Re: [TCML] VTTC filament getting brighter

[Ed Phillips <evp@xxxxxxxxxxx>]

Note at end:

"Hello Folks.

First thanks to all who advised concerning the staccato controller,
I am currently waiting for the parts I ordered.
But in the meantime some more beginners questions:

I just changed from self-rectified AC mode to a level shifter using
3 MO an a 2nF MMC caps in parallel and a MO diode, the level shifter
itself does not seem to be the problem, output increased significantly.
But at a price: After 2 seconds you can see the tube getting brighter,
I would say it is the filament getting brighter, but as I can only observe it
for a very short moment and cannot get my head too close to the tube I still might be
wrong. I use 47nF bypass caps, two from the filament connetions to ground and
one between the connections ( its a GU81 with center tapped filament ) directly on
the socket.
On the first occasion it happened I realized it to late and there was some kind of
sparkling/arking inside the tube for a second or so. However, with lower voltage
the tube still runs smooth and seems to be OK.

What could be the problem? Am I pushing things too hard? Of course  I am pushing
beyond factory values, but others have done that, too, without too much problems.
I would have expected a red plate, but I can't even run the thing long enough to get
to that point.

Further specs that might help in finding an answer:

Setup type: impromptu lashup  ;-) 
Tube: russian GU81M, new ( well, until this evening  ;-)  in triode configuration
Plate xformer is a MOT, 2.1KV feeding a doubler,
estimated plate voltage around 4 or 5 KV
L1 10inch, 21turns of 6sqmm ( about 9AWG ), tapped at 19 ( tank ) and 19 1/3 (
supply )
L2 7" by 15" wound with awg23, 20mH
Feedback coil below L1, 16turns awg17 / 1sqmm, tapped at 15 turns.

Any suggestions are welcome.

best regards

Christoph Bohr"

	How much filament voltage are you running and how much plate current?  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.

Ed


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