High power plasma tweeters
were all the rage on the 4hv forum a few years ago, so I decided to
join in on the fun and bought myself a GK-71 power pentode. Cue
several years, later I finally slapped together the simple circuit
this Tesla coil consists of, and had a working
high-frequency-vacuum-tube-Tesla-coil, or HFVTTC. This project is
quite simple and easy to get working, but absolutely lethal, so take
care if you attempt something similar. The GK-71 can operate at anode
voltages up to 1500V, which coincidentally is the voltage a microwave
oven transformer will output once full-wave rectified. For the
longest time I fussed over how to connect the transformer, as MOTs
have one secondary wire connected to the core which should be
grounded. For full-wave rectification to work neither end of the
secondary winding can be grounded, but to simply disconnect the
grounded end of the secondary might cause flash-overs from the
secondary to the core. It stands to reason that the winding
insulation is only designed to cope with low voltages near the core,
so the threat of flash-overs is very real. Despite this I disconnected
the grounded wire, rectified the output, and grounded only the core,
all without
problems. Which honestly, was just luck. The transformer could have
just as likely gone up in flames, but in my case the insulation was
sufficient.
Some bolts at various stages of heating. The same bolt after getting the unit working correctly, melted like wax.
To filter the microwave
oven transformers output, two
microwave oven
capacitors were used in parallel for 2uF. When powering vacuum tube
amplifiers from a supply with filter capacitors, it is recommended to
use a "glitch resistor" to limit the surge current in the event of an
internal arc in the tube. This will protect the tube from damage, often
at the price of the glitch resistor. I used a 30W, 22R sandbar
resistor. To power the filament I used an old laptop charger, which
provides a shade over 19V DC at several amps. The high frequency noise
from the coil doesn't seem to bother it.
The estimated power
dissipation of the 11k7 resistor is 85W,
so be sure to use something that can handle a lot of power. I used some
5W and 10W resistors wired together for 20W, but they melted through
the plastic bubble-wrap in just a few seconds. The
feedback capacitor
seems critical to the design. I initially used cut brass sheet which
had sharp edges, and breakouts would occur along these edges unless
the two plates were spaced a few centimeters apart. With the plates
so far apart the power from the coil was severely limited, and the
breakout was only a few cm tall, similar to a regular plasma tweeter.
I then used two metal jar lids with smooth edges and enamel, which
seems to have forced the breakout to occur only at the breakout
point. This has allowed close proximity of the jar lids, and higher
feedback capacitance. Which in turn has increased the plasma streamer
to roughly 16cm! It should be possible to use purchased capacitors
instead of making your own with jar lids. I haven't measured the
capacitance, but I assume it is in the range of 5-15pF. I haven't run
this HFVTTC for more than 30s at a time,
but so far the tube has shown no signs of turning red from losses.
Disclaimer:
I do not take responsibility for any injury, death, hurt ego, or other
forms of personal damage which may result from recreating these
experiments. Projects are merely presented as a source of inspiration,
and should only be conducted by responsible individuals, or under the
supervision of responsible individuals. It is your own life, so proceed
at your own risk! All projects are for noncommercial use only.
This work is licensed under a
Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.