Output Stages
The basic choice of output valves will lie between a triode, a beam tetrode, or a pentode. If large output powers are required—say, in excess of 2 W—triode output valves are unsuitable because the physical spacing between the control grid and the anode must
be small, and the grid mesh must be relatively widely spaced, in order to achieve a low anode current resistance and a high practicable anode current level. This closely packed
type of construction will lead to the almost complete stripping of the space charge from the region between the cathode and the grid. Experience shows that the life expectancy of cathodes operated under such conditions is short, and the only way by which this problem can be avoided is by the use of a directly heated (filament type) cathode construction, which is much more prolific as a source of electrons, and this leads to other difficulties such as hum intrusion from the AC heater supplies, and the awkwardness of arranging cathode bias systems.
So, if it is required to use a triode output stage, at anything greater than the 50-mA anode current obtainable from a parallel connected 6N7 double triode (the 6SN7 has a smaller envelope and, in consequence, a lower permissible anode dissipation), a directly heated valve such as the now long obsolete 6B4 or PX25 would need to be found. Therefore, in practice, the choice for output valves will be between output beam tetrodes or pentodes. Although a fairly close simulation of a triode characteristic can be obtained in both of these valve types if the anode and G2 are connected together, this approach works better with a beam tetrode than a power output pentode because the presence of the suppressor grid in the pentode somewhat disturbs the anode current flow.
The required grid drive voltage for typical pentode or beam tetrode output valves, at Va = 300 V, will be in the range of 20–50 Vp–p for each output valve, and whether or not the valve is triode connected has little effect on this requirement. The triode connection does, however, greatly affect the anode current impedance, which is reduced, in the case of the KT88, from 12 kΩ to 670 Ω, and the need for a lower turns ratio greatly simplifies the design of the necessary, load-matching, output transformer with low half-primary to half-primary and primary to secondary leakage inductances.