12 „ Fig* 30 shows the frequency response of the 120B amplifier < 

 complement is as follows: 



Its tube 



120B Amplifier: One 6c6, replaceable vdth type 1603 or 77 



i One 1620, replaceable Tilth type 6J7 or W*E„ type 348A 

 Input and Output Noise Voltages of 120B Amplifier 



13 o To determine the input and output noise voltages of the 120B pre- 

 amplifier using a 121A amplifier and a vacuum tube voltmeter, apply the 

 following procedure; 



(a) Set channel volume control at 0„ 



(b) Connect a Ballantine or Hewlett-Packard vacuum tube voltmeter 

 across the output socket of the 12 1A amplifier and place a 600-ohm 

 terminating resistor across the meter terminals „ 



(c) Determine the over-all gain of the 120B and the 12 1A amplifiers 

 together. This should be between 106 and 108 db 



(d) Throw the channel input switch to and observe the 

 voltmeter reading across the 121A output,. Evaluate this reading, 

 using one volt as the reference points 



(e) To determine the input noise to the 120B amplifier add the 

 voltmeter reading to the actual gain of the system referred to one 

 volt, 



(f) To determine the output noise of the 120B amplifier, subtract 

 the voltmeter reading from the gain of the system referred to one 

 volto Sketch 5 is an illustrative example of the sort of readings to 



\ 



600 Si. 

 Term, 



Switch on 

 Front Panel 



Channel Volume Set 

 at »0» 



Over-all gain = 107 db 



Sketch 5 - Input and Output Noise Voltages of 

 120B Amplifier 



- 5 - 



600 



Su 



Meter reads 

 25 db below 

 1 volt 



