5651(A) 85Kr

[jechentau]

i was looking into a nice power indicator, and thought of a 5651 voltage reference, which between 1.5 and 3.5 mA through it, varies only little in voltage.

apparently it (or the A only, contradicting sources) contains some krypton 85, which with a half time of some 10.7 years decays into rubidium 85, according to wiki.

and these tubes are very old already, so maybe only 1 in several 10 parts of it is left, and the rest turned in rubidium.

where is it, and in howfar does it influence the tube?

[kevinkr]

Interesting question, I use 5651 as references in my power amplifier psu (all actively regulated) and in the psu of my phono stage as well. I haven't observed any significant deviation from the published specifications as a result of non use related aging phenomena.

I have a bunch of both 5651 and 5651A and really haven't been able to tell them apart in terms of their electrical performance.

As references go they're quite good - better thermal stability than high voltage zeners, and good precision as well. Their dynamic impedance is a lot higher however.. (I started a thread on this subject a while back, I'll search for it.)

Edit: Gas Reference Tube (5651/85A1/etc) Internal Impedance

I'd be interested in any answers relating to the krypton 85 question.. I haven't noticed anything that makes me believe this is an issue, but still I am curious.

I think using one as a power indicator is an interesting idea, although IMO they aren't really all that attractive.

[jechentau]

ah, nice.

which color does it have?
i have seen many have quite opaque tops.
but not one operating.

the krypton 85 may be present in trace amounts, to facilitate dark striking.
i seem to have read somewhere normal neon bulbs have a notably higher striking voltage in the dark, and that some have slightly radioactive electrodes emitting electrons to reduce the voltage required to strike in the dark.

thus if there is enough headroom in the circuit the tube will strike anyway.

but, i would gladly know for sure ;p and what happens to the rubidium.

the name stems from the gorgeous ruby red emission lines afaik.

a rubidium vapour VR/indicator tube would work, i guess, if heated. (ah lol i see such exists ;p)

i also see any rubidium will at least add to the getter function too ;p

[kevinkr]

They glow with a neon orange glow, but often the getter makes it look kind of dirty..

[jlsem]

the krypton 85 may be present in trace amounts, to facilitate dark striking.
i seem to have read somewhere normal neon bulbs have a notably higher striking voltage in the dark, and that some have slightly radioactive electrodes emitting electrons to reduce the voltage required to strike in the dark.

The Kr85 increases the regulation by a factor of about ten. That is, it goes from about 5% variation to about .5%. Don't ask me how though.

John

[jechentau]

ah, i see,

does also the noise decrease by it? that would indicate a more even discharge,
possibly by the beta emitting of the Kr85 (which would also effect the striking then).

i mean that makes sense.

also it would only require very very small amounts of it in the mixture, maybe way less than even these days is present in those already decennia old tubes.

i guess one could tell the presence and amount of it as there must be a higher leakage current below striking (still exceedingly small, and completely irrelevant to the normal operation) than in tubes without.

same principle of the way such radioactive smokedetector works.

[jechentau]

here some more info: http://www.orau.org/ptp/collection/c...ctrontubes.htm