be expected* In this case, the input noise = 107 db -4- 25 db s 132 db 

 down, and the output noise - 10? db ~ 25 db - 82 db down,, 



121A Amp lifier 



14, The 121A amplifiers are provided with 500-cycle low-pass filters to 

 improve their signal-to-noise ratios A 1500-cyele and a 200-cycle filter 

 are included in the spare parts and may be substituted for the 500-cvcle 

 filter if operating conditions require a change. The installed filter is 

 located on the right rear corner of each 121A amplifier.. Its pass-band 



is stamped on the top, as :■ , etc. Figs, 29, 36 and 37 are 



response curves of the three filters „ 



15 , The normal over-all gain for the 121A amplifier is 75 db and the 

 normal undistorted output is approximately 20 volts. The tube complement 

 is as follows: 



121A amplifier : Two 1620 s s 



: One 6F6, replaceable with W»E. type 349A 



Servicing 



16 , Extra precaution should be observed if the input tube of the 120B 

 preamplifier (type 1603 or 6C6) requires replacement,, It has been found 

 that considerable variation in inherent noise level can be caused by 

 tubes picked at random from stock. When replacing this tube, try several 

 stock tubes, using the one giving the lowest noise reading., Use the test 

 circuit of Sketch 1, watching the output meter for the lowest possible 

 reading. Exercise care in replacing all other tubes as well, 



17, If further servicing is necessary remove the cables connected to 

 the amplifier (either the 120B or the 121A), All cables except the fila- 

 ment leads can be reached by removing the rear end plate; the filament 

 leads are reached by removing the front panel of the rack cabinet, ' Then 

 take the entire chassis out of the Rack by removing the four nuts holding 

 the chassis and the Lord Mounts to the tray. The nuts are located under the 

 tray. When the amplifier has been serviced and is replaced make certain 

 that the cables do not interfere with the shock mounting and that the 

 amplifier floats freely. 



Gain and Undistorted Output Measurements 



18, At least once each week each channel should be checked for over-all 

 gain and maximum undistorted voltage output. To do this a signal generator 

 should be connected to the terminals on the top panel of Rack 1 marked 



, Pushing the proper channel control switch down will place 

 the signal generator across the input of the channel under test. The signal 



-&- 



