Name:    Steve Kennedy
Email:   steve@pacair.com  
Subject: Self Biasing amps?
Thread:  109
Time:    Thu, 19 Jul 2001 00:33:35 UTC

Here is my take on this...

The output stages of most Music Man amplifiers are designed the same... that is they are cathode driven and cathode-biased.

There are only two basic methods of output tube bias used in "modern" guitar tube amps. The "fixed-bias" type (like most Fenders) grounds the Cathode of the output tubes and applies a "fixed" (unchanging) negative voltage bias to one of the grids. It is this negative grid bias voltage that you are adjusting when you tweak the bias pot in one of these amps. Once adjusted, the current through the tubes is "fixed" to a level determined by the negative grid bias voltage value.

In cathode biasing, the cathode goes to ground through a resistor and no negative biasing of the grids is required. As the tube is driven harder, more current tries to flow through the cathode resistor. This has the effect of inducing a positive voltage across the cathode resistor, so the cathode now looks less negative to the grid than it did when there was no drive. This has the effect of limiting the bias current... the harder you drive it, the more it resists increased current flow. This is why cathode-biased amplifiers are ALSO known as "self-biasing". They don't automatically bias themselves and match the tubes (as some people may imply), they simply allow a "dynamic" changing bias that happens automatically while you play.

The "bias" adjustment that is used in the older Music Man amps is not the same as that found in fixed-bias designs. The MM bias control adjusts the bias of the driver transistors, which sets the base "quiescent" bias current (what the amp draws at rest). The bias is still dynamic and changes while you play (becoming more self-limiting the harder you drive it), it just returns to a "base" level that is set by the "bias" pot.

To change the bias range in this sort of amp, you would have to manually play with resistor values, in effect you would be redesigning the amplifier output stage. Because these resistor values are in series with total current the output tubes draw and are very low in resistance value, it is not typical to see them replaced with adjustable pots. You should use matched tubes in these amps, as there are no inherent matching or balancing adjustments.

Steve