REFERENCES 



case the effective output impedance of the device is 1-2 megohms. Clearly 

 a 1 per cent change in resistance of a 100 ohm coil in series with 1-2 megohms 

 has a quite negligible effect on the coil current. 



'Stopper' resistors 



In many practical circuits, particularly those containing a power valve, re- 

 sistors connected immediately in series with the control grid, screen grid or 

 anode sometimes appear. These were not discussed in Part I. 



The wiring associated with the valve has stray capacitance and a small 

 amount of inductance. Under certain circumstances an unwanted LC 

 oscillator is formed. These 'parasitic' oscillations are extremely rapid, of the 

 order of 100 Mc/s and may be difficult to detect as such ; but the normal oper- 

 ation of the circuit is impaired. To ensure they cannot occur it is a sound pre- 

 caution to damp heavily any possible such spurious LC circuits by the 

 inclusion of 'stopper' resistors. Anode and screen stoppers may be of the 

 order of 100 ohms, and control grid stoppers 10 k; they should be soldered in 

 as close as possible to the relevant valveholder tag. 



Stoppers appear in Figures 37.8, 37.10, 37.13, 37.23, etc. 



REFERENCES 



^ Luke, R. J. Electronic Engineering 28 (1956) 100 



^ Philips Technical Review, February 1941 



^ Valley, G. and Wallman, H. Vacuum Tube Amplifiers. Radiation Lab. series, 

 McGraw-Hill 



* Attree, V. H. Electronic Engineering 27 (1955) 174 



5 Walker, A. H. B. Wireless World 58 (1952) 374 



^ Perry, B. J. Electronic Engineering 28 (1956) 517 



' Scroggie, M. G. Wireless World 54 (1948) 543 



® Dickinson, C. J. Electrophysiological Technique, Electronic Engineering 



» Cherry, L. B. and Wild, R. F. Proc. Inst. Radio Engrs. N. Y. 33 (1945) 262 

 10 Attree, V, H. Electronic Engineering 25 (1953) 71 

 " Dickinson, C. J. Wireless World 57 (1951) 70 

 ^^ Barron, J. Electronic Engineering 26 (1954) 393 

 ^^ Lowe, A. E. Electronic Engineering 27 (1955) 85 

 ^* Ackland, R. G. Electronic Engineering 26 (1954) 143 

 ^^ Benson, F. A. and Seaman, M. S. Electronic Engineering 28 (1956) 260 

 i« SowERBY, J. McG. Wireless World 55 (1949) 395 



601 



