What you need really is my ideal Bias point chart:
This is the Bermuda Triangle of Tube Biasing:
There is really no single ideal bias setup,
however, you can intelligently select operating points and ranges based on intended use.
Here are some examples:
(1) Hi-Fi setup: High Voltage B+ (500v) and Large Load (420k),
to minimize distortion and maximize voltage transfer to load (this is
not a power circuit, so voltage is more important than power transfer
efficiency). Although higher voltages raise risks, they give more
horizontal loadlines, which means current remains stable (think CCS).
Headroom is not paramount here, because typical input signals will be
played at low to mid-level volumes, to further limit system distortion,
and protection against high voltage swings can be built in via -db pads
for input of stage.
A lower bias point is selected, to better center swing in zone of maximum linearity.
(2) Universal Soldier Setup: Slightly Higher Voltage B+ (400 v
range) Here is the mid-zone. Reasonable voltage and current excursion
is expected, and more current can give both stability and a current
source for subsequent stages which might need draw. a bias-point of
-2v gives a nice centering for a balance of headroom and linearity.
(3) Guitar Maniac Setup: Here overloading is expected and harmonic
distortion (non-linearity) is actually desired. Lower plate load gives
a nice steep 'dive-bomb' loadline, while lower plate voltage (330 B+)
protects tube from HV shorting, and allows pushing tube into cut-off safely. The higher bias
point (-2.5 to even 3.5) is chosen to give maximum headroom for wild
guitar antics and easy sliding into non-linear 'sweet-spots', without
driving tube into grid-conduction, which is a non-musical type of distortion.
One thing that many people will notice, is that a large number of circuits load and bias 12AX7s nowhere near the design-center Triangle depicted above.
It will be a good exercise in fact to take both your favorite circuits
(and your notorious pet peeves) and put them on my chart, to see how and
where they are screwing up.
As I maintained in another thread, few people know how to properly set up a tube for the intended purpose.
Some things to observe:
(1) proper bias and setup is first dictated by intended use of the circuit. This must be nailed down first.
(2) Next, appropriate B+ voltages and loads are chosen, to fix the slope and position of the load line.
(3) Now, the bias-point is selected based on the balance desired
between headroom/input range and non-linearity/harmonic distortion.
(4) To force the bias-point, the correct self-biasing cathode resistor
is chosen, or better, several tubes are set up in a rig, and the
resistor is selected by experiment to put the bias-point in the best
compromise position between the acceptable range of tube samples.
(5) The performance of the circuit is tested under realistic
conditions, including input signals, and output loads from following
stages. Attenuation or amplification is adjusted at the input, and
impedance matching is done at the output, to conform to expected
Finally, notice that the ideal bias-point slides along the load-line to
the left, as the load-line tilts toward horizontal and slides to the
right. The Bias Point traces the beautiful mathematical curve known as the sea-shell spiral: