New to the List and pulsed DC-VTTC (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 04 Oct 2007 14:14:55 +0200
From: Martin Damev <mdamev@xxxxxxxxxx>
To: tesla@xxxxxxxxxx
Subject: New to the List and pulsed DC-VTTC

Hello together,

My name is Martin Damev. I'm an 32-year old
coiler from Zurich, Switzerland. I've been
into coiling for more than ten years but my
recent interests are  specifically the  VTTCs.
Having a nice collection of transmitting tubes
from 1 watt to 100 kW RF output, and some good
knowledge about RF transmitters and industrial
RF heating it was obvious to build a nice tube coil.
Here is some data of my VTTC:

oscillator:  Hartley, parallel fed (plate RF choke: 6.8 mH,
                   3 nF /20 KVDC coupling cap. from plate to tank)
tube        :  BBC (now ABB) T 380-1 glass transmitt. triode
                   with 5V / 15 Amps filament and 380 watts plate
                   dissipation.  Very  sturdy graphite anode.
                   Tube often used in RF generators for plastic-
                   welding.
primary :   20 turns of  #18 AWG special high-temp.
                  stranded copper wire on a 8 inch plexiglas
                  former. Taps at 12.5 and 16.7 percent for
                  feedback adjustment (connected to ground in
                  a Hartley osc. circuit).
                 700-1100 pF of primary cap. Adjustable with a
                 vacuum capacitor.
secondary: 3.0 inches diameter. Winding length: 16.5 inches
                   with #26 AWG magnet wire. 10.5 mH inductance.
                   No topload.

Tests with 3000 volts filtered DC gave hot and intense
"brush" discharges, actually plasma flames of approx.
3.5-4 inches length. Measured  DC Input Power was
1200-1300 watts.
These tests proved the excellent efficiency of the
Hartley design. But I wanted long sparks and not
a plasma burner...
I have known  John Freau's excellent experimental work
with pulsed VTTC for quite some time.
He reported very good results with the now widely used
AC-fed staccato-controlled VTTC.  Attempts to pulse a
DC-fed VTTC were not so promising, except for the tetrode
version, which proved to have an excellent efficiency.
Could a similar efficiency be achieved with a triode instead of
a tetrode ?
I really wanted to run my VTTC as grid-pulsed oscillator with
high plate voltage. Grid-pulsing an oscillator is often used to
regulate the power of large RF generators for industrial
RF heating and welding. Luckily I got a complete grid-pulsing
unit from an 12 kW-generator. Driven by a signal generator
it produces an rectangular output voltage  of  -850 volts (adjustable)
with any duty cycle and frequency.

I connected the the grid-pulsing device into the the grid circuit, and
built a DC supply from 0...6200 volts DC with a 5 microfarad/6.3 kV
filter cap.
The results were impressive:

Spark length with filtered DC (5660 V) was at least as high as with
corresponding AC (4000 volts), the sparks being approx. 16-18 inches).
Power consumption was 20..30 percent  compared to VTTC operation
with 4000 VAC(no staccato).

Experimenting with the VTTC in this mode (grid-pulsed, filtered DC)
is real fun. You can easily change the spark's appearance by changing
the pulse repetition frequency and/or the duty cycle.
I found that in my setup the spark needs approx. 1.5 - 2 milliseconds
to grow to full length. Shorter "on"-times of the oscillator give thin
and weak sparks, longer "on"-times give bright,fat and hot sparks and
of course higher power consumption.
Best efficiency was achieved with low rep. rates, not more than 20...25
pulses/sec.). With 16 pulses/sec. DC input was not more than 200 watts
for 16-18 inch sparks.

A lot of measurments will follow. I feel that there is some potential to
pulsed DC-fed VTTCs. I will post some more my ideas and theories to
this mode of operation  soon.

Martin