Eliminating Hum

Author: N.H. Crowhurst

Heater may be a source of hum, noise and instability which can be eliminated by connecting heater system to ground.

The heaters are a possible source of hum, noise, and instability in the amplifier, particularly at high frequencies. Owing to the capacitance between cathode and heater, audio can be transferred from the cathode in an output tube to the heater wiring and from there to the cathode of the input stage, which would cause oscillation. The remedy for these troubles is to connect the heater wiring to a ground point, so that any capacitive transfer from the output-stage cathode to the heater wiring is immediately conducted to ground. Then, although minute currents may flow between the cathode and heater wiring, there will be no audio voltage corresponding to them.