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THEORY AND WIRING OF AMPLIFIERS 555
the notation, "45 Fil." which indicates that this winding heats the filaments of the type 45 amplifying tubes (top right of the drawing). Glancing at those tubes we see that their filaments also terminate in arrowheads. This is a common method of showing filament circuits. Drawing in the connecting wires would complicate the diagram without giving any additional information. Let us return to the transformer. The winding that supplies the "45" filaments is bridged by a center-tapped resistor, the function of which will be seen later. The lowest secondary of this transformer supplies power to the heaters of the other four tubes. This coil also is shunted by a resistor, the center-tap of which is grounded. Tracing the wire from the center-tap we see that it runs without interruption to the ground connection. An opencircuit in this ground connection might cause the amplifier to hum.
The rectifier tubes shown just to the left of the plate secondary of T-3 happen to have two plates each. One of these tubes is sometimes used alone as a "full-wave rectifier. But in this circuit the plates are connected together and act as a single plate; each tube acts as a half-wave rectifier, the two being combined in a fullwave rectifying circuit. (Page 501.) The positive side of the d. c. circuit leading from these tubes is, of course, the wire leading from their filaments, and the negative side is the wire running to the center-tap of the plate or high-voltage secondary. Tracing one branch of this circuit from positive to negative: from the filaments of the rectifier tubes left about % inch, up one inch, left J/S inch, down y^ inch, and left about one inch to the filter choke coil, L-l. At both sides of this choke coil the line is bridged by filter condensers C-9. We continue to follow the positive line through R-20 and R-19, to the point where it turns downward about an inch and a half left of R-19. Thence the circuit runs through Resistors 15, downward and right, through R-16 and still further right, then upward for two inches, right for two inches, and down an inch to the center terminal of the speaker terminal strip. Thence down about an inch, left a quar