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Re: My VTTC fun begins...
Original poster: FutureT@xxxxxxx
In a message dated 1/26/07 9:18:24 PM Eastern Standard Time,
...especially since I have started a collection of
power rf triodes (and others) in my parts closet, and
all the filaments check out good. I want to make a
vttc test platform that can adapt to the insertion of
a new tube and have the capability to (with a little
tuning) start producing sparks with a completely
If a particular tube has weak emission, you can often
raise the filament voltage some and run the tube that way.
For example for a 10V filament use up to 11, 12, or 13
volts as needed. It will shorten the filament life, but for
intermittent VTTC it's OK, and otherwise the tube is not
of much use anyway except for making a wimpy coil.
It's been said that one can run the filament at a higher
than normal voltage for awhile and sometimes full
emission will be restored permitting one to then lower
the filament to normal again. I haven't had any luck
with such rejuvenation however. I'm looking forward to
seeing your VTTC results. It all looks fine so far in
your mock up photo.
I like to keep very good notes of all my tests, showing
tune points, cap values, current input, grid coil turns,
grid coil height, coupling height, spark length, spark
appearance, coil sound, etc. Certain
adjustments will give a raspy sound which is generally
a bad thing. Some adjustments give a sizzling sound
at low power. In some cases the coil may even sound
like it's oscillating somewhat erratically. When tuning the
coil, the goal is to
simultaneously obtain; longest sparks, lowest input
current, and minimal tube redness. Coupling, grid
feedback level, grid resistance, primary tune point, and
tank impedance all have an effect. This can make tuning
a VTTC a slow process unless you happen to hit all the
right parameters by chance early. It's much easier to get
long sparks than it is to get minimal tube redness.
Some designs seem to be more finicky than others.
If you're copying an existing design, that makes it easier.
I'm sure you're familiar with these issues anyway but it
may benefit other new VTTC'ers in any case.
An interesting tube to try is the 3-1000A (4-1000A?) This has
a plate voltage rating of 6kV I think. It's a tube that I never
tried yet. I think someone on this list tried one but I can't
remember the results.