You are building a preamp? 300Vdc is typical in a tube amp and yes, that would be the B+ (high voltage) tap from a power transformer. But you will also need some form of rectifier (tube or solid state) and a heater (filament) circuit (6.3v)to make the tubes work. And if you intend to build a complete amp, you’ll need a power section.
Check out: http://www.valvewizard.co.uk/
There is info there that addresses all these topics. Also, Dave Hunter’s book “The Guitar Amp Handbook” explains how a tube amp works in very user friendly terms. Study the Two Stroke schematic found in the Wiki section of the TAN site. It is a very basic tube amp design that illustrates the various parts of a tube amp and how they connect to each other.