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Mar 20 2014

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Fixed + Auto = Fixed Auto Bias ?

{{ Update: 2014-03-26 }}

.. Come to think about it, especially in terms of the preamp sections, there is YET another type of bias … a Constant Current type “bias”, or setup, if you will.

Imagine that you load the cathode of a differential input stage with a Constant Current Sink.  Is this “Bias” ? Obviously, it IS.  Is this “voltage” bias ?  Well, strictly speaking, not necessarily, as the sinking circuit strives to keep the CURRENT constant, the cathode current flowing through the tube.  OK, so what about the “voltage” value of the bias?  The simple answer is: does not “matter”.  The grid-to-cathode voltage, or “bias”, if you prefer, shall under such conditions shall adjust “by itself” to such a specific value so as to “enforce” the possibility of current flow, of a value as defined by the Constant Current Sink. So, depending on the specific characteristics of this tube or that tube, it shall adjust by itself to such a value of bias voltage, so that the programmed current shall flow. Even within a pair of “unmatched” tubes, you shall observe that the current is the same, but … the bias voltage have adjusted “themselves” to slightly different values, so as to facilitate such even flow …

Hmmmm …. since we are at this particular idea …. interesting … would it be possible to build a negative-current-feedback push-pull output stage, on say 2x KT88, the cathodes of which are routed in antiphase via two totally independent, equal-turn-rate secondaries of the output transformer, and then loaded with two independent, finely tuned/trimmed Power-Current-Sinks … hmmm … need to ponder about that a bit. … or maybe even a centrally taped secondary winding, with the cathodes being “sunk” by a single power-constant-current-sink ? … hmmmm ….

{{ /update }}


Have you ever pondered about the idea as to what type of bias to choose ?

Fixed bias, or Auto bias ?

Well, each of them has it’s pro’s and it’s cons.

The auto-bias scheme is definitely a feature that introduces a bit of local degenerative cathode feedback.  This tends to “stabilize” the working conditions of the tube. Especially the output tube. This works both in the AC as well as in the DC domain, provided that no bypass cap is available. It is far less probable for a tube to thermally “run away” in an auto bias scheme, as in comparison to a fixed bias scheme.

The auto-bias scheme, especially in the output tubes section, has it’s drawback in the sheer loss of power that is wasted on the resistors, through which flows quite a significant current, as some tube types require.

Some people also complain about the specific sound signature (compressed?), that goes with the auto-bias scheme.

bias fixed

Above: some examples of fixed bias. Obviously, the choice and exact manner of application depends on the place within the circuit and specific requirements.

bias fix auto fix-auto


Above: a simple “fixed” bias scheme, a simple “auto” bias scheme, and a simple “Wash-And-Go” scheme.

Contrary to the auto-bias, you have the fixed bias, with the punchy bottom line, with the increased output stage gain, and with an increased fear of thermal runaway, if something goes astray. But have you ever considered the third option ?

Mainly, a bit of BOTH ?

Lets say that you need some -60 Volts of grid bias on a specific tube.

How about making it 50/50 ?   With say, 30 Volts sourced from a voltage source, and the other 30V dropped on a cathode resistor ?

Or some other proportion, depending on your particular situation ?

Please note that by using a smaller resistor, for the “auto bias” part of the scheme, you actually also have the side effect of changing the degenerative feedback for the tube.

Please note that a zener diode, or a LED, can, under certain circumstances, perform essentially the same function as a “fixed” voltage source.  Having said that, you need to be aware that such solutions tend to increase the THD a bit. They may also introduce a certain sound flavour to the sound, which you may like, but then, you may not.

To muddle the waters even further, you can go yet another step forward and modify the cathode circuit of your (output) tubes, so that the cathode current is routed through a secondary winding of the transformer, prior it hits the common ground.  This would also introduce some Negative Current Feedback from the transformer’s secondary. NCF is a good option, because it is based on current actually flowing through the secondary, and not just but voltage relationships.

If you have a push pull stage, with two even-count secondary windings on the output side (such as in the case of a series of 0 – 4 – 16 ohm taps), it is best to route the cathode currents of the respective tubes directly through the secondary speaker windings, provided that you have in advance provided for appropriate central tap within.

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