RE: VTTC provisional results

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "James by way of Terry Fritz <twftesla-at-uswest-dot-net>" <mustang3-at-home-dot-com>

I had the same problem with my VTTC. When I ran the power up I was getting a
severe flashover from the last (highest) turn of the primary to the
secondary. I had left 10 inches of the 8 in. PVC former(pri coil form) to
prevent this. The arc would smoke the primary and short it (large carbon
track). So, I rewound the primary as a funnel. 60 deg. angle, useing 6 plexi
pieces with holes to hold the wire with tye-wraps. Easy as pie, and that
fixed those pesky pri. to sec. arcs.
 I was unlucky in that the arcs killed one of my tubes. I use 25 turns of
#10 THHN wire for the primary. It will be a pain to do 65 turns. If you want
a picture, email me (off list).
										Later,
										  James



-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Thursday, March 15, 2001 8:58 AM
To: tesla-at-pupman-dot-com
Subject: VTTC provisional results


Original poster: "Herwig Roscher by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <herwig.roscher-at-gmx.de>


Hi all,

After having finished a series of tests I would like to inform you of
the results and , of course  :-)  , I have a few questions as well.

My system:

primary coil:
diameter    : 8"
inductivity : 640 µH
capacity    : 700 pF
turns       : 67
coupling    : primary 1.2" above secondary

secondary coil:
diameter    : 4"
inductivity : 35 mH
turns       : 1370

toroid/corona shield 1.2" x 6", 1.2" above the secondary coil,
discharge pin

grid coil:
diameter    : 8"
turns       : 8
coupling    : grid coil 1.4" below primary coil
grid leak R : 15 kOhm
grid leak C : 3.75 nF

The oscillator frequency  is about 200 kHz.
I am using a 4PR1000A tetrode, driven from a 10 kVdc (filtered)
power supply.

Output (the most important thing of course):
I am getting a bunch of branched streamers which are
accompanied half of their length by thick blue seams. Occational
thick arcs to a grounded rod at a distance of 17". Should I be
satisfied?

Measured full load values:
Plate voltage       : 8500 Vdc
Plate current       : 375 mA
Screen grid voltage : 1280 Vdc
Screen grid current : 140 mA
Control grid voltage: 200 V
Control grid current: 20 mA

Unfortunately (some of) these values are probably wrong, as I could
not find a really "cold" reference point within the system. The tube
oscillates in C-mode and there are spikes all around,
superimposing the values to be measured. The only point
connected to the rf-ground is the base of the secondary coil. All
other "grounds" are connected to the ground of the house wire
system. I have connected both grounds of course but could not see
any difference.

Unfortunately there was another flashover between the primary and
the secondary coil: From the flush screw (correct expression?) I
am using to fix the upper end of the primary winding there was a
thick carbon track (more than 0.1" wide and deep) to the upper end
of the coil form. From here the spark jumped to the 6" protection
tube I have placed between the coils and furtheron (without
puncturing this tube!) to the secondary coil. It hit it at the same
place(!) as it did before(one time). I am suspecting that this is not
an accident. Any hints to prevent my secondary from being hit any
more?

As tetrodes require high plate resistances I would like to increase
the number of the primary turns even more. But there are some
drawbacks:
- More turns means a higher coil (if the wire diameter was not
changed) and a greater menace of flashovers - see above.
- More turns means a smaller tank capacity and a higher influence
of the ambient capacities and capacity-changes. Has anybody
used tank capacities smaller than 700 pF?
- More turns means a lower turns ratio primary/secondary and a
lower secondary peak voltage therefore. This could be contrary to
an improvement of the output. Is this correct?

Critics and hints are very appreciated.

Herwig