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.
Shielding is an effective means of keeping out RFI noise. Shielding is a good thing in a heavy RF environment. It does nothing to the noise on the line feeding the cord.