[TCML] Re: VTTC tuning

[Futuret <futuret@xxxxxxx>]

 Wm, 

Some VTTC's seem to just fall into place nicely, tuning wise.  Where
I faced the greatest challenges was with unusual designs having
flat primaries, grid coils closely coupled to the secondary, etc.  In some
of these designs, the amount of grid-leak resistance, the grid coil placement,
the coupling, and main tuning were very critical.  

VTTC's will function over a wide range of tuning, but there will be a best
tune point nevertheless.  An important thing for VTTC tuning is to obtain
longest sparks with minimum tube plate redness.  If you're using 833C,s
they may resist getting red.  In a way this can make it harder to tune (harder
to judge the best tune point).  Graphite plate tubes (such as the 833C) should not be
allowed to run with red plates, although I do it anyway.  The 833A tubes with
the tantalum plates are designed to run somewhat red/orange.  If a coil is
far out of tune, the redness will form a small red bright spot at the center of the
plate.  As the tuning is improved, the red spot will spread out and become 
larger.  

In my coils I've found the best results by varying only the tank C for tuning,
rather than the tank L.  Tank L should usually be held constant at some
optimal point.  Others may have had different results in that area.  

In a VTTC, the ratio of C to L in the tank is important.
Higher voltages require higher L for best matching (best efficiency).  This
seems to be more critical in some coils for some reason.  

A trick I use to help in tuning is to use various sized toroids as tuning aids.
If adding a larger toroid makes the sparks longer, then a larger tank C is
needed.  If adding the larger toroid makes the sparks shorter, then a smaller
C might be needed.  For tuning purposes, the toroid can be just a round 
piece of metal with a rolled edge such as the cover from a cookie tin.  
Provided a long spike point is used on top, the spark will emit from the spike,
not from the cookie tin cover.  

I use a variable capacitor in parallel with the fixed capacitors to optimize my
tuning.  It's possible to tweak the tuning such that whereas spark length will
stay the same, the tube plate will run less red.  That's the tune point I use.
For the final setup, I replace the variable capacitor with a fixed capacitor of
the same value that gave this optimal tuning.  

A lower value grid resistor will give you longer sparks, but the tube will run
redder.  Many adjustments are a compromise in a VTTC.  If the grid coil and
primary are placed too close together, a destructive arc can jump between
the coils.  I like to make the grid coil so that it can be slid up or down along
the top of the primary form (above the primary) to optimize the grid feedback.
Some folks like to place the grid coil below the primary, but it can be a 
little trickier construction-wise.  

The redder the tube plate is running, the more likely the tube will be
destroyed by internal arcing if the tube is over-volted.   

The use of staccato will allow for using a slower pulse-rate, thereby
keeping the tube plate cool, and lowering the input power, yet maintaining
the full spark length.

http://futuret.110mb.com/

Please let us know how your project progresses.

Cheers,
John






I am also interested in your
> view's on tuning.
>
> Wm A Tickhill

 

 

 



 
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