its been out of production for about 20 years. The tooling was destroyed with the plant
6C33 overloads the socket connections which can fail within the lifetime of the tube itself.
I’ve asked Mike Matthews for his views on this, from what I understood his factory in Russia has all the dies and tooling, and sell them on New Sensor and the sockets at reasonable prices much cheaper if you join and login.
https://shop.ehx.com/item/6c33/tubes-vacuum-sovtek/
https://shop.ehx.com/item/7big/