An Elementary Superregenerative Circuit

amplification to the plate circuit. With grid rectification as shown, the increase in grid current when a carrier signal is applied causes an increase in grid voltage in the negative direction, consequently the average plate current of the grid detector decreases when a signal is applied.

Grid detection is generally used in amateur receivers of limited r.f. amplification because grid detectors are capable of greater sensitivity for small signals than plate detectors, using similar tubes. Plate detection is more commonly used where detector sensitivity is of minor importance, since a larger signal can be handled with less distortion than with grid detection.

Regenerative Detectors

0 With both the grid and plate detectors just described it will be noted that a condenser is connected across the plate load circuit to bypass radio-frequency components in the output. This radio-frequency can be fed back into the grid circuit, as shown in C of Fig. 512, and re-amplified a number of times. This regeneration gives a tremendous increase in detector sensitivity and is used in most amateur receivers. If the regeneration is sufficiently great the circuit will break into oscillation, which would be expected since the circuit arrangement is almost identical with that of the oscillator shown in Fig. 507-A. Therefore a control is necessary so that the detector can be operated either regenerating to give large amplification without oscillation, or to oscillate and regenerate simultaneously. Methods of controlling regeneration are described in Chapter Six.

Superregeneration

0 The limit to which regenerative amplification can h'e carried is the point at which the tube starts to oscillate, because when oscillations commence, further regenerative amplification ceases. To overcome this limitation and give still greater amplification, the superregen-erative circuit has been devised. Essentially, the superregenerative detector is similar to the ordinary regenerative type but with a comparatively low, but super-audible (above audibility) signal introduced in such a way as to vary the detector's operating point at a uniform rate. As a consequence of the introduction of this quench or interruption frequency the detector can oscillate at the signal frequency only when the moving operating point is in a region suitable for the production of oscillations. Because the oscillations are constantly being interrupted, the signal can build up to relatively tremendous proportions, and the super-regenerative detector therefore is extremely sensitive. An elementary form of superregenerative circuit is shown in Fig. 514.

Superregeneration is relatively difficult to attain at ordinary frequencies, and does not possess the property of discriminating between signals of different frequencies characteristic of other types of detectors — in other words, the selectivity is poor. For this reason the superregenerative circuit finds its chief field in the reception of ultra-high-frequency signals, for which purpose it has proved to be eminently successful.

Multi-Element Tubes

0 So far only tubes with two and three elements have been considered. Other elements may be added to the structure to make a tube particularly suitable for certain specialized applications; likewise two or more sets of elements may be combined in one bulb so that a single tube may be used to perform two or three separate functions.

Tubes having four elements are called tetrodes, while if a fifth element is added the tube is known as a pentode. Many element

* Output

* Output

B" SUPPLY

FIG. 515 —A TUNED RADIO-FREQUENCY AMPLIFIER CIRCUIT USING A SCREEN-GRID TETRODE

B" SUPPLY

FIG. 515 —A TUNED RADIO-FREQUENCY AMPLIFIER CIRCUIT USING A SCREEN-GRID TETRODE

combinations and structures become possible as the number of electrodes is increased, but only a few have practical applications. Of the possible four-element arrangements, the only one in general use is that known as the screen-grid type.

Screen-Grid Tetrodes

• In the section on tube oscillators it was explained that oscillations could be sustained through transfer of energy from the plate to the grid through the electrostatic capacity existing between plate and grid, the circuit of Pig. 511-B being used as an illustration. This circuit without the feed-back condenser is exactly the one we would want to use if the tube is intended to amplify, but not oscillate, at radio frequencies; that is, the input and output circuits must be tuned to the same frequency. However, the grid-plate capacity of the triode returns so much energy to the grid circuit from the plate that it is impossible to prevent the tube from oscillating. Consequently a triode cannot be used as an amplifier at radio frequencies without the use of special circuits. These are not very satisfactory when a considerable frequency range is to be covered, as in a receiver.

If a second grid, made in the form of an electrostatic shield between the control grid and plate, is added to the tube the grid-plate capacity can be reduced to a value which will not permit oscillations to occur. The screen grid, as it is called, has a definite effect on the characteristics of the tube. It increases the amplification factor and plate resistance of the tube to values much higher than are attainable in triodes of practicable construction, although the mutual conductance is about the same as that of an equivalent triode. The screen grid is ordinarily operated at a positive potential about one-third or less that placed on the plate, and is by-passed back to the cathode so that it has essentially the same a.c. potential as the cathode. A typical screen-grid receiving amplifier is shown in Fig. 515.

Large screen-grid tubes of the power type are used as amplifiers in transmitting installations. The screen-grid tube can be used as both plate and grid detector, generally showing greater sensitivity than the triode types. It has very little application in audio-frequency amplifiers, however.

