Re: First VTTC light, and a few questions

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

Tesla list wrote:
> >>      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.
	Hmm, I use a network of 2 470uH inductors and 2 200nF capacitors to
help block RF from getting back into the filament supply, it's also
possible variation of heating inside the tube is causing the temperature
of the filament to vary, causing the resistance to vary, and hence
upsetting the voltage. I have observed this before when running the tube
DC ( no RF present ), if the plate current is very high, then for some
reason the filament cools down.

> >       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.
	The grid coil is very weakly coupled on mine, the plate coil is 2.7"
diameter and 3" tall, and the grid coil is 4.3" diameter and 2" high,
the top turn of the plate coil is 0.5" below the bottom turn of the
plate coil.

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