Audio Engineering

Valve (Tube-Based) Amplifi ers 365

given for 1 and 2% THD values by a push–pull pair of U/L-connected KT88s in
relation to the anode to anode load impedance chosen by the designer. These data are
courtesy of the GEC^2 Since the distortion can also alter in its form as a function of load
impedance, Figure 11.17(b) shows the way these circuit characteristics change as the load
resistance changes. The fi gures given for a single-ended 6AK6 output pentode are due to
Langford-Smith.^3

11.9 Available Output Power ........................................................................................

The power available from an audio amplifi er, for a given THD fi gure, is an important
aspect of the design. Although there are a number of factors that will infl uence this, such
as the maximum permitted anode voltage or the maximum allowable cathode current,
the fi rst of these that must be considered is the permissible thermal dissipation of the
anode of the valve. These limiting values are quoted in the manufacturers ’ handbooks,
and from these it is possible to draw a graph of the kind shown in Figure 11.18 , where
the maximum permitted combinations of anode current and anode voltage result in the
curved (dashed) line indicating the dissipation limits for the valve, and the load line for its
particular operating conditions can then be superimposed on this graph to confi rm that the
proposed working conditions will be within these thermal limits.

Anode voltage

Anode current

Load line

Dissipation limit

Figure 11.18 : An anode dissipation curve.