What are OA2 tubes and why are there more gear

using them?

I have a First Sound Presence Deluxe preamplifier which I bought used. This preamp as well as the JOULE ELECTRA use the OA2 tubes. What sort of tubes are these? When where they made and why are they not more prevalent in other preamp except in a handful of them such as the JOULE and the LAMMs, as well as the First Sound gear? What particular sound do these produce that make them appealing or unappealing for designers to incorporate them in their designs? Is there a designation that is different than the 0A2 that I am not aware off?
My knowledge is limited regarding the OA2 tube and I’m sure there are others that read this forum that are much more knowledgeable about tubes then myself. I’m responding because I also own a Presence audio preamp. The 0A2 tube serves as some type of a regulator control agent. Your preamp will work with or without the 0A2 tubes however it will sound best with the tubes. The First Sound preamp seems to perform and sound the best with the Sovtek 0A2 tubes.
OA2 (actually all the O** series I believe, like OD3, etc.) are cold-cathode voltage regulator tubes. Generally filled with mixes of neon/argon/xenon. They glow pretty colors, which I think is why they are used. Generally they conduct at a certain voltage level and can be run in series for higher voltage regulation. (e.g. OA2 @ 105v x 3 in series = 315v regulated.). They are not really in the audio signal path at all, so tube rolling them probably won't affect sound at all.

Enjoy the glow,

The OA2 used in the First Sound is different from the one used in the Joule Electra. The Joule Electra will take any domestic OA2, but the First Sound needs one designed for the Russian military that can handle a 3ma current draw. These are getting hard to find. The domestic OA2 will either crack or burst when used in the First Sound.
The OA2's are used in voltage regulators. One setup is to offer a 150-volt anchor to the cathode of the regulating tube. Another, it's used as a straight regulator. Usually the regulator itself is a dual-triode and each cathode of the regulator has an OA2 attached (one per channel). Your amp definitely needs an OA2 to work, especially if it's a shunt-type regulator, because removing it disconnects the high voltage mains.

The reason they're not popular is because tubed voltage regulation is not. It's much easier, and more accurate, to do with solid state than tubes, if it's even done at all. One problem with the OA2 is that it does not present a pure resistance to the regulator - instead its resistance varies with frequency making it slightly more complex to wire up as a stabilizer. Also, I read in some tube data sheet that some OA2's have radioactive stuff inside them to make them heat up evenly (is this from where the cool glow arises?). If that's true, I would not want to be around one of these should they break.
UHHHH!!!! I had one 0A2-tube break, and both channels continued to work however it did not sound as good as usual.
I wondered why I started to lose my hair!
The cool glow on O** series tubes is from the gas fill - argon, neon, xenon, etc. Not from any radioactive elements. Though some larger (high voltage transmitting type) tubes do give off Xrays in minor amounts.

Hm. Well I have 5 of these. I'm selling off much of my collection, anybody interested?

They are RCA and pulled in about 92 from research lab gear being thrown out. Probably very little use. Will test on my TV7 later.

OA2s work much the same as Zener diodes but have a set of issues that I would think would make the Zener a preferred design means. One of them is that the tube makes noise when it fires (which is continuously) so you have to bypass them and isolate them from the regulator that is using it as a voltage reference.

Zeners make noise too but they are less problematic, plus you can choose zeners to make a variety of voltages. None of these issues present much challenge to a competent designer- consequently their use would be primarily for show (although they are functional) than anything else.
Contrary to reported above, different samples of gas regulator tubes do sound different, so it makes sense to try a variety. Fortunately, you can buy these off the internet at quite low prices since there is relatively low demand. In my experience, using the larger OA3, OB3, OC3 and OD3, different brands all sound different and older versions of the same brand usually sound better. The good news is that when used properly gas tubes have a virtually unlimited lifetime.