Tuning A Class C Rf Amplifier

In general, the same adjustments are made in tuning different class C r-f amplifiers, irrespective of the type of tube or circuit used. Although the tuning of a triode r-f amplifier is described in the following paragraphs, the procedure applies almost equally well to tetrode and pentode amplifiers. In the following explanation, it is assumed that the triode has been correctly neutralized.

The filament of the amplifier tube is lighted, the positive plate-supply lead disconnected**, and r-f excitation from the driver stage applied. The plate circuit of the driver is tuned to resonance, which is indicated by a dip in the driver plate current or by maximum d-c grid current in the amplifier stage. If the amplifier has a tuned grid circuit, the latter must also be tuned to resonance (indicated by the grid-current reading). After a maximum amplifier grid current has been obtained by these tuning processes, the coupling between the driver and the amplifier may be adjusted to give still more amplifier grid current, if this can

• Sec reference No. 7 in the READING LIST.

•• The screen voltage should also be removed, if the tube is a tetrode or a pentode.

be done without overloading the driver stage. The plate circuit of the driver should be retuned to resonance eve'ry time the coupling is changed, because of the interaction between the various circuits.

After the interstage-coupling adjustments have been made, the amplifier plate tank should be set as near to resonance as possible. A protective resistance of adequate size should then be placed in series with the positive plate-supply lead, as explained in TRANSMITTING-TUBE INSTALLATION. In the case of large, high-power tubes which are protected by d-c overload relays, this protective resistor can be omitted, especially in those installations where the d-c plate voltage can be reduced to about 50 per cent of its rated value by means of taps in the primary circuit of the plate-supply transformer. The plate voltage is now applied and the plate tank circuit quickly tuned to resonance (indicated by a sharp dip in the d-c plate current of the amplifier). The plate current at resonance will usually drop to a value between 10 and 20 per cent of the rated full-load value (see Fig. 13), if no load is coupled to the plate circuit. In case the plate tank condenser does not have an adequate voltage rating, the high r-f voltage developed across the unloaded plate tank circuit may cause the condenser to flash over. This effect should not occur with the d-c plate voltage reduced 50 per cent, if the condenser is suitable for the purpose. If it does occur, however, the load circuit can be coupled to the plate tank in order to reduce the r-f voltage developed.

If the plate tank cannot be tuned to resonance, the reason will usually be found in improper tuned-circuit constants. Either the tank inductance L, or the tank capacitance C, or both, may have to be increased or reduced, depending on whether the circuit is found to tune higher or lower than the desired frequency. An absorption-type wave-meter is useful in checking trouble of this kind. The "off-resonance" plate current of an amplifier may be quite high, even with a protective resistor in the plat supply lead. For this reason, a tube should not be operated with its plate circuit out of resonance, except for the very short time required to make the proper tuning adjustment. If the plate current does not dip normally with no, load coupled to the plate tank, the trouble may be due to insufficient r-f grid excitation, to excessive tank-circuit losses, or to improper neutralization. Because the minimum plate current under no-load conditions depends on the Q of the tank circuit, on the biasing method used, and on the excitation voltage, the minimum plate-current value should not be considered a definite indication of the efficiency of an amplifier.

When the tuning procedure described has been completed, the load circuit may be coupled to the amplifier. The load may be an antenna, a dummy antenna (for test purposes), or the grid circuit of a following r-f amplifier stage. When the load is applied, the amplifier plate current will rise. The plate circuit of the amplifier should be retuned to resonance to guard against the possibility that the load has caused detuning. The plate current will still dip, but its minimum value will be considerably higher than under no-load conditions. Full plate voltage should now be applied and the coupling of the load made tighter, until the minimum plate current (at the dip) reaches the normal value given in the typical operating conditions tabulated under the tube type. Of course, if the required power output can be obtained with a lower value of plate current, the load-circuit coupling can be loosened or the d-c plate voltage reduced. In no case should

Fig.13

the d-c plate input exceed the value given under MAXIMUM RATINGS for the particular class of service involved.

