Re: First VTTC light, and a few questions

["Grayson B Dietrich" <electrofire-at-juno-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

On Wed, 19 Jul 2000 07:58:48 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
> Original poster: "Steve" <legion-at-bigfoot-dot-com> 
> 
> 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.

Great!...   ...just about the 4" I thought it could do.


> 	The tubes are very conservatively rated, you can overdrive 
> them by a
> very large amount without damage

So I've ben told. I'll see what I can do..  ..worst case scenario, the
tube destructs, and I have a spare waiting. It is a different brand, AMC
as opposed to Cetron I'm currently using. But that ought not to matter
too much, should it? It looks a little less robust, with less heat
sinking on the plate, though.
> 
> > 
> > Plate Voltage: 1030VAC
> 	You can go slightly higher with this, 1100VAC will not hurt 
> the tube

If I had a 1100VAC transformer, I would try it. I may be able to get a
220V MOT, and run it off of 110V for that 1100VAC...   

>> 	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

My transformer is 6.6V...   ...I could replace it. But, the question was
more as to why the voltage was changing, when the line voltage remained
the same? It seemed to be effected by how the circuit was running. The
filament V has no adjust yet, but the plate uses a variac, and when I
bring the plate V up, there is a measured increase of V accross the
filament. Right now it gets to a little less than 7V. I have a small
variac I will install to fine tune the filament voltage, I guess.

> 	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

I've heard from others that a small topload seems to help the discharge
behavior and length a little bit (it certainly isn't hurting it), and I
like the way it looks up there, anyways ;-)


> 	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. 

Wow...  ...I didn't think that could be done with a VTTC. Maybe I'll try
it.... you would need very little coupling between the primary and
feedback, right? Or the other way around?....   ....I'm a little tired,
and not thinking too clearly.

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. 

?? I've read and heard that the actual value isn't critical. Just
something to provide the DC bias...   ....004µF is a common value for
many coils. 

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.

I kept it small in order to get a larger primary coil...   ...closer to
the most common 20 turns or so. But the frequency would remain the same,
as the primary would have to be adjusted to match the reso Fr of the
secondary. I admit that the Fr is rather high, 773,000Hz calculated (sans
sphere), so I may wind a different secondary. I didn't excpect a short,
fat coil to work as well as it appears to be doing so far.



> 	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

Alright! Time to start the stress tests.....  I'll let the list know how
it all comes out.

> 
> 		Steve
> -- 
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> *Steve Rodway                        * Heisenberg : We are not sure 
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> 

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