So according to checkout page on manual the plate voltage(pin3 on 6B4G) should be 308V +/- 5%. But looking at the datasheets of NOS 6B4G, the max rating for class A single tube operation, plate voltage is 250V. Not so good for the lifespans of 6B4G's, is it? Or am I missing something here?
http://www.hebertech.com/datasheet/tung-sol/6b4g.html