Pentodes and tetrodes are tuned in the same manner as triodes. Because neutralization is ordinarily not required for screen-grid tubes, the circuits of these tubes are relatively simple and easy to adjust. It is quite important in a screen-grid r-f amplifier to prevent stray coupling between the input and output circuits. Although the use of a screen grid in a tube substantially eliminates internal feedback within the tube, self-oscillation and unstable operation may be caused by external feedback due to stray capacitances. Complete shielding of the input and output circuits from each other, and in some cases from the tube itself, is generally advisable.

The value of the d-c potential on the screen usually has an important effect on power output; adjustment of this voltage after the circuit has been tuned may result in better efficiency and more power output. Care should be observed, however, that the maximum rated d-c power input to the screen is not exceeded.

As the load on an r-f amplifier is increased, the d-c grid current will decrease, more so for triodes than for tetrodes and pentodes. After the load has been adjusted to the desired value, the d-c grid current should be checked. If it has dropped substantially lower than the normal value, insufficient r-f grid excitation or excessive d-c grid bias may be the cause.

The process of tuning other types of amplifiers will vary somewhat, depending on the class of service in which the tube is used.

PARASITIC OSCILLATIONS*

A parasitic, as the term is used in radio work, is any spurious oscillation taking place in a vacuum-tube circuit other than the normal oscillation for which the circuit is designed. Parasitic oscillations may occur in either audio- or radio-frequency amplifiers.

Parasitics, like normal oscillations, are generated when the conditions necessary for oscillations exist and may be of either audio or radio frequency. In many cases, circuit troubles which may be attributed to other causes are actually due to parasitics. They may cause the radiation of spurious carriers and side bands, voltage flashover, loss of efficiency, instability, and premature failure of vacuum tubes and other circuit elements.

Unfortunately, parasitic oscillations cannot always be forseen and eliminated in the design of a new type of radio transmitter. It is usually necessary to remove any existing parasitics after a transmitter has been constructed. The location of the parasitic circuit often requires considerable study and may involve the use of "cut-and-try" methods. Detuning and damping of the offending circuit to stop the oscillation are often quite simple, once the undesired oscillating circuit has been located. The occurrence of parasitics during the development of a complex, modern transmitter, especially one of high power using several tubes in push-pull or in parallel, is not necessarily indicative of poor design. Such an occurrence is often to be expected.

The most detrimental parasitics are probably those which cause flashovers, spurious radiations, and low amplifier efficiency. The tubes and associated circuits in a transmitter may have damped or undamped parasitics, depending on the feedback coupling, the circuit losses, and the grid and plate potentials, as well as on the reactance and tuning of the parasitic circuit. Damped oscillations, or "trigger" parasitics, occur as the result of modulation transients, keying transients, •Part of the material in this section is adapted from reference No. 2 in the READING LIST.

or flashovers in vacuum tubes due to peak voltage effects. These parasitics may exist only during a part of the modulation cycle, when the plate or grid voltage is at a high positive value. When one parasitic is eliminated, it is quite possible that an entirely different one may start. Vacuum tubes can oscillate simultaneuosly on more than one frequency, but one oscillation may prevent one or more other oscillations from starting.

The tuned-plated-tuned-grid oscillator circuit has been found to be the basic circuit for the most common forms of parasitic oscillations. To satisfy the conditions for oscillation, there must be a grid circuit and a plate circuit tuned approximately to the same frequency together with capacitive feedback through the grid-plate capacitance of the tube. Oscillation can usually be stopped by heavy damping or by detuning of the circuits. It is generally preferable to detune a grid parasitic circuit to a much higher frequency than the corresponding plate parasitic circuit in order to stop the spurious oscillation.

Ultra-high-frequency parasitics may be generated if the leads from the amplifier tube to the plate tank condenser are long. This type of oscillation can be eliminated in a number of ways. Resistors in the order of 10 to 50 ohms may be inserted in the grid lead, plate lead, or both, close to the socket terminal. The resistors should be of the non-inductive, wire-wound type, or preferably, of the carbon-stick type. When large tubes are employed, especially in class B r-f service, it is not desirable to add very much series resistance in the grid circuit. Too much resistance tends to limit the positive modulation peaks, due to the flow of grid current through the grid resistor. A suitable method is the use of a grid resistor shunted by a low-resistance r-f choke; the latter carries the d-c grid current.

