All about Tubes, Tube Circuits, Tube Gear

Thursday, June 14, 2012

Tube Biasing 101 (Pt 2): 12AX7 bias chart!

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 conditions.

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:

No comments:

Post a Comment