(from TM 11-866)
20. AF AMPLIFIER.
a. The AF amplifier has three stages using one VT-65 triode (V13) and three VT-66 pentodes (V14, V15, and V16). The grid of the first tube (V13) is connected to the moving arm of the AUDIO GAIN control (R26) through a blocking capacitor (C24). Its plate is coupled to the grid of the second AF amplifier (V14) by means of a capacitor C25, plate resistor R28, and grid leak R29. The second amplifier tube (V14), while a pentode, is operated as a triode by connecting its plate and screen together. It drives the output tubes (V15, V16) through a push-pull input transformer (T7). The output tubes (V15, V16) are also triode-connected and are operated as class AB2 amplifiers, which means that grid current flows during some part of the input cycle. For a power output up to approximately 3 watts no grid current flows, and harmonic distortion is negligible. Above 3 watts, and up to 10 watts (maximum output) grid current steadily increases causing a corresponding increase in distortion.
b. The output transformer (T8) has two secondary windings; a 600-ohm secondary (4-5) for power output, and a secondary for headphones (6-7) designed to deliver about 3% of the output power into an 8000-ohm resistive load when the 600 ohm secondary is connected to a matching load. The turns ratio and resistance of the headphone winding are such that the power delivered to any load between 8000 ohms and 80 ohms varies less than 6 db, and the power input into a 250-ohm load is but 2 db greater than that to a 4000-ohm load.
|