Ultra-high-frequency parasitics can also be eliminated by tuning a grid parasitic circuit to a much higher frequency than the corresponding plate parasitic circuit. This detuning can be accomplished by mounting the grid tank capacitor close to the tube in order to make the grid-to-filament circuit as short as possible. Small r-f chokes placed in series with the plate lead, next to the socket, are often helpful. In some cases, resistors should be shunted across the chokes.

Spurious oscillations are sometimes caused if the leads to the neutralizing capacitor are long. At high frequencies, long leads may have considerable inductance. A non-inductive resistor of low value placed in the lead from the tube to the neutralizing capacitor may remedy trouble from this source.

It is common practice to use a split-stator capacitor with the rotor grounded, in push-pull circuits and in single-ended circuits of the balanced type. If the

Push Pull Amplifier

-B tECj ti

(b) tuning taps

(a) loading taps

Fig. 14

-B tECj ti

(b) tuning taps

(a) loading taps

Fig. 14

capacitor is not grounded for r-f potentials, a parasitic oscillation may be the result. In such a circuit, with the rotor grounded for r-f voltages, the center tap on the tank inductance usually should not be bypassed to ground (or to the filament), because the use of a double r-f ground may unbalance the circuit and create parasitics. An r-f choke in the high-voltage lead to the plate tank inductance may prevent this condition.

When taps for loading (Fig. 14A) or tuning (Fig. 14B) are used, additional circuits for parasitics are formed. If the parasitic is caused by the use of tapped coils for loading or excitation, detuning of the coupling circuits by the addition of reactance or a change to inductive coupling may be required. The use of a tuning capacitor across a part of an inductance, as shown in Fig. 14B, creates a complex circuit which is resonant at more than one frequency. In general, this method of obtaining vernier control of tuning is undesirable, especially if the capacitor is shunted across a relatively small portion of the tank inductance.

If "shunt feed" is used for both the grid bias and the plate-voltage supply, considerable trouble may result from the complex circuits thus formed. The choke coils tend to resonate at various frequencies with the tank elements, and cause parastics of the tuned-plate-tuned-grid variety. For this reason, it is desirable to eliminate shunt-feed chokes wherever possible. If shunt feed is used in one circuit, it is preferable to use series feed in the other. In case two chokes are used, whether in shunt-feed or in series-feed circuits, parasitics thus caused can often be eliminated by using a plate choke having about 100 times the inductance of the grid choke. This arrangement prevents the parasitic oscillating circuit from receiving sufficient excitation to continue in oscillation.

When tubes are paralleled, intertube parasitics having a very high frequency may exist. They may be eliminated by means of small resistors (in the order of 10 to 50 ohms) connected in series with each grid lead at the socket; or, the grids may be connected together with as short leads as possible and small choke coils placed in series with each plate lead.

In the checking of a transmitter for parasitics, an all-wave receiver is quite useful. The receiver will respond not only to parasitics but also to normal harmonics at integral multiples of the operating frequency. The latter are to be expected and need cause no confusion. If the receiver is a superheterodyne and is located near the transmitter, it is also important that signals due to image-frequency response not be mistaken for parasitics. An oscillating detector or a beat oscillator is a valuable aid in this method of testing. A pure tone should result from an unmodulated carrier and from its various harmonics. A rough tone usually indicates the presence of a parasitic.

Fig. 15

Fig. 15 illustrates an r-f amplifier circuit with several circuit elements introduced to eliminate parasitic oscillations. Ri and R2 are non-inductive resistors having a small resistance. Li and Lj are very small r-f chokes. One or more of these damping elements may be found necessary. In some cases, R2 may be replaced by a variable capacitor having a very small maximum capacitance. Thus, a tuned circuit or "parasitic trap" is formed for the elimination of ultra-high-frequency parasitics.

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  • kasey
    How to indicate insufficient excitation in class c amplifier?
    6 years ago

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