Pentodes

# The addition of the screen grid in the tetrode causes an undesirable effect which limits the usefulness of the tube. Electrons striking the plate at high speeds dislodge other elec trons which "splash" from the plate, this phenomenon being known as secondary emission. In the triode, ordinarily operated with the grid negative with respect to cathode, these secondary electrons are repelled back into the plate and cause no disturbance. In the screen-grid tube, however, the positively charged screen grid attracts the secondary electrons, causing a reverse current to flow between screen and plate. The effect is particularly marked when the plate and screen potentials are nearly equal, which may be the case during part of the a.c. cycle when the tube is delivering high output voltage.

To overcome the effects of secondary emission a third grid, called the suppressor grid, is inserted between the screen and plate. This grid, being connected directly to the cathode, repels the relatively low-velocity secondary electrons back to the plate without obstructing to any appreciable extent the regular plate-current flow. Larger undistorted outputs therefore can be secured from the pentode than from the tetrode.

Pentode-type screen-grid tubes are used as radio-frequency voltage amplifiers, and in addition can be used as audio-frequency voltage amplifiers to give high voltage gain per stage, since the pentode resembles the tetrode in having a high amplification factor. Pentode tubes also are suitable as audio-frequency power amplifiers, having greater plate efficiency than triodes and requiring less grid swing for maximum output. The latter quality can be indicated in another way by saying that the power sensitivity — ratio of power output to grid swing causing it — is higher. In audio power pentodes the function of the screen-grid is chiefly that of accelerating the electron flow rather than shielding, so that the grid often is called the accelerator grid. In radio-frequency voltage amplifiers the suppressor grid, in eliminating the secondary emission, makes it possible to operate the tube with the plate voltage as low as the screen voltage, which cannot be done with tetrodes.

As audio-frequency power amplifiers pentodes have inherently greater distortion (principally odd-harmonic distortion) than triodes. The output rating usually is based on a total distortion of 10%.

Multi-Purpose Types

• A great many types of tubes have been developed to do special work in receiving circuits. Among the simplest of these are full-wave rectifiers, combining two separate diodes of the power type in one bulb, and twin-triodes, consisting of two triodes in one bulb for Class-

13 audio amplification. To add the functions of diode detection and automatic volume control — described in Chapter Six — to that of amplification, a number of types are made in which two small diode plates are placed near the cathode, but not in the amplifier-portion structure. These types are known as duplex-diode triodes, or duplex-diode pentodes, depending upon the type of amplifier section incorporated.

Another type is the pentagrid converter, a special tube working as both oscillator and first detector in superheterodyne receivers. There are five grids between cathode and plate in the pentagrid converter; the two inner grids serve as control grid and plate of a small oscillator triode, while the fourth grid is the detector control grid. The third and fifth grids are connected together to form a screen-grid which shields the detector control grid from all other tube elements. The pentagrid converter eliminates the need for special coupling between the oscillator and detector circuits.

Another type of tube consists of a triode and pentode in one bulb, for use in cases where the oscillator and first detector are preferably separately coupled; while still another type is a pentode with a separate grid for connection to an external oscillator circuit. This "injection" grid provides a means for introducing the oscillator voltage into the detector circuit by electronic means.

Receiving screen-grid tetrodes and screen-grid pentodes for radio-frequency voltage amplification are made in two types, known as "sharp cut-off" and "variable-^" or "super-control" types. In the sharp cut-off type the amplification factor is practically constant regardless of grid bias, while in the variable-^ type the amplification factor decreases as the negative bias is increased. The purpose of this design is to permit the tube to handle large signal voltages without distortion in circuits in which grid-bias control is used to vary the amplification, and to reduce interference from stations on frequencies near that of the desired station by preventing cross-modulation. Cross-modulation is modulation of the desired signal by an undesired one, and is practically the same thing as detection. The variable-^ type of tube is a poor detector in circuits used for r.f. amplification, hence cross-modulation is reduced by its use.

Receiving Tubes — Types of Cathodes

9 In the practical construction of receiving tubes there are two types of envelopes or "enclosures", glass and metal. Glass bulbs have been the rule since the early days of tube manufacture; recently, however, welded metal envelopes have been introduced. The metal envelope can be utilized to act as an electrostatic shield for the tube elements.

Receiving tubes can be divided into groups according to the type of cathode used. Cathodes have been the subject of much research and development, so it is but natural to find that several tube types more or less duplicate each other except for the type of cathode.

