USA 6SJ7. It is a metal tube with characteristics similar to 310A etc. You can drive 300B with just one of these. It is sometimes used in place of the 310A of the 91 type amplifier.
This is a relatively popular ball, so we recommend getting it while it's still cheap. There is a red ball 5693 that is a highly reliable tube for this ball. There are many different manufacturers, so please check.
It appears to be NOS, but since it is unknown, I will list it as a used item.
One bottle costs 3000 yen. Brown or white boxed