644 BELL SYSTEM TECHNICAL JOURNAL 



TUBE UNDER TEST 



Fig. 2 — Schematic amplifier circuit for measuring fluctuation noise in vacuum tubes. 



The input circuit consists of the tube under test together with the 

 variable grid resistor, external load resistor, and batteries for furnishing 

 the required filament, grid, and plate voltages. Because of the high 

 value of amplification required and the wide frequency range covered 

 by the amplifier, this circuit required shielding from external dis- 

 turbances arising from electrical, mechanical, and acoustical shock. 

 Accordingly the tube under test was suspended by means of rubber 

 bands, the whole circuit with the exception of batteries placed inside 

 a tightly sealed lead lined box, and this box in turn suspended by 

 means of a system of damped springs. The box with its cover re- 

 moved and the tube in place is shown in Fig. 3. This shielding was 

 sufficient to reduce the noise from outside disturbances to such a low 

 level that no correction had to be made for it at any time. 



The high gain amplifier ^^ consists of two separate resistance coupled 

 units each containing three stages. Each unit is so designed and 

 shielded that the effect of external disturbances is eliminated. The 

 total gain obtainable is about 165 db (constant to within 2 db from 

 10 cycles to 15,000 cycles). Since this gain is in excess of that required 

 for the study of thermal and tube noises, an attenuator having a 

 range of 63 db was inserted between the two units. In order to limit 

 amplification to certain desired frequency bands, specially designed 

 electric filters were inserted between the first amplifier unit and the 

 attenuator. Three such filters were used of which one is a low -pass 

 filter with cut-off around 205 cycles, and the other two are band-pass 

 filters with mid-frequencies at 1750 and 11,000 cycles respectively. 

 The frequency characteristic of the amplifier with no filter and with 

 each filter inserted is shown in Fig. 4. 



The recording instrument is a 600-ohm vacuum thermocouple and 

 microammeter. Conveniently, the deflection of the microammeter is 

 closely proportional to the mean square voltage applied to the couple. 

 The procedure in making a measurement of tube noise is as follows: 



1- The essential parts of this amplifier were designed by Mr. E. T. Burton. 



