HEATER POWER SUPPLIES 



A better method is to use a 'hum-dinger.' This is a low resistance poten- 

 tiometer — some 50 ohms — connected as in Figure 37.16. The shder is then 

 adjusted until the 'hum' is minimized. The effect of inductive coupling is 

 reduced by tightly twisting the heater wiring, so that the magnetic fields 

 due to the two wires as nearly as possible cancel {Figure 37.17) and by the 



A^/ A^/ ''^«^' 



3 



50fl 



Figure 37.16 



X 



/ 



Figure 37.17 



use of valves having heaters of special construction aimed at minimizing 

 magnetic field production, such as the Mullard EF86, Osram Z729, etc. 

 In the highest class of amplifier it may be safer to excite the heaters of 

 low-level stages from direct current, and if the apparatus is direct coupled, 

 a stabilized heater supply may be advisable as well. The simplest d.c. 

 heater supply which also is fairly stable is the lead-acid accumulator, but 

 the cells should be large — i.e. a 6 V car battery — so that the method is bulky 

 and, in so far as accumulators need careful maintenance, not very con- 

 venient. It is possible to make heater supplies by transformation, 



+ o- 



dc. 

 out 



— o- 



J. '"^ 



11.000 pF 



1H 



Figure 37.18 



rectification and smoothing of the mains, but here again a bulky apparatus 

 results because adequate smoothing of large currents calls for chokes 

 which are large (because the wire must be thick) and capacitors which 

 are large (because of the magnitude of current they have to supply between 

 rectifier conduction periods without significant fall in potential difference). 

 Thus Dickinson^ suggests, in a d.c. heater supply of 1 to 2 amps, the use 

 of a 1 H choke in each lead and 1,000 /xF input and smoothing capacitors 

 (Figure 37.18). 



In general the heater current demands of apparatus is constant so that 

 in order to stabilize such a supply it is only necessary to stabilize against 

 variations in mains voltage. A simple device for doing this, which gives 

 a modest but useful output, is due to Cherry and Wild^. Stabilizer tubes 

 are used in a bridge circuit on the primary side of the transformer {Figure 

 37.19). With this device a 10 per cent change of mains voltage produces 

 only a 0-35 per cent change of output voltage. 



595 



