VACUUM TUBES AS HIGH-FREQUENCY OSCILLATORS 99 



Effect of Energy Losses on Performance 

 A tube operating in the range where frequency afifects performance 

 must withstand energy losses, and the resulting heating within its 

 structure, which occur to only a negligible degree at the lower fre- 

 quencies. Some of these losses are due to dielectric hysteresis in the 

 insulating materials of the tube, particularly in the portions of the 

 glass supporting stem or bulb which lies between the tube leads. 

 The glass is sometimes so softened by the heat thus developed that it is 

 punctured by the outside air pressure. Losses also occur in the 

 auxiliary metallic parts of the tube structure due to the increased eddy 

 currents that occur at high frequencies. Losses in the tube electrodes 

 and their lead-in wires are also greatly increased due to skin eflfect 

 which increases their resistance, and due to the increased charging 

 current required by the inter-electrode capacitances. The increased 

 lead temperature, depending upon its amount, will cause a more or 

 less rapid deterioration of the lead-to-glass seals which may ultimately 

 destroy the vacuum. In order to protect the tube from damage 

 because of these new types of energy dissipation, the operating 

 potentials and currents must be reduced to values less than those 

 established for low-frequency operation. Some manufacturers are 

 now giving special ratings on such of their standard tubes as may be 

 used at ultra-high frequencies. These ratings should be adhered to 

 when operating in this range. 



Effect of Circuit on Performance 

 The decrease in output power and plate efficiency which sets in with 

 the increase in frequency, while due in part to the losses described 

 above, and to the rapid increase in radiation losses, is also due to two 

 additional effects of fundamental importance. The first to become 

 evident, with increasing frequencies, is circuital in nature. This can 

 be explained by reference to the conventional oscillator shown in 

 Fig. 2. The frequency of such an oscillator is given by 



/= ' 



LC 



where L and C are the effective inductance and capacitance of the oscil- 

 lating circuit. In the lower frequency range, the LC product which 

 determines the frequency is substantially equal to I-oCo, that is, to 

 the product of the inductance and capacitance of the external circuit. 

 The inductance of the tube leads and the capacitance between its 

 electrodes (indicated by the dotted lines in the figure) play a negligible 

 role. In order to tune the circuit to higher and higher frequencies. 



