By default the Automatic Bias System comes with a fixed resistor, which will
make it output a fixed bias to the tube. But I wanted to be able to adjust
the bias so that I could swap between different types of tubes (e.g. EL34
and KT88) while still taking advantage of the ABS control.
The ABS package includes a table that shows what bias you get for a given
resistor value, so I picked three key values as being important:
2.7KΩ - gives a bias current of 35mA
3.9KΩ - gives a bias current of 45mA
4.7KΩ - gives a bias current of 55mA
I chose these because 35-45mA is about the right range for an EL34 and
45-55mA is about right for a KT88.
I already had a 22KΩ potentiometer, so wanted to use this to adjust the bias.
First I thought of putting it in parallel with a 7.5KΩ resistor, which would
result in this:
In this chart, the horizontal axis is the setting of the bias pot from 0Ω
to 22KΩ. The vertical axis is the total resistance, so the blue line shows
what resistance you get for the setting of the bias pot. The red lines
show the key values described above, so it can be seen that for an KT88
the bias pot needs to be set between about 8KΩ and 12.5KΩ.
This isn't too bad, but I wanted a wider adjustment range to make it more
accurate. By using a 3.3KΩ resistor in parallel to the 22KΩ pot and then
adding a 2KΩ resistor in series with them I get:
This now gives me a usable range from 4.5KΩ to 15KΩ which is perfect for
a KT88, although the range for an EL34 has actually been reduced slightly.