Instead of measuring the total cathode current (in mA) for the pair of
output tubes, you can measure the voltage across the current sensing resistor.
First, you must accurately measure the resistance of the current sensing
resistor(s) within the output tubes' cathode circuit. Using ohms law, (E= IxR),
you can calculate the correct voltage. Finally, adjust the bias for the
correct calculated voltage.
You can prolong amp and tube life by
properly adjusting biasing or even adjusting biasing a tad (10 %) cooler then normal (higher
negative grid bias).
But, be careful, if you adjust the bias to operate too cool, the amp's
sound quality may degrade.
As an example, let' say the spec for the Scott 299D,
calls for an output grid voltage of -20.3 volts DC when correctly biased.
By adjusting the bias down 10%, to -22 volts DC, the amp still sounds great.
The benefit is the transformers all run a lot cooler, and the
expensive, out-of-production, 7591 output tubes last