Cathodes are of two types, directly and indirectly heated. Directly-heated cathodes or filaments used in receiving tubes are of the oxide-coated type, consisting of a wire or ribbon of tungsten coated with certain rare metals and earths which form an oxide capable of emitting large numbers of electrons with comparatively little cathode-heating power. In modern receiving tube types, directly-heated cathodes are confined to audio power-output tubes, power rectifiers and the groups intended for operation from dry-cell batteries, where economy of filament current is highly important.

When directly-heated cathodes are operated on alternating current, the cyclic variation of current causes electrostatic and magnetic effects which vary the plate current of the tube at supply-frequency rate and thus produce hum in the output. Even though the hum can be reduced considerably by proper circuit design, it is still too high in level to be tolerated in multi-tube amplifiers, since the hum appearing at the first tube is amplified through the whole set. Hum from this source is eliminated by the indirectly-heated cathode, consisting of a thin metal sleeve or thimble, coated with electron-emitting material, enclosing a tungsten wire which acts as a heater. The heater brings the cathode thimble to the proper temperature to cause electron emission. This type of cathode is also known as the equipoten-tial cathode, since all parts are at the same potential. The cathode ordinarily is not connected to the heater inside the tube, the terminals of the two parts being brought out to separate base pins.

The first receiving tube filaments were intended to be operated from a six-volt storage battery through a rheostat, hence we find them designed for a terminal voltage of five volts d.c. These and a few early dry-battery types have now been superseded. The first tubes for a.c. heating of the cathodes were designed for 2.5 volts; a very large number of tubes having this cathode voltage are available, some directly and some indirectly heated. When auto radio sets first became popular, a new series of tubes designed for operation at 6.3 volts was made available. This voltage later was adopted for a.c.-operated tubes, and is now standard. All recent types except dry-battery types operate at this cathode voltage. The battery series operates with a terminal voltage of two volts.

In addition to grouping by cathode voltages, it is also necessary to make some distinction between older and newer types of 2.5-volt tubes according to the heater current consumed, and also to differentiate between glass and metal tubes. In each series will be found general-purpose triodes, sharp cut-off screen grid tubes, variable-^ screen grid tubes, power amplifiers of the triode or pentode type, and special purpose tubes. There are also rectifier tubes for the power supply. The logical groupings of tubes are given in the form of tables with the essential characteristics and operating conditions of each type.

Ratings and Characteristics

• The tables give maximum ratings for the various types of tubes listed. In the interests of long tube life, filament or heater voltages should be maintained as nearly as possible at the rating given (variations not more than 5 % either above or below rated voltage) and the maximum plate-supply voltage indicated should not be exceeded. It is important, of course, that the tube be operated with the proper negative bias, as indicated by the tables, applied to the grid. Methods of obtaining bias will be treated in the chapters on receiver and transmitter design.

The important characteristics of the tubes, such as amplification factor, mutual conductance, etc., also are given. In addition, the inter electrode capacitances are listed in the tables of transmitting tubes. Since transmitting tubes often are large in physical structure, these capacities can be quite high with some types of tubes, limiting their application in very high frequency transmitters, since the tube capacity acts as a shunt across the tuning condenser. The important tube capacities are those between grid and cathode (input capacity), grid and plate, and plate and cathode (output capacity). Input and output capacities of receiving tubes usually are quite small — a few micromicrofarads for most tubes.

Base Connections and Pin Numbering

# The older tube bases will be found to have from four to seven pins for element connections. In all except the five-prong type, the two cathode pins are heavier than the others, making them readily distinguishable. The pins are numbered according to the following system: Looking at the bottom of the base or the bottom of the socket, the left-hand cathode pin is No. 1, and the others are numbered consecutively in the clockwise direction, ending with the right-hand cathode pin.

When metal tubes were brought out, a universal-type 8-pin or "octal" base was introduced. Usually only those pins needed for

FIG. 516 — TUBE-BASE PIN NUMBERING SYSTEM

These drawings sho tu the pins looking at the bottom of a tube base or socket. Pins are numbered in the clockwise direction, starting ivith the left-hand cathode pin as No. 1 with glass tubes; with the shield pin as No. 1 with metal tubes. On the 4-, 6- and 7-pin bases thecathodepins are heavier than the others; on the 5-pin and octal bases the No. 1 pin is readily identified from the drawings above.

FIG. 516 — TUBE-BASE PIN NUMBERING SYSTEM

These drawings sho tu the pins looking at the bottom of a tube base or socket. Pins are numbered in the clockwise direction, starting ivith the left-hand cathode pin as No. 1 with glass tubes; with the shield pin as No. 1 with metal tubes. On the 4-, 6- and 7-pin bases thecathodepins are heavier than the others; on the 5-pin and octal bases the No. 1 pin is readily identified from the drawings above.

connections are actually molded into the base, but the design is such that a single type of socket will handle any tube equipped with an octal base. The base and pin-numbering diagrams are shown in Fig. 516.

