The ideal solution to fixed bias stability is to use an electronic circuit to actively sense current flow through the tube.
Benefits: tube life is extended, saturation of the output transformer is avoided, amplifier always operates optimally, better bass performance, more dynamics, danger of runaway tube current when amplifier is unattended is minimized. Also since most bias voltage supply filter networks tend to be very simple single diode or half wave, there is usually a great deal of ripple in the most sensitive signal path in the amplifier – the grid! And active bias system is also an opportunity to design a bias supply with extremely low levels of ripple. This imbues a sense of stillness and low-level detail not obtainable with standard bias voltage supplies.
Challenges: since the music signal its self effects a change in current through the tube, how can we devise a tube current sensor that responds only to changes in standing current but not respond to dynamic changes in current due to the musical signal on the grid? Also, since the circuit used to control the bias voltage will need to have some solid state devices, how can we make sure that we isolate the sound of solid state from our lovely tube’s sound?
Through creative engineering and careful design this has been achieved with the DIYHFS Active Bias Supply (ABS module)