How to configure power amplifier input circuits. Lugansk Association of Radio Amateurs - output loop system

Transcript

1 392032, Tambov Aglodin G. A. P CONTOUR Features of the P circuit In the age of the victorious march of modern semiconductor technologies and integrated circuits, tube high-frequency power amplifiers have not lost their relevance. Tube power amplifiers, like transistor power amplifiers, have their own advantages and disadvantages. But the undeniable advantage of tube power amplifiers is that they operate on a mismatched load without failure of vacuum devices and without equipping the power amplifier with special mismatch protection circuits. An integral part of any tube power amplifier is the anode P circuit (Fig. 1). In the work r Methodology for calculating the P circuit of a transmitter, Konstantin Aleksandrovich Shulgin gave a very detailed and mathematically accurate analysis of the P circuit. Fig. 1 To save the reader from searching for the necessary journals (after all, more than 20 years have passed), below are the formulas for calculating the P circuit borrowed from: fo = f N f B (1) geometric mean frequency of the Hz range; Qn X r = loaded quality factor P of the circuit; the intrinsic quality factor P of the circuit is mainly determined by the quality factor of the inductive element and has a value within (in some sources it is designated as Q XX); own losses in the circuit, mainly in the inductor, cannot be accurately calculated, since it is necessary to take into account the skin effect and radiation losses along the field. The indicated formula has an error of ±20%; N = (2) transformation coefficient P of the circuit; equivalent resistance of the anode circuit of the power amplifier; load resistance (feeder line resistance, antenna input resistance, etc.); Qn η = 1 (3) P circuit efficiency;

2 X = N η η (Qn η) N 1 Qn (4); X X = Qn X η (5); Qn X X = (6); η 2 2 (+ X) 2 10 = X 10 = 6 12 pf (7); X µgn (9); 10 = 12 pf (8); The X P circuit, on the one hand, is a resonant circuit with a quality factor Qn, on the other hand, a resistance transformer that converts a low-resistance load resistance into a high-resistance equivalent resistance of the anode circuit. Let's consider the possibility of transforming, using a P circuit, different values ​​of load resistance into the equivalent resistance of the anode circuit under the condition =const. Let's say it is necessary to implement a P circuit for a power amplifier assembled on four GU-50 pentodes connected in parallel according to a circuit with a common grid. The equivalent resistance of the anode circuit of such an amplifier will be = 1350 Ohm (for each pentode 5400 ± 200 Ohm), the output power will be approximately R OUT W, the power consumed from the power source R PO W. According to the given conditions: range 80 meters, fo = f f = = , N V =1350 Ohm, Qn=12, =200 using formulas (1) (9) we will calculate for five values: =10 Ohm, =20 Ohm, =50 Ohm , =125 Ohm, =250 Ohm. The calculation results are shown in Table 1. Table 1 range 80 meters, fo= Hz, =1350 Ohm, Qn=12, =200 SWR N pf μgn pf,78 5.7 20 2.5 67.5 357.97 5.8 50 1.0 27.0 333.04 6.5 10.8 302.98 7.94 972.4 273.80 9.56 642.2 Similar calculations must be made for other ranges. More clearly, the changes in the values ​​of the elements and the load resistance are shown in the form of graphs as a function of Fig. 2.

3 400 C1 pf μg 8.8 7.2 5, pf Fig. 2 Let us note the characteristic features of the graphs: the value of capacitance C1 decreases monotonically, the value of inductance increases monotonically, but the value of capacitance C2 has a maximum at = 16 20 Ohms. This must be paid special attention to and taken into account when choosing the tuning range of capacitance C2. Moreover, the load resistance is of a purely active nature quite rarely; as a rule, the load (antenna) resistance is complex in nature and to compensate for the reactive component, an additional margin is required in the tuning range of the elements of the P circuit. But it is more correct to use an ACS unit (antenna matching device) or an antenna tuner. It is advisable to use ACS with tube transmitters; for transistor transmitters, ACS is mandatory. Based on the above, we come to the conclusion that in order to coordinate when the load resistance changes, it is necessary to rearrange all three elements of the P circuit in Fig. 3. Fig. 3 Practical implementation of the P circuit Since the mid-60s of the last century, the P circuit diagram Fig. 4 has been circulating, which seems to have taken root and does not arouse much suspicion. But let's pay attention to the method of switching the inductive element in the P circuit. 1 2 S Fig.4 T Fig.5 S Whoever tried to switch a transformer or autotransformer in a similar way, Fig.5. Even one short-circuited turn can lead to complete failure of the entire transformer. And with the inductor in the P circuit, without a shadow of a doubt, we do exactly the same!?