In indicating which element is connected to which base pin, it is customary to use the letters F, F, or H, H for filament or heater, C or K for cathode, P for plate, etc. In multi-grid tubes the grids are numbered according to the position they occupy, the grid nearest the cathode being No. 1, the next No. 2, etc. Some tubes are provided with a cap connection on top, especially when it is desired that the elements connected to the cap have very low capacity to other tube elements.

Tube Numbering

• Until recently arbitrary numbers were assigned to tubes as they were placed on the market. For the past few years, however, a numbering system has been in effect which to some extent indicates the nature of the tube. These designations consist of a number, a letter, and a final number. The first number indicates the cathode voltage, the letter the individual tube of the series, the first being designated A, the second B, and so on, except for rectifiers, which start with Z and go back wards; the last number indicates the number of useful elements brought out to pin connections. Cathode voltages are indicated by 1 for 2-volt tubes, 2 for 2.5 volts, 5 for 5 volts, 6 for 6.3 volts, 12 for 12 volts, and 25 for 25 volts. In the final number, the filament or heater counts as one element, although always having two connections.

For example, the 2A6 is a 2.5-volt tube hav ing six elements brought out to connections (cathode, heater, triode grid, triode plate, and two diode plates) and is the first six-element tube of the 2.5-volt series, designated by the "A". The 5Z3 is a five-volt rectifier having three elements (cathode and two plates) brought out to connections, and is the first rectifier numbered according to this system. Other examples readily can be worked out.

FIG. 517 — BASE DIAGRAMS OF GLASS RECEIVING TUBES These views are of the bottoms of the bases or sockets. F, filament; H, heater; C, cathode; G, grid; S, screen; Sup, suppressor; /', plate. G1, G2, G3, etc., denotes grids numbered in order from the cathode outward; Gi, G2, Pi, /'2, etc., denote grids and plates of multi-pur-pose or twin tubes having separate sets of elements; elements having the same subscripts belong together. A top cap on the tube is shown by an external unnumbered connection.

FIG. 517 — BASE DIAGRAMS OF GLASS RECEIVING TUBES These views are of the bottoms of the bases or sockets. F, filament; H, heater; C, cathode; G, grid; S, screen; Sup, suppressor; /', plate. G1, G2, G3, etc., denotes grids numbered in order from the cathode outward; Gi, G2, Pi, /'2, etc., denote grids and plates of multi-pur-pose or twin tubes having separate sets of elements; elements having the same subscripts belong together. A top cap on the tube is shown by an external unnumbered connection.

Multi-Grid Tubes — Element Connections

• A number of receiving tubes are so constructed that one type can be made to serve several . different purposes simply by re-arranging the element connections. Thus we find power amplifier tubes with two or three grids, which can be connected in various ways to make the tube suitable for use as a Class-A triode power amplifier, as a Class-B triode amplifier, or as a Class-A pentode amplifier. The Type 59, a triple-grid tube, is an example. If the inner grid, No. 1, is used as the control grid while Nos. 2 and 3 are connected to the plate, the tube is a triode suitable for Class-A power amplification. If, however, No. 1 grid is connected to the middle grid, No. 2, while No. 3, the outer grid, is connected to the plate, the tube can be used without bias as a Class-B amplifier. Still a third method of connection makes the 59 a Class-A pentode; Grid No. 1 is the control grid, No. 2 the screen or accelerator, while No. 3, connected to the cathode, becomes the suppressor. The connections to be used with the several types of tubes falling in this classification are indicated in the tables.

"G" Tubes and Preferred Types

0 The tremendous number of receiving tube types available is likely to appall the neophyte who wants to pick out the most suitable tube for the application he has in mind. However, despite the fact that additions are made to the lists very frequently, actually the trend is now towards simplification of the status of receiving-tube types. We do not hesitate to predict that eventually all tubes in current use for design purposes will have octal bases; it is an accomplished fact that such tubes are now universally equipped with 6.3-volt filaments in the a.c. series; other filament voltages have been discarded.

Practically all the now-used glass tubes can be obtained with octal bases. Such tubes have the suffix "G" attached to the type number. In some cases these tubes duplicate in characteristics types in the metal series; when this is so, the tube carries the same number as the corresponding metal tube, but with the suffix "G". For example, the glass equivalent of the 6K7 metal tube is known as the 6K7G. Other "G" tubes duplicate existing types of glass

Superregenerative Receiver Tube

FIG. 518 —BASE DIAGRAMS OF METAL AND 6.3-VOLT RECEIVING TUBES *

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