Re: First VTTC light, and a few questions

["Steve" <legion-at-bigfoot-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Grayson,
	I too have just completed an 811A based VTTC, and am getting an output
of 86mm when running from full wave AC and 66mm when running smoothed DC
comments below.

Tesla list wrote:
> At first it barely gave the tiniest spark, but with quite a lot of
> fiddling, rearrangement, and finally adding metering of  filament
> voltage, plate current, and grid voltage (the meter wired for grid
> current seems to be nonfunctional :p ) It now runs smoothly, apparently
> well within the specs of the tube.
	The tubes are very conservatively rated, you can overdrive them by a
very large amount without damage

> 
> The sparks are only 2" long at the very best, and I've read that an 811A
> coil ought to be able to give twice that....   ....so here are the
> pertinent specs as best I see them, and a request for advice.
> 
> (values measured)
> Plate Voltage: 1030VAC
	You can go slightly higher with this, 1100VAC will not hurt the tube

> Current Draw: 120mA (I'd like to draw more, as the plates aren't even a
> dull red yet, but I don't want to reduce the grid leak resistor much
> more... it is already only 3K)
> Grid Voltage: -120VDC
> Filament Voltage: 6.6VAC (the meter reads a rising voltage as i increase
> the variac controlling plate voltage...  ...when the coil was ill-behaved
> it would increase to as much as 10V, now it only reaches 7V...what is
> happening?)
	You should lock the filament voltage at 6.3V, the filament has enough
emmisivity at 6.3V to support a plate current of more than 400mA,
increasing the filament voltage beyond this just shortens it's life with
no performance gain

> Primary: 4" thin wall PVC form, PVC insulated #18 wire, 23turns, tapped
> at 18, windings are 1 7/8" tall.
> Feedback: on same form as primary, Cottenamel insulated #24 wire,
> 19turns, not tapped just wound on and off (excess spirals in solenoidal
> forms to reduce space taken up)
> Secondary: 3.5" dia. thick wall PVC, #28(a guess, worked right with
> WinTesla) enamel insulated wire, 5.75" winding height.
> Terminal: 2.25" aluminum ball with a 1" wood screw attached, pointing
> strait up.
	You may wish to experiment with a taller, thinner secondary, and remove
the sphere , a VTTC does not require a topload at all. I do use one
myself sometime to lower the resonant frequency, but performance is the
same with or without

> Grid Leak Resistor: 3K to 4K, on a 20K 25W rheostat.
> Grid Leak Capacitor: 4000pF, 1600VDC PP film/foil (4 1000pF caps in
> parallel)
> Tank Capacitor: 1000pF, 3200VDC PP film/foil (mini MMC, 4 1000pF caps)
	I'm using a grid leak system composed of a 2K2 25W rheostat in series
with a 100R resistor, at times I have run the system with no grid leak
resistance at all, this results in very high plate currents, up to
300mA, and means run time must be kept short. Also I am not using a grid
capacitor at all, I did experiment with one, but found that it just made
a 3rd circuit to tune. You can remove the capacitor and increase the
grid coil coupling to get the same effect with less fiddling
	You may also want to consider greatly increasing the plate capacitor,
I'm using 3000pF , the lower frequency this produces results in less
corona and a longer arc.

> and....    ....Oh, yeah. Just how long ought I to let this thing warm up
> before I apply Plate Voltage? the filament gets orange-white in seconds,
> and doesn't appear to get any brighter if I let it sit there for a while.
	I find 15 seconds is long enough

> Also, is it acceptable to run an 811A with a dull-red glowing plate, or
> to what extent is such a thing allowed?
	The tube can be run with a bright red plate, and if you keep run times
down below 30 seconds or so, an orange hot plate will be OK, but if it
gets to yellow it will be rapidly damaged, this will show up as a smooth
glassy patch on the plate

		Steve
-- 
*********************************************************************
*Steve Rodway                        * Heisenberg : We are not sure *
*Legion-at-bigfoot-dot-com                  *  which side of the road the  *
*http://www.legion-elec.demon.co.uk/ *chicken was, but it was moving*
*Tel. +44 (0)1761 300615             *          very fast           * 
*********************************************************************
        Please reply using RC2-128bit encryption if possible