Serial No. 1934/17 MAXIMUM OUTPUTS OF SCREENED GRID RECEIVING VALVES.

Transcription

1 Research Department. REPORT No. G.008,. 18th July, Serial No. 1934/17 \ ;, Invest1gation by C.H.SMITH and R.PARSONS... ~~;~;:' Drawings Nos. G.OOS.l to G.OOS.21. MAXIMUM OUTPUTS OF SCREENED GRID RECEIVING VALVES. SUMMARY. A series of measurements has been made to determine the maximum permissible grid swing and voltage output for various types of S.G. valves, including H.F. pentode and variable-mu valves. The results obtained are shown at the end of the report in the form,of a series of curves and a table of maximum permissible input and output values. I~TRODUCTION. Little information is available in regard to the maximum input and output voltages allowable in the use of screened-grid valves. Measurements have therefore been made in this ~respect on all classes of screenedgrid valves and H.F. pentodes of Mazda and Osram manufacture. METHOD. A direct method of measurement was employed using the circuit shown in Fig.' 1, in which the R.M.S. values of the input and output voltages of a sinusoidal oscillation applied to the valve were observed. To facilitate IIWI~wii~ifilIIIM~ Y

2 -2- the method and to ensure greater accuracy, this was done at an audiofrequency. The output 'of a variable oscillator was kept at oonstant level as observed on a Moullin voltmeter, and passed through a variable attenuator (TI) to the grid of the valve under test. The anode impedance of the valve comprised a tuned circuit resonant at about 1.7 k.c. shunted by a 200,000 ohms resistance in series with the primary of a step-down transformer feeding a second variable attenuator. The whole system at the resonant frequency of the tuned circuit provided an anode impedance of 100,000 ohms. attenuator T2 fed an aperiodic amplifier terminatin~ The output of the in a diode detector and galvanometer. For calibrating purposes a double pole double throw switch S was provided, which put the valve out of circuit and connected the input voltage direct-ly across the anode load. The procedure adopted for taking the valve curves was as follows. The amplification of the valve for a low value of input was first measured. The switch S was placed in the calibrating position, and with a oonvenient value of input from the tone source attenuation was introduced by means of Tl to give a convenient reading on the amplifier deteotor meter. The switch was then restored to the normal position in which the output of the attenuator Tl is connected to the grid of the screened-grid valve, and additional attenuation was introduced in this. attenuator uhtil the reading on the detector meter corresponded with that previously obtained. The additional attenuation measures the amplification of the stage. The amplitude characteristic of the valve was then obtained by simultaneously decreasing the attenuation of the box Tl and inserting

3 attenuation in the box T2 to maintain constant reading of the detector.,.3- meter. Over the linear part of the valve characteristic the reading of the detector meter is unchanged when attenuation is transferred from the boxt l to the box T 2 When the limits of linearity are exceeded. however, this relationship does not hold. It was found to be a convenient and accurate method to note the values of input and output at which the amplification had been reduced or increased by exactly t db., i db., ~ db. etc., as indicated by the change in the total attenuation of the two boxes for constant reading of the detector meter. RESULTS. Figs. 2 to 21 show the amplitude characteristics measured under the above conditions of the following valves:- (1) 2-vo1t Normal S-G Valves. Mazda. SG.215 S.215A S.215B Osram. S.23 S.24 (2) 4-vo1t A.C. Mains Normal S-G Valves. AC/SG AC/S2 MS4B MS4 (:3). 4-volt A. C. Mains H. F. Pentode. AC/S2/Pen. MSP4 (4) 2-volt Variable-mu S-G Valves. S.215.VM VS24 (5) 2-vo1t Variable-mu H.F.Pentode. VP21

4 -4- (6) 4-volt Variable-mu S-G Valves. Mazda. AC/SG7VM AC/SI/VM Osram. VMS4 VMS4B (7) 4~volt Variable-mu R.F.Pentode. VMP4 The points marked.25,.5 etc. on the curves represent the points at which the output has diverged from linearity by the value marked in decibels. It should be noted that, for a carrier reaching a peak value when modulated to 100% equal to the peak value of the input voltage for which the change in output is 1 db., the distortion of the modulation would be of the order of 2~. An inspection of the curves shows that excessive values of input usually cause a relative diminution in output in the case of the normal screened-grid valves and R.F. pentodes. For the variable-mu valves. however. this effect is only noticed for small values of grid bias. At high bias values the opposite effect takes place, i.e., amplification increases with input. At medium values of bias these two effects produce a partial cancellation so that relatively high values of output are obtainable without distortion. Instances of this are the Osram VMS4 at Vg = 20 v. and the Mazda AC/SG/VM at Vg : 7.5 v. The normal screenedgrid valves show this effect when the bias is higher than the recommended value, e.g., Mazda S.215A at Vg : 3.2v. The maximum permissible inputs and outputs for the normal soreened

5 -5- grid valves under standard operating conditions are given in Table I. It is not, however, practicable to give tabular results for the variable-mu type, and reference must be made to the curves themselves. The attached results were all obtained with an anode impedance of 100,000 ohms. Since this figure is considerably less than the normal values of anode impedance of the valves under test, it is prdbable that the limiting condition is anode current variation. It should follow, therefore, that with other anode impedances not differing very widely from the above value, the maximum permissible output vqltage should be directly proportional to the anode impedance used, while the maximum permissible input voltage will be independent of anode impedance. CHS/CHF.

6 TABLE I. 1fuximum Inputs and Outputs for Normal S-G Valves under standard Operating Conditions. / Anode Screen Grid R.M.S. R.M.S. Carrier Peak Volts for Valve. Volts. Volts. Bias Input for Output for 2~ Distortion at 100% Volts. 2~% Dis- zi% Dis- Modulation: tort ion. tortion. 11 Input. Output I 2 v.s-g Valves: Mazda SG2l S2l5A S215B Osram S v. A. C. Ma ins S-G Valves: Mazda AC!8G AC/S Osram MS4B M v. A.C.Mains ~.F.Pentodes: Mazda AC/SZ/Pen Osram. MSP

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