Audio Engineering

Valve (Tube-Based) Amplifi ers 345

In use, a small-signal pentode amplifying stage will give a much higher stage gain than
a medium impedance triode valve (250 ̆ in comparison with, say, 30 ̆). It will also have a
better HF gain due to its lower effective anode–grid capacitance. However, a triode gain
stage will probably have a distortion fi gure, other things being equal, which is about half
that of a pentode.

The second solution to the problem of anode current nonlinearity in tetrodes, particularly
suited to the output stages of audio amplifi ers, was alignment of the wires of the control
grid and screening grid so that they constrained the electron fl ow into a series of beams,
which served to sweep any secondary electrons back toward the anode—a process
that was helped by the inclusion within the anode box of a pair of “ beam-confi ning
electrodes, ” which modifi ed the internal electrostatic fi eld pattern. These are connected to
the cathode internally and take the form shown in Figure 11.5. These valves were called
beam tetrodes or kinkless tetrodes and had a lower distortion than output pentodes. Valves
of this type, such as the 6L6, the 807, and the KT66 and KT88, were widely employed in
the output stages of the high-quality audio amplifi ers of the 1950s and early 1960s.

Beam-forming plate

Cathode

Grid

Screen

Plate

Figure 11.5 : Construction of a beam tetrode.

(Courtesy of RCA.)