The reference voltage is developed across a resistor.
The control loop adjusts the bias extremely slowly, rejecting dynamic variations that are a function of the music.
During startup the Active Bias System applies maximum bias voltage for around 15 Seconds, effectively shutting down any current flow in the output tubes. During this time the "Error" Leds are lit up (they may briefly flicker on/off as the control loops stabilise).
Then the Active Bias System starts ramping up the output tube cathode current slowly, reaching full preset current within around 45 Seconds.
If any one valve connected to the Active Bias System cannot be biased to specified current, but continues to increase, the "Error" LED will re-light and the (open collector) "Error" output will be pulled low to allow external protection circuitry to protect the Amplifier from the defective tube.
regards
Brian
Take control! DIY.
