A typical triode audio power amplifier will have a driver that provides voltage gain, coupled to a triode ( like 2A3 and 300B ) or a pentode or kinkless tetrode such as EL34 or KT88 connected as a triode, connected to the loudspeaker through an audio transformer in a common cathode arrangement.
22.
Audio transformers for vacuum tube equipment were coded blue for the finishing lead of the primary, red for the B + lead of the primary, brown for a primary center tap, green for the finishing lead of the secondary, black for grid lead of the secondary, and yellow for a tapped secondary.
23.
The resulting audio signal extracted by the diode from the RF signal is coupled back into the grid circuit by audio transformer " "'T " 1 "', " "'T " 2 "'whose iron core serves as a choke to help prevent RF from getting back into the grid circuit and causing feedback.
24.
Valve circuits for quality reproduction have long been produced with no other ( inter-stage ) audio transformers, but an output transformer is needed to couple the relatively high impedance ( up to a few hundred ohms depending upon configuration ) of the output valve ( s ) to the low impedance of a loudspeaker . ( The valves can deliver a low current at a high voltage; the speakers require high current at low voltage . ) Most solid-state power amplifiers need no output transformer at all.
