Class-B Operation

Author: N.H. Crowhurst

Calculations for class B push-pull. Load line is for each tube, but only one works at a time on alternate halves of waves.

If we wish to use an extreme economy measure, known as class-B operation, we bias each tube back approximately to zero current. (Actually it does not go quite to zero current because of the curvature, but it goes to where zero current would be if the tube characteristics were all straight.) Using the same load line, the operating point for each tube would (in theory) be 400 volts at zero milliamperes. This means the plate potential voltage will swing from 100 volts to 700 volts. Plate current will swing, during one half-cycle, from zero to 60 milliamperes and back, while in the other half-cycle, no current iows in that tube.

The permissible maximum voltage on the plate is considerably increased by this method of operation (from 400 volts to 700 volts). There is less danger of breakdown between the plate and some other electrode when no plate current is flowing. There is, however, a maximum rated voltage even under this condition, which sometimes restricts the amount by which this method of operation can improve efficiency.

Push-pull class B amplifier operation

In this case, the peak voltage and current swing are now 300 volts and 60 milliamperes in each direction, with one tube providing one half-wave and the other tube providing the other half-wave. This represents a peak power of (300 X -06) or 18 watts for the complete arrangement, or an average power of 9 watts - just double that of class-A push-pull operation.

Class B operation is suitable only where continuous skilled supervision is available

This method is not too often used because it involves very careful adjustment of the operating point. Tubes vary in individual characteristics due to production differences, with the result that the zero plate current point (the point where the grid bias cuts off plate current) is apt to differ noticeably between individual tubes. This variation requires a critical adjustment in the amplifier to make sure that both tubes are at the correct point for producing undistorted output. Not only is this adjustment critical when replacing tubes, it can also involve continuous attention if the plate voltage changes at all due to line voltage supply fluctuation or due to the fact that tubes sometimes change their characteristics as they get older.