WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
If you pop over to Audio Asylum and search the Bottlehead forum for posts containing "parafeed" by Paul Joppa or Doc B or Voltsecond, you'll find all you need.