Re: [TCML] VTTC filament getting brighter including some mini-eyecandy

["Christoph Bohr" <cb@xxxxxxxxxxxxxxx>]

Hello Ed.

> How much filament voltage are you running and how much plate current?
Filament voltage of the tube is 12.6V, safe limits between 11,8 and 13.4
I am running 13,2 at the moment ( set it to 12.8 but it somehow drifted away )

>  Don't know what kind of filament the tube has but sometimes undervoltage can cause
sparking at the cathode.
> Safer to be over voltage - that will just shorten the life but if you're running a
lot of plate current undervoltage can
> cause immediate damage.
Oh, good point, I did not know that could happen, just did not stress tubes that bad
in my other tube related projects.
If I understand that right, you are saying that running more current than the cathode
emission can deliver will
disintegrate the filament due to excessive emission in some spots?

Unfortunately I can not measure the plate current, the only meter I have here at the
moment goes crazy
when the coil runs, but my setup should be within limits of what can be run or has
been run by others.

However, I had to try something, so I fetched up a variac from the shop and gave it
one more try.
Up to 40% or so everything ran smooth, that the one side of the filament went
brighter and I heard some
arcing, so I shut the coil down and tried to find the place where it arced... with no
success.
I ran the coil in comlete darkness several short runs, still trying to find where
that arcing came from.
There mus have been some 5 or so runs of some seconds each when I noticed, that my
tank cap
got seriously hot ( its a rather rare type but was recommended for VTTC use iirc, and
bought new )
So I changed the cap for a MMC of the same rating and that seemed to solve the
problem.
now I can run full blast and at least I can not see any problems inside the tube....
I am still not sure I this was the solution, this case needs further investigation,
the original 5nF tank cap measured 4,7nF new and now 4,6nF...., but during my
experiments I
changed the feedback coil from 19 to 16turns as well as the grid leak resistor from a
reostat to
a fixed value of 1K... I am just afraid to change back and see what happens as I
might destroy the
tube... rather stay dumb and keep the parts *g*, I will have to decide.

Another thing I tried is to change the grid leak cap and noticed that between 2nF and
"nothing at all"
I can hardly see any difference... but not problems either.....so whatever.

OK, just to keep the mood up and to proove I actually have something halfway working:
http://www.luebke-lands.de/media/IMG00152.JPG

The setup is VERY quick and dirty, just a proof of concept before I assemble
everything nice and tidy.
However, the output seems very acceptable for a lashup at 12" ( calculated from
foto )

Thanks a lot, I will try to mesure the plate current when I get the chance.

regards

Christoph

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