the circuit to be used. Reconnect the 121A input leads after removing 

 the 500-ohm resistor and remove the voltmeter from the 121A output , replac- 

 ing it with 500 ohms., Connect the voltmeter across the input leads to the 

 121A amplifier, thus bridging the systems Push the proper channel input 

 switch up into the position*, With the channel volume control 



at the position the voltmeter should read at least ,00008 volts or 82 

 db below 1 volt 



10„ The next step, in case the noise has not been located, is to eliminate 

 the Daven 40-db attenuator as shown in Sketch 4 by removing the rear end 



600 Si. 



Term 



ti n 1 QqCfr-J- 



Switch on 

 Front Panel 



500 -TL 



Sketch 4 - Fourth Step in Self -Noise Measurement 



plate of the 120B amplifier and disconnecting the output leads (left-hand 

 red and blue leads), l^hen the unterminated vacuum tube voltmeter is now- 

 connected across the output of the 120B preamplifier it should read o 00003 

 volts as before, or 82 db below 1 volte 



Ho If noise is present in the channel amplifiers its source will usually 

 be in the 120B preamplifier, the input stage of which is very susceptible 

 to hum pickupo Care should be exercised in grounding this amplifier, carry- 

 ing the cable shields to ground if necessary „ It may also be necessary to 

 run a ground wire from Rack 1 to a good external ground to obtain minimum 

 noise . This will ground all three racks c Note (Fig* 39) that an E,F„ filter 

 has been provided to suppress radio-frequencv interference and that a 6l8B 

 VLEo input transformer (T-101) has been used to obtain maximum low-frequency 

 response o The normal over-all gain of this amplifier is 35 db and the normal un~ 

 distorted output is approximately 5 volts „ 



- 4 - 



