POWER PACKS 



More output and better regulation may be obtained with a device described 

 by Attree^" wliich is primarily a stabilizer having an a.c. output, but which 

 can easily be arranged to give d.c. This uses two beam tetrodes arranged as 

 a screen-coupled multi-vibrator to generate square waves at 2 kc/s. These 

 are transformed to the voltage required for the output, and are also used to 



Mains in 

 oN 



Figure 37.19 



heat the cathode of a saturated diode, the anode current of which controls, 

 via a valve, the power supplied to the multi-vibrator {Figure 37.20). In this 

 manner the output R.M.S. voltage is both back-stabilized (output impedance 

 0-03 ohms for 6 V output) and forward-stabilized (0-1 per cent change in 

 output for 10 per cent change in mains): the a.c. power output available 

 is 10 W. Because of the high frequency and the square waveform this 



Figure 37.20 



supply is particularly easy to rectify and smooth ; Attree suggests the use of 

 a single 100 mH choke (actually, the secondary winding of a small loud- 

 speaker transformer) and capacitors of 50 and 1 ,000 /j,F. At 2 A output the 

 ripple is then below a millivolt; unfortunately 3 W are lost in the recti- 

 fication and smoothing process. 



A recent device of promise is the electrolytic smoothing and stabilizing 

 cell, manufactured by L'Accumulateur fitanche S. A. of Brussels. A heater 



596 



