Hummm

Causes
- the tube is pulling more current than the cathode emission can handle.
- the tube is resonating along with one of the transformers by means of the chassis or wiring.
- the ripple on the filament or B+ power supply is too high.
- the last capacitor in the filament supply is too far away from the tube itself.

Prevention
- use a seperate transformer for the filament supplies to prevent capacitive coupling between windings.
- make sure the power supply for the filament has a ripple of no more than 5-10mV
- make sure the B+ supply does not have a ripple greater than 20mV
- use star point grounding for all components and inputs/outputs

Damage Control
- place a cathode bypass cap of about 1000uF...
- use battery heating on the filament...
- add more capacity to the B+ supply...
- remove the ground point from earth/chassis and connect the two by means of a 100 ohm resistor...