RESISTANCE-COUPLED, audio frequency voltage amplifiers utilize simple components and are capable of providing essentially uniform amplification over a relatively wide frequency, range.
Suitable Tubes
In this section, data are given for 48 types of tubes suitable for use in resistance-coupled circuits. These types include low- and high-mu triodes, twin triodes, triode-connected pentodes, and pentodes. The accompanying key to tube types will assist in locating the appropriate data chart.
Circuit Advantages
For most of the types shown, the data pertain to operation with cathode bias; for all of the pentodes, the data pertain to operation with series screen grid resistor. The use of a cathode-bias resistor where feasible and a series screen-grid resistor where applicable offers several advantages over fixed voltage operation.
The advantages are:
(1) effects of possible lube differences are minimized; (2) operation over a wide range of plate supply voltages without appreciable change in gain is feasible; (3) the low frequency at which the amplifier cuts off is easily changed; and (4) tendency toward motorboating is minimized.
Number of Stages
These advantages can be enhanced by the addition of suitable decoupling filters in the plate supply of each stage of a multi-stage amplifier. With proper filters, three or more amplifier stages can be operated from a single power supply unit of conventional design with-Rk out encountering any difficulties due to coupling through the power unit. When decoupling filters are not used, not more than two stages should be operated from a single power-supply unit.
Symbols Used in Resistance Coupled Amplifier Charts
C = Blocking Capacitor (uF).
Ck = Cathode Bypass Capacitor (uF).
Cgs = Screen-Grid Bypass Capacitor (uF).
En, = Plate-Supply Voltage (volts).
Voltage at plate equals plate supply voltage minus drop in R, and Rk .
= Cathode Resistor (ohms).
= Screen-Grid Resistor (megohms).
= Grid Resistor (megohms) for following stage.
= Plate Resistor (megohms).
V.G.= Voltage Gain.
Eo = Output Voltage (peak volts).
This voltage is obtained across RK (for following stage) at any frequency within the flat region of the output vs. frequency curve, and is for the condition where the signal level is adequate to swing the grid of the resistance-coupled amplifier tube to the point where its grid starts to draw current.
Note: The listed values for E,. are the peak output voltages available when the grid is driven from a low-impedance source. The listed values for the cathode resistors are optimum for any signal source. With a high impedance source, protection against severe distortion and loss of gain due to input loading may be obtained by the use of a coupling capacitor connected directly to the input grid and a high-value resistor connected between the grid and ground.
Structure of a Miniature Tube:
1-Glass Envelope
2-Internal Shield
3-Plate
4-Grid No. 3 (Suppressor)
5-Grid No. 2 (Screen)
6-Grid No. 1 (Control Grid)
7-Cathode
8-Heater
9-Exhaust Tip
10-Getter
11-Spacer Shield Header
12-Insulating Spacer
13-Spacer Shield
14-Inter-Pin Shield
15- Glass Button-Stem Seal
16-Lead Wire
17- Base Pin
18- Glass-to-Metal Seal
--------- KEY TO CHARTS
General Circuit Considerations
In the discussions which follow, the frequency (f2) is that value at which the high-frequency response begins to fall off. The frequency (f1) is that value at which the low-frequency response drops below a satisfactory value, as discussed below. A variation of 10 percent in values of resistors and capacitors has only slight effect on performance. One-half-watt resistors are usually suitable for Rgs, Rg, Rp and Rk resistors.
Capacitors C and Cg--should have a working voltage equal to or greater than Ebb.
Capacitor Ck may have a low working voltage in the order of 10 to 25 volts.
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Triode Amplifier
Heater-Cathode Type
------------ Diagram No. 1
Capacitors C and Ck have been chosen to give an output voltage equal to 0.8 E„ for a frequency (f,) of 100 Hz. For any other value of fj, multiply values of C and Ck by 100 uf,. In the case of capacitor Ck , the values shown in the charts are for an amplifier with dc heater excitation; when ac is used, depending on the character of the associated circuit, the gain, and the value of fi, it may be necessary to in crease the value of Ck to minimize hum disturbances. It may be desirable to operate the heater at a positive voltage of from 15 to 40 volts with respect to the cathode. The voltage output at f, of "n" like stages equals (0.8)" X E,„ where E„ is the peak output voltage of final stage. For an amplifier of typical construction, the value of f2 is well above the audio-frequency range for any value of R„.
-------- Diagram No. 2
Pentode Amplifier Heater-Cathode Type
Capacitors C, Ck, and have been chosen to give an output voltage equal to 0.7 X E„ for a frequency (fi) of 100 cycles. For any other value of fi, multiply values of C, Ck, and Cs» by 100/f,. In the case of capacitor Ck, the values shown in the charts are for an amplifier with dc heater excitation; when ac is used, depending on the character of the associated circuits, the voltage gain, and the value of f,, it may be necessary to increase the value of Ck to minimize hum disturbances.
It may be desirable to operate the heater at a positive voltage of from 15 to 40 volts with respect to the cathode. The voltage output at ft for "n" like stages equals (0.7)" X E„ where E„ is peak output voltage of final stage.
For an amplifier of typical construction, and for R,, values of 0.1, 0.25, and 0.5 megohm, approximate values of f2 are 20000, 10000, and 5000 Hz, respectively.