(1)Yes it's microphonics alright. Using a pencil-eraser, gently (very very gently) tap on the tube envelope to find out which is the offender.
(2)Let's HOPE that the tubes exhibit a thermionic emission behavior; that's normal & is how they all work! Filament heats the cathode which emits electrons past the control-grid toward the collector plate. Without any thermionic emissions you'll get no usable output.
(2)Let's HOPE that the tubes exhibit a thermionic emission behavior; that's normal & is how they all work! Filament heats the cathode which emits electrons past the control-grid toward the collector plate. Without any thermionic emissions you'll get no usable output.