[turretslug]

Some high-impedance thermionic diode circuits with cathodes that are "live" to audio can be very prone to hum resulting from stray loops of heater filament protruding from the ends of the cathode cylinder which are at high enough temperature to emit electrons, and comms receiver noise limiters are a classic example. One remedy is to slightly reduce the heater temperature with a series resistor- the cathode cylinder, being coated with an emissive potion of low work function, is less affected by this lower temperature than the plain stray heater ends. Later AR88s got this resistor in series with the 6H6 noise limiter's heater. Another remedy, notably adopted in many Eddystone sets, is to use a separate heater winding for the noise limiter diode which is elevated to a higher (positive) DC potential than the maximum AF excursion expected.

Colin