Radio Broadcast (May 1929-Apr 1930)

An Apparatus of Improved Design for A. F. Work A SIMPLE TWO-TUBE V. T. VOLTMETER iiiiiiiiiiiiiiiiiiiiiiii By HOWARD E. RHODES Technical Editor View One of the most useful instruments to be found in a radio laboratory is the vacuum-tube voltmeter — it is practically an indispensable piece of apparatus in many measurements on circuits and parts used in radio receiving sets. In its simplest form it consists of a tube and meter connected as indicated in Fig. 1a, the B and C potentials being such that the tube is operated on the lower bend of its Ip-Eg characteristic, so that any voltage impressed on the grid produces an increase in plate current. However, unless a very sensitive measuring instrument is used in the plate circuit, this arrangement has the disadvantage that its sensitivity is not very great; with a 200-microampere meter it is generally impossible to read, with good accuracy, any potential below 0.6 volt. A second disadvantage is that the calibration of the instrument is quite sensitive to changes in the A, B, or C battery voltages. An unusual form of vacuum-tube voltmeter which does not have these two disadvantages was constructed recently in the Laboratory. This instrument has been used with very satisfactory results for some time and its construction is described in this article. The voltmeter was designed especially for making measurements on phonograph pick-up units, but it is equally suitable for any measurements at frequencies between about 60 and 8000 cycles. However, the usefulness of this meter is limited to audio frequencies because it incorporates a one-stage a.f. amplifier which has a flat characteristic between 60 and 8000 cycles. Features of Meters The circuit diagram of the voltmeter is given in Fig. 4. It consists essentially of a simple one-tube voltmeter with a stage of audio-frequency amplification ahead of it. From the stage of amplification, a voltage gain of about eight is obtained, and the sensitivity is consequently increased by a factor of eight, so that when using a 200-microampere meter at M we can now read potentials down to about 0.1 volt of the two-tube breadboard-type vacuum voltmeter constructed in the Laboratory. and, by means of multipliers, up to about 4 volts — a voltage range of 40, corresponding to about 32 db. By adding a stage of amplification we have, therefore, overcome one of the disadvantages of the simple voltmeter, i.e., lack of sensitivity. Independence of changes in A, B, and C voltages is obtained by supplying all of these potentials from a single battery. -tube 1.2 0.6 1.0 0.5 0.80.4 1 1 1 1 RADIO BROADCAST LiADUix /oltmeter C alibrations * * • —/ / V . / f Fig. I— (A) a simple v.t. voltmeter circuit; (B) Ip-Eg curve of tube showing operating point of meter. <£ 0.6 0.3 o > 4 0.4 02 3 2 0.2 0.1 1 0 0 0 (J) ® <J0 20 40 60 80 100 120 140 160 180 200 RANGE METER DEFLECTION Fig. 2 — Calibration curves of the v.t. voltmeter described in this article. By adjusting a single resistor, R3, it is always possible to set accurately all the voltages so that a single calibration is correct over a long period of time. The voltmeter uses two 199-type tubes with their filaments connected in parallel and supplied from a single 45-volt B battery — in the Laboratory a 45-volt storage battery was used. In the plus lead from the battery are connected three resistors, Ri, R2, and R3, the total resistance of these three units being such as to permit about 120 mA. to flow through the circuit — each tube filament takes 60 mA. The voltage drop across the 300-ohm resistor, Ri, supplies a plate potential of 300 X 0.12 or 36 volts to the plates of the tubes. A C-bias potential of 2.4 volts for the grid of the first tube is supplied by R4, a 20-ohm resistor in the Aminus lead to the tube. A 66ohm resistor, R5, supplies a C bias of 8 volts to the grid of the second tube. The tubes are resistance coupled, the plate resistor being R6, a 50,000-ohm unit, the grid resistance, R7, 1 megohm, and the coupling condenser, C, with a capacity of 0.01 mfd. The steady plate current from the last tube is about 150 microamperes, and, in order that the entire scale of the meter may be used, it is necessary that the steady current be balanced out. This is accomplished by connecting a rheostat, R2, in series with the plus lead and utilizing the voltage drop across it to send a current around through R8, a 4000-ohm resistor, and the meter, M. The direction of this current is opposite to that of the plate current and, by adjusting the position of the slider on the rheostat R2, we are able, therefore, to balance out the steady plate current and bring the pointer of the meter back to exactly zero. In operating the voltmeter it was found that if the resistance of R3 was slightly reduced, so that more current flowed through the circuit, the reading of M increased; if the resistance was increased the pointer on M moved back passed the zero point. This fact affords a simple and accurate method of adjusting the instrument to the correct operating point. The battery potential may vary from 40 to 50 volts and it will always be possible to adjust the voltmeter correctly by simply adjusting R3 so that the pointer is at exactly zero. It has been found possible to set up the voltmeter in the Laboratory and, with this single adjustment, exactly to duplicate a calibration made several months ago. Voltage Range The voltage range of the instrument was increased by connecting several resistances, R9, Rio, and Rn, in series across the input terminals. With the lead from the grid connected to terminal No. 1 a calibration corresponding curve 1 in Fig. 2 was obtained. Curves 2 and 3 were obtained by connecting the grid lead to terminals 2 and 3, respectively. </>0.4 go.3 5 0.2 UJ t— Lvj = 0.1 radio broadcast' laboratory" Frequency Calibration [ Input Voltage Constant at 0.3 Volt 100 1000 10,000 FREQUENCY IN CYCLES PER SECOND Fig. 3 — Frequency characteristics of the v.t. voltmeter designed by the writer. • JUNE 1929 • 107