What are OA2 tubes and why are there more gear

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,

-Ed

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!