On 08/06/2011 04:44 PM, Michael wrote:

In tube and transistor and tube amplifiers I am familiar with the
definition of a class A, class B, and class C amplifiers. Until
recently I had assumed class AB1 and AB2 must be somewhere in between
class A and class B, however after reading some post in other news
groups I'm beginning to suspect my understanding of class AB1 and AB2
may not be accurate or complete. So what is the accurate definition of
a class AB1 and a class AB2 amplifier?

Thank you,

Michael Rawls
KS4HY

Technically all class "B" amplifiers are really class AB. A class B
amplifier has a conduction angle of 180 degrees. A class AB amplifier
has a conduction angle of between 360 degrees and 180 degrees. You
can't run the amp at EXACTLY 180 degrees because tubes (and transistors)
are not perfect devices and will distort (go non-linear) in the fuzzy
region just before cutoff. Therefor the devices must be biased to draw
some resting current to make sure that at least ONE of the two devices
in a push-pull circuit are running in the linear region at all times.

With tubes (valves) the amount of bias required is a function of the Mu
(gain) of the device (at least for triodes). Tetrode and beampower
tubes are always biased so the device will draw enough resting current
to avoid crossover distortion. Low Mu triodes are biased in the same
manor as tetrodes (but will require MUCH higher bias). High Mu triodes
(or triode connected tetrodes) will not draw very much current at zero
bias. These tubes are run in what is called class B. Since the tubes
are run at zero bias they require driving power over 180 degress of the
conduction angle, which meets the definition of class B.

Most tetrodes do not have enough gain in their screen grids so that the
tube can be driven to full output in class AB1. The 6L6 for example
when biased at about -35 volts with 300 volts on g2 and a plate voltage
of 400-500 volts will not be driven to it's maximum current of 110ma
(per plate) without driving the grids positive. In fact the difference
between class AB1 and AB2 for the 6L6 is ONLY the driving voltage. The
g2 and bias voltages are the SAME for class AB1 and AB2, only the g-g
peak signal voltage is different. The type 6146 tube has enough gain in
the screen grid that the difference between AB1 and AB2 max ratings is
very small, in AB1 it delivers a max of 120w RMS output (max ICAS) while
it can go to about 132w in AB2. The 807 tube (same curves as the 6L6)
maxes out at 90W in AB1 and 120W in AB2 (ICAS ratings).

With solid state devices the terms AB1 and AB2 have no meaning since the
device will only conduct with the control element biased in the forward
direction (bipolar transistors and enhancement mode mosfets). A power
JFET could, in theory operate in class AB2, but there are very few
examples of power JFET's in production (ditto for depletion mode
mosfets). So we only use AB with no modifier.