Ultimate screen and bias voltage source: series 9v batteries in parallel with B+?

[exclamationmark]

Just as a side note/inquiry... Anyone know WHY the standards of 50Hz/60Hz frequency?

Something about the inductance of motors which tend to disagree with high frequency AC. Probably a myriad of other reasons too.

[J M Fahey]

As with many other things, there's a compromise involved.
Lower frequencies require *huge* transformer cores, higher ones increase losses dramatically, plus real higher ones (such as 400 Hz) have extra losses into long transmission lines.
400 Hz has a wavelength of 750Km, so a 375Km line is an *excellent* and very efficient 1/2 wave antenna !!!!
You'll read all about it at: Utility frequency - Wikipedia, the free encyclopedia