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Tech Talk with
Steve Wilson:
Biasing Your Amp
"Bias" is to your amp like "idle" is to
your car. It sets the RANGE or CLASS of operation for the tubes in that
amp like the idle sets the range of operation for your engine. The bias is
adjusted when the amp is not playing (no signal), and the idle is adjusted
when the car is not moving (no motion). Too much current flowing (idle too
high, wears parts and wastes fuel) and the amp runs hot and burns up tubes
and transformers. Not enough current (idle too low, sluggish take off) and
the tubes don't turn on long enough to amplify the signal without a lot of
distortion. This gritty, grainy, nasty distortion is easily recognizable
once you know what you're listening for.
CLASSES OF OPERATION: The class of
operation for a tube amp is determined by the amount of time the tube is
conducting signal. What determines the amount of time the tube conducts
signal? Bias. The two main classes of operation used in guitar amps are
class "A" and class "AB1". This discussion will be limited to them.
CLASS A: In a class A amp, the tube is
biased on the linear part of it's dynamic characteristic curve and the
signal driving the grid is small enough so that it remains on the linear
part throughout the full cycle.
This ensures the output wave form closely
resembles the input signal. Since the output is similar to the input, there
is little or no distortion. The input signal never drives the grid
sufficiently negative to "cut-off" the tube. This allows plate current to
flow for the complete cycle. Have you noticed how "hot" a class A amp seems
to run? That's because the tube never "shuts off" and is always dissipating
a lot of power. This dissipated power is developed across the tube and not
the load, and is wasted as far as output power is concerned. This means that
class A amps are low efficiency. Also, the amplitude of the input signal is
less than the bias voltage, so the grid is never driven positive. This keeps
distortion producing grid current from flowing.
Class A amps are characterized by the
following:
- little or no distortion
- no grid current
- plate current that flows for the whole cycle
- low efficiency
CLASS AB1: For simplicity, we'll loose
the subscript on class AB1 and just call it class AB from now on. In a class
AB amp, plate current flows for slightly more than 1/2 of the input signal
cycle, but less than the whole cycle. This allows the tube to "turn off"
occasionally, and not run as hot as class A. Like class A, the input signal
is less than the bias voltage, so no grid current flows. In fact, that's
what the subscript "1" stands for - no grid current flow. The bias for class
AB is set somewhere between "cutoff" and the linear part of the dynamic
transfer characteristic curve. Part of the negative 1/2 cycle of the input
signal drives the tube into cutoff, turning it off, and clipping its output
signal.
During this part of the cycle where the tube
does not conduct, no current flows through the tube. This means it doesn't
have to dissipate any power (heat), and cools somewhat. Now, with only 1/2
of the signal present at the output, it's going to sound pretty awful! This
is the "Push" of the Push-Pull topology that is always associated with class
AB. The "Pull" is the other tube, or set of tubes, which are driven 180
degrees out of phase by the phase inverter . As the Push goes into cut off,
the Pull starts conducting. The two signals are mixed in the output
transformer and sent to the speaker so we hear a continuous sound.
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