HARD VALVES 



Hard valves are those in which the electrodes are sealed in a glass envelope 

 containing a vacuum. The object of this is to exclude as far as possible the 

 complicating effects of ions in the valve. Hard valves are described according 

 to the number of electrodes they contain: diode, triode, tetrode (or screened 

 grid), pentode, hexode, heptode (or pentagrid), octode and nonode. Of 

 these the screened grid, hexode, heptode, octode and nonode valves are of 

 little interest in general electronic work, being used for specialized radio 

 appHcations; the important hard valves for electrobiology are the diode, 

 triode, beam tetrode and pentode. 



Hard valves employ thermionic emission from a heated cathode. The 

 cathode is heated by current supplied from a special auxiUary circuit which 

 for clarity is usually left out of circuit diagrams. The heating may be achieved 

 in either of two ways. 



DIRECTLY HEATED CATHODE 



The cathode is in the form of a thin wire or ribbon of tungsten which is 

 heated by the passage of a suitable current through it (Figure 8.1). It is 

 usually coated with a material which emits electrons richly at relatively low 



Tensioning 

 spring 



Cathode 



Support 

 wire 



Symbol 



Lead-in 

 wires 



Figure 8.1 



temperature — dull red heat — thus reducing the amount of heater power which 

 has to be supplied. This material is usually a mixture of barium oxide and 

 strontium oxide. The thermal capacity of the cathode is low so that to 

 achieve a steady emission the heater current has usually to be direct; a few 

 valves having particularly robust cathodes — capable of holding more heat — 

 are designed for direct heating from a.c. They are mostly power rectifier 

 diodes, for in power rectification there is ripple in the output anyway, and a 

 little more due to 100 cycle fluctuation in cathode temperature does no harm. 



132 



