FU811 this tube is commonly used in medical equipment, not commonly used in the bilevel amplifier circuit, the main reason is that the linearity of this tube is not very good, made of amplifier distortion may be larger, so this tube is used to do bilevel amplifier not much.
Circuit diagrams are also not much, looked at two drawings, do chassis amplifier screen pole voltage is generally used about 560V ，，，，，
The above is for reference only!