What are OA2 tubes and why are there more gear

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.

-Ed

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.

Jim

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.