4 Firstly, the magnetic field of the open part of the inductor creates a short circuit current I SC in the closed part of the coil Fig. 6. For reference: the amplitude of the current in the P circuit (and in any other resonant system) is not so small: I K 1 A1 = I Qn = 0.8A, where: I K1 is the amplitude of the resonant current in the P circuit; I A1 amplitude of the first harmonic of the anode current (for four GU-50 I A1 0.65A) Fig. 6 And where will the energy of the short circuit current be spent (I short circuit Fig. 6): for heating the short-circuited turns themselves and for heating the contact nodes of the switch S (Fig. 4). Q-meter Fig. 7 Q-meter Q =200 Q Short circuit 20 a) b) Secondly, if it is possible to use a Q-meter (quality factor meter), take readings from an open inductor and with partially closed turns Fig. 7a, fig. 7b Q of the short circuit will be several times less than Q, now using formula (3) we determine the efficiency of the P circuit: Qn 12 η = 1 = 1 = 0.94, 200 Qn 12 η short circuit = 1 = 1 = 0.4?! kz 20 At the output of the P circuit we have 40% of the power, 60% went to heating, eddy currents, etc. Summarizing the first and second, we end up with not a P circuit, but some kind of RF crucible. I Short circuit What are the ways to constructively improve the P circuit: Option 1, the circuit according to Fig. 4 can be modernized as follows: the number of inductive elements should be equal to the number of ranges, and not two or three coils as usual. To reduce the magnetic interaction of nearby coils, their axes must be placed perpendicular to each other, at least in space there are three degrees of freedom, coordinates X, Y, Z. Switching is carried out at the junction of individual coils. Option 2: use tunable inductive elements, such as variometers. Variometers will allow you to more finely tune the P circuit (Table 1 and Fig. 3). Option 3: use a type of switching that excludes the presence of closed or partially closed coils. One of the possible options for the switching circuit is shown in Fig. 8.

5 M M M Fig. 8 Literature 1. Shulgin K. A. Methodology for calculating the P circuit of a Radio transmitter, 7


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Let's continue the conversation about the features that any radio amateur faces when designing a powerful RA amplifier and the consequences that can occur if the amplifier structure is installed incorrectly. This article provides only the most necessary information that you need to know and take into account when independently designing and manufacturing high-power amplifiers. The rest will have to be learned from your own experience. There is nothing more valuable than your own experience.

Cooling the output stage

The cooling of the generator lamp must be sufficient. What does this mean? Structurally, the lamp is installed in such a way that the entire flow of cooling air passes through its radiator. Its volume must correspond to the passport data. Most amateur transmitters are operated in the “receive-transmit” mode, so the volume of air indicated in the passport can be changed in accordance with the operating modes.

For example, you can enter three fan speed modes:

  • maximum for contest work,
  • average for everyday use and minimal for DX work.

It is advisable to use low noise fans. It is appropriate to recall that the fan turns on simultaneously with the filament voltage turning on or a little earlier, and turns off no less than 5 minutes after it is removed. Failure to comply with this requirement will shorten the life of the generator lamp. It is advisable to install an aero switch along the path of the air flow, which, through the protection system, will turn off all supply voltages in the event of a loss of air flow.

In parallel with the fan supply voltage, it is useful to install a small battery as a buffer, which will support the fan operation for several minutes in the event of a power failure. Therefore, it is better to use a low voltage DC fan. Otherwise, you will have to resort to the option I heard on the air from one radio amateur. He, supposedly to blow the lamp in the event of a power outage, keeps in the attic a huge inflated chamber from the rear wheel of the tractor, connected to the amplifier by an air hose.

Amplifier anode circuits

In high-power amplifiers, it is advisable to get rid of the anode choke by using a series power supply circuit. The apparent inconvenience will more than pay off with stable and highly efficient operation on all amateur bands, including ten meters. True, in this case the output oscillating circuit and the range switch are under high voltage. Therefore, variable capacitors should be decoupled from the presence of high voltage on them, as shown in Fig. 1.

