One of the major drawbacks in vacuum tube amplifer design and testing is a power supply which is suitable for the job. Before any experimentation can take place you have to light the tube filaments, supply a bias voltage for the grids (if they're not cathode biased) and high voltage for the plates of all the tubes (preamp and power amp). There just isn't a power supply on the market which will supply these voltages and currents.
This throws you into having to build your own when you build your amplifier. When you do that it is typically never adjustable without having to change passive components within the supply itself. This is far from ideal for testing and experimenting with new ideas.
Enter the BirdStone Tube Amp Power Supply. This power supply will supply amplifiers from 15 to 50 watts. It will supply up to 150mA. The first two B+ voltages are not adjustable. Their voltages will be mostly determined by the current draw of the selected power tubes. The thrid, fourth, and fifth B+ voltage supplies are adjustable using rotary switches to select different resistances between the outputs.