Re: VTTC output

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

> I've just finished another series of tests: Using 15 kOhm/1.6 nF as
> grid leak combination and a Vg2 of 1200 V, the plate current was
> 480 mA at a plate voltage of 8600 Vdc. Seems that I reached the
> limits of my tube  ;-)
> 
> The tank caps became barely warm and the primary coil reached
> about 30 deg C after a few minutes of testing with various loads.
> But the lower part of the secondary became so hot that I hardly
> could touch it (more than 60 deg C ?).

	Any idea what the temperature distribution is along the secondary?  If
the bottom is much hotter than the top that would imply a very
non-uniform current distribution; would wonder if you are running at a
harmonic of the fundamental frequency of the thing.
 
> Trying to find a possibility to "measure" the relative output power
> instead of guessing it by eye, I've used
> - the a.m. 5 Ohm resistor
> - a pickup coil of 4 turns wound on the form of the primary at a
> distance of 3.5" and connected to a 230 V/100 W bulb. The voltage
> across this bulb is 130 Vpp and nearly independent(!!) on the power
> output.

	Gather you are running with the bulb AND the secondary connected
simultaneously.  Wonder if the power into the secondary streamers
increases so rapidly with increasing input power that the system acts as
a non-linear voltage regulator, with more or less constant primary
voltage and hence induced current into your bulb which, of course, has a
positive temperature coefficient of resistance.

> I was really surprised that the system is so insensitive. Could the
> reason be that I'm using a tetrode which has a very high plate
> resistance in comparison to triodes?

	I don't think so, and not sure your system is as "insensitive" (to
power input?) as you think.  
 
> If I calculated the tank voltage from 67 turns of primary / 4 turns of
> pickup coil and multiply it by 130 Vpp, I get 2200 Vpp. IMO this is
> low, taking into consideration the plate voltage of 10 kVdc.

	That measurement can't be right.  If you really had only 2200 Vpp
across the primary, that would mean that the minimum voltage would be
almost 7500 (8600 - 2200/2) and the plate dissipation of the tube would
be AT LEAST (7500 x 0.48) = 3600 watts.  Better way to measure the
primary voltage would be with a tube rectifier (1B3GT or any later
version of high-voltage rectifier intended for CRT power supply)
connected across the primary and a FLOATING HV DC meter, suitably
bypassed with a high-voltage capacitor.  You could run the tube filament
from a small battery - I've used a single D cell to run a 1B3 for
similar measurements.  Solid-state rectifiers might also be OK, but not
sure about their high frequency response.

> Any hints or comments?
> 
> Bye             Herwig

	Should point out that I have no experience at all with such a big
tube-driven coil and hence am "winging it" based on other RF
experience.  Perhaps some of the "big boys" will have more informed
comments.  I'm beginning to get the urge to dig out some of the
high-powered stuff in the attic and start building!!!!!

Ed