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Old 18th March 2009, 02:20 PM   #1
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Default Ballast Tube Current Source

Anyone tried ballast tubes as current source for filaments?
They are available as NOS in a wide range of currents.
Will they generate some form of thermal noise making them impossible to use in eg. a linestage? I suppose that some of you veterans have some hands on experience with them.
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Old 18th March 2009, 02:21 PM   #2
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Do you mean, Barretter or a voltage-regulator tube...?
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Old 18th March 2009, 02:32 PM   #3
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The question if they are useable for audio has been debated before on diyaudio.com

Ballast tubes usually consist of an iron wire in hydrogen gas. They are very slow devices and as such not useable as current sources for audio application, the reaction time can be measured in seconds or 10s of seconds so for audio they would look almost like a short circuit or very low impedance.

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Old 18th March 2009, 02:34 PM   #4
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These tubes:
http://www.antiqueradios.com/features/ballast2.html

I was under the impression that they were fixed current regulators?

(A typical set up would be: Transformer -> 3-pin voltage regulator -> ballast current regulator -> ECC88 filament.)
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Old 18th March 2009, 02:47 PM   #5
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Quote:
I was under the impression that they were fixed current regulators?
Yes, but they are 2 terminal devices built with a hot wire. The principle is that a wire increase its resistance when it gets warm thereby the current decreases, the wire cools off and finally you reach a stable operating point with an almost constant current. An ordinary lightbulb works almost the same way but the iron wire in hydrogen gas give a more constant current.

As the working principle is based on the heating and cooling of a hot wire they are very slow devices.

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Old 18th March 2009, 02:50 PM   #6
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Quote:
Originally posted by tubetvr


Yes, but they are 2 terminal devices built with a hot wire. The principle is that a wire increase its resistance when it gets warm thereby the current decreases, the wire cools off and finally you reach a stable operating point with an almost constant current. An ordinary lightbulb works almost the same way but the iron wire in hydrogen gas give a more constant current.

As the working principle is based on the heating and cooling of a hot wire they are very slow devices.

BR Hans

Thanks for clarifying the principle, Hans. I just thought I had a bright idea... for once.
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