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.
There is absolutely NO reason to do this - unless there is a decoupling cap on the powersupply line that is after the diode - the diode then is acting as a "decoupler" so that ripple on the main supply from heavy load current demands doesn't affect the circuit being isolated.