Immediately following the bass boost circuit is a treble tone control having four positions: After the selector switch there 18 & bass tone control which gives a range of 15 db variation at 100 cycles. This high impedance, low gain input is intended for the audio portion of television, magnetic reproducer, or similar use. A four position selector switch is supplied to switch between AM - FM, phonograph, and an external connection which is labeled television. (3) A single stage audio amplifier is provided with the necessary equalization for using a variable reluctance or similar type phonograph pickup. This trap provides optional attenuation at any portion of the band so that the signal from a strong interfering local station may be reduced to a point where other weaker stations may be received without interference. The second section covers the range from 900-1800 kilocycles. One section of the trap covers the range from 500-1000 kilocycles. A dual wave trap is provided at the input of the AM section, and is inserted by means of a link on the antenna terminal strip. A separate 605 / G5 Electron Tuning Indicator is used as an AM tuning indicator. 18 obtained by means of a IN34 Crystal and is applied to both RF stages. The detector is of the infinite impedance type and the audio output is obtained across a portion of the cathode resistance. Complex coupling networks are used between the various networks to provide nearly constant gain and band width. It is of the tuned radio frequency type, employing two 6AU6 type tubes in two stages. (2) The AM Tuner covers the band of 514-1740 kilocycles. Maximum sensitivity of this unit is 5.5 trovolts with a quieting sensitivity of 12 microvolts. A half wave dipole antenna, having an impedance of 300 ohms 18 supplied with a sixty foot transmission cable. Accurate tuning is aided by the use of a 605/665 electron tuning indicator. The triode mixer is used since it has high gain and low noise compared to a pentagrid converter. The balanced antenna transformer, used as the coupling medium into this stage, gives a high degree of rejection to unwanted interference signals picked up by the antenna lead-in. The modified cascode circuit, a wartime " radar development which is used as the RF amplifier, produces high signal gain with very low noise.
Delayed AVC gives good small signal sensitivity and is applied to the RF and first IF stages. The output of the second IF stage feeds the ratio detector which incorporates & 6AL5 tube. Two stages of IF amplification, having a frequency of 10.7 megacycles, use GBA6 tube8. A 6AU6 tube is employed as a separate oscillator, the voltage being injected into the grid circuit of the second half of the 12AT7 tube, which operates as a mixer. The RF stage consists of a 6ABL tube and one half of a 12AT7 tube connected in "cascode". The antenna is coupled to the RF stage through a broad band transformer having a high degree of balance. (1) The FM Tuner employs six miniature tubes.