The phase inverter
The phase inverter used is of the type in which the out-of-phase voltage is obtained from the cathode circuit. Actually, the 25,000-ohm resistor at the ground end of the 6J5 cathode is really part of the plate circuit. Where additional amplifier gain is not needed, this form of phase inverter is preferable (from the standpoint of simplicity) to the double-triode type, and was used for this reason.
Notice the 250,000-ohm resistors connected from the plates of the 6L6-G tubes to the plates of the 6C8-G tubes. These are the feedback resistors furnishing negative feedback which contributes so much to the excellent frequency response, and to the hum-free operation.
This method of feedback was purposely used, so that the constructor would have no trouble in getting the amplifier to operate properly the first time it is tried out. The polarity or magnitude of the feedback voltage is not dependent on the output transformer, so the constructor need not fear that the amplifier will squeal and howl like a banshee when first turned on. Incidentally, no output transformer was incorporated on the chassis, because the speaker used by the writer has the transformer built onto it.
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