[Rob N]

There appears to be only four brands of these available - Ken Rad,Sylvania,NU and Raytheon.Are there any others?

When did production cease on these? Most seem to be 1940's vintage

 

[ical]

There's also Western Electric.

Yes, they're all made in 1940's tubes other than NU is I'm not sure.

 

[pabbi1]

These were military WWII radio tubes, and most are stashed in Europe - I've had good luck with one guy in France who lists them occasionally on eBay. Pete has an extensive discussion about the 307a (aka vt-225).

TubeData lists Western Electric also, but no idea who really manufactured, and who rebranded.

 

[Leny]

I know someone who has built a 307A based amp and he said that getting matched tubes was quite difficult. I hope TTVJ are keeping a stash for their customers!

 

[FrankCooter]

If you're "rolling your own" amp, probably be a good idea to find a substitute. The 10y is pretty close.