Variable bias

[Mikael Abdellah]

How will variable bias change the sound of a tube amp or preamp? It's fairly easy to add variable bias to any preamp but I don't think I have ever seen it. Is it no good?

[ray_moth]

By "variable bias" I take it that you mean adjustable "fixed" bias? (Aaah, the joys of the English language) If that is what you mean:

(a) Varying the bias on OP tubes in class AB PP allows the operating point to be positioned somewhere between class A (higher quiescent plate current) and class B (lower quiescent plate current), to determine the available output power and according to taste. The nearer to class B, the more power is available. However, hotter biasing (closer to class A) is often found to give a "fuller" tone, while cooler biasing can produce a "thinner" sound. This is discussed at a few sites on the Internet, including this one.

(b) "Variable" bias is not generally used in voltage amplification stages, because it's normally easier to provide the required bias using a cathode resistor or LEDs. The operating point of the tube is still important, because it can affect the linearity of the stage, but it's not something that needs to be tuned in the same way as PP output stages.

[Mikael Abdellah]

In my current case its the setup you described in (b). Since I am already running the tube in class A I guess (a) is irrelevant for me at the moment. Right now I have a 2.2k resistor from cathode to ground but I have seen scehamtics with 1k and similar operating voltages so I was thinking about putting a 1k resistor and a 1.5k rheostat instead. But I have searched the internet for variable bias and now adjustable fixed bias and still haven't found anything that simply explains what it really does sonically. I guess I could hook up my 1k rheostat and find out but I am not sure what I am looking for. Sometimes the impact can be so small it is hard to hear on a quickly built test setup with wires going all over the place.