348 Chapter 11
exceed 200 V. Moreover, there must always be a resistive path, not exceeding 250 k Ω ,
between the cathode and heater circuits.
As a practical point, the wiring of the heater circuit, which is usually operated at 6.3 V
AC, will normally be installed as a twisted pair to minimize the induction of mains hum
into sensitive parts of the system, as will the heater wiring inside the cathode tube of low
noise valves, such as the EF86. With modern components, such as silicon diodes and low-
cost regulator ICs, there is no good reason why the heater supplies to high-quality valve
amplifi ers should not be derived from smoothed and stabilized DC sources.
It has been suggested that the cathodes of valves can be damaged by reverse direction
ionic bombardment if the HT voltage is applied before the cathode has had a chance to
warm up and form a space charge, and that the valve heaters should be left on to avoid
this problem. In practice, this problem does not arise because gaseous ions are only
formed by collisions between residual gas molecules and the electrons in the anode
current stream. If the cathode has not reached operating temperature there will be little
or no anode current and, consequently, no gaseous ions produced as a result of it. Brimar
specifi cally warns against leaving the cathode heated, in the absence of anode current,
in that this may lead to cathode poisoning because of chemical reactions occurring
between the exposed reactive metal of the cathode surface and any gaseous contaminants
present within the envelope. Unfortunately, the loss of electron emissivity as the cathode
temperature is reduced occurs more rapidly than the reduction in the chemical reactivity
of the cathode metals.
Indirectly heated HT rectifi er valves have been used, despite their lower operating
effi ciency, to ensure that the full HT voltage was not applied to the equipment before
the other valves had warmed up. This was done to avoid the HT rail overvoltage
surge that would otherwise occur and allow the safe use of lower working voltage and
less expensive components, such as HT reservoir, smoothing, or intervalve coupling
capacitors.
11.1.9 Microphony
Any physical vibration of the grid (or fi lament, in the case of a directly heated cathode)
will, by altering the grid–cathode spacing, cause a fl uctuation of the anode current, which
will cause an audible ringing sound when the envelope is tapped—an effect known as
microphony in the case of a valve used in audio circuitry. Great care must therefore be