some sort of plug in
connectors.  This makes it easy when you change components such
as tube sockets, etc.  Solid wire can be good because it stays where
you put it, and is less likely to sag against another wire and short out
due to the high voltages on the wire.

The variac to control the coil should be placed at some distance from
the coil for safely.  Meters for voltages, currents, etc, can be installed
onto the coil as needed or desired.

For my largest tube coils, I build a base on wheels, with 3 levels.  The
filament transformers and perhaps the main transformers and any 
staccato (pulse) circuits are placed at the bottom.

When the sparks exceeds 28" or so, it may be necessary to use a
thin corona ring or toroid at the top of the secondary to prevent the
sparks from flaring off from the side of the winding.

Small, high Q secondaries tend to give sharp pointy sparks, larger
low Q secondaries tend to give fuzzy type sparks.  However, the look
of the sparks is also affected by tuning and other factors.  I just use
#12 stranded pvc insulated wire for all my primaries.  For small coils,
I use a cylinder primary, for larger coils I use a flattish primary. 

The sparks from a large tube TC take on a very powerful roaring sound,
it's quite eiree in a way.  For long sparks, you need plenty of power.
For instance for a 19" spark, you need about 1200 watts.  For a 36"
spark, you need about 5500 watts.  Recently I tore apart my small
tube TC that gave 19" sparks because I didn't make it modular enough,
and it became so messy with wires all over the place that it was
unmanageable.  I plan to rebuild it using the principles mentioned above.
I also plan to build an intermediate sized tube TC, that I'm hoping will
give good performance.

An excellent approach for great efficiency is to use the level shifted
(AC voltage doubler) design pioneered by Dave Sharpe.  I plan to
incorporate this technique in my new tube coils, since I was very
impressed by the performance of the level shifted design when I 
tried it on my small tube TC.

Just some thoughts,
John Freau