Fig.1.

The presence of an anode choke, if its design is unsuccessful, can also cause the above phenomena. As a rule, a well-designed amplifier using a series-powered circuit does not require the introduction of “antiparaeits” either in the anode or in the grid circuits. It works stably on all ranges.

Separating capacitors C1 and C3, Fig. 2 must be designed for a voltage 2...3 times higher than the anode voltage and sufficient reactive power, which is calculated as the product of the high-frequency current passing through the capacitor and the voltage drop across it. They can be composed of several parallel-connected capacitors. In the P-circuit, it is advisable to use a variable-capacity vacuum capacitor C2 with a minimum initial capacitance, with an operating voltage no less than the anode voltage. Capacitor C4 must have a gap between the plates of at least 0.5 mm.

The oscillating system, as a rule, consists of two coils. One for high frequencies, the other for low frequencies. The HF coil is frameless. It is wound with a copper tube with a diameter of 8...9 mm and has a diameter of 60...70 mm. To prevent the tube from becoming deformed during winding, fine dry sand is first poured into it and the ends are flattened. After winding, cutting off the ends of the tube, the sand is poured out. The coil for the low frequency ranges is wound on a frame or without it with a copper tube or thick copper wire with a diameter of 4...5 mm. Its diameter is 80...90 mm. During installation, the coils are positioned mutually perpendicular.

Knowing the inductance, the number of turns for each range, can be calculated with high accuracy using the formula:

L (μH) = (0.01DW 2)/(l/ D + 0.44)

However, for convenience, this formula can be presented in a more convenient form:

W= C (L(l/ D + 0.44))/ 0.01 - D; Where:

  • W is the number of turns;
  • L - inductance in microhenry;
  • I - winding length in centimeters;
  • D is the average diameter of the coil in centimeters.

The diameter and length of the coil are set based on design considerations, and the inductance value is selected depending on the load resistance of the lamp used - table 1.

Table 1.

Variable capacitor C2 at the “hot end” of the P-circuit, Fig. 1, is connected not to the anode of the lamp, but through a tap of 2...2.5 turns. This will reduce the initial loop capacitance on the HF bands, especially on 10 meters. The taps from the coil are made with copper strips 0.3...0.5 mm thick and 8...10 mm wide. First, they need to be mechanically secured to the coil by bending a strip around the tube and tightened with a 3 mm screw, having previously tinned the connection and outlet points. Then the contact point is carefully soldered.

Attention: When assembling powerful amplifiers, you should not neglect good mechanical connections and rely only on soldering. We must remember that during operation all parts become very hot.

It is not advisable to make separate taps for WARC bands in coils. As experience shows, the P-circuit is perfectly tuned on the 24 MHz range in the 28 MHz switch position, on 18 MHz in the 21 MHz position, on 10 MHz in the 7 MHz position, with virtually no loss of output power.

Antenna switching

To switch the antenna in the “receive-transmit” mode, a vacuum or ordinary relay is used, designed for the appropriate switching current. To avoid burning the contacts, it is necessary to turn on the antenna relay for transmission before the RF signal is supplied, and for reception a little later. One of the delay circuits is shown in Fig. 2.

Fig.2.

When the amplifier is turned on for transmission, transistor T1 opens. Antenna relay K1 operates instantly, and input relay K2 will operate only after charging capacitor C2 through resistor R1. When switching to reception, relay K2 will turn off instantly, since its winding, together with the delay capacitor, is blocked by the contacts of relay K3 through the spark-extinguishing resistor R2.

Relay K1 will operate with a delay, which depends on the capacitance value of capacitor C1 and the resistance of the relay winding. Transistor T1 is used as a switch to reduce the current passing through the control contacts of the relay located in the transceiver.

Fig.3.

The capacitance of capacitors C1 and C2, depending on the turnips used, is selected within the range of 20...100 μF. The presence of a delay in the operation of one relay in relation to another can be easily checked by assembling a simple circuit with two neon bulbs. It is known that gas-discharge devices have an ignition potential higher than the combustion potential.

Knowing this circumstance, the contacts of relay K1 or K2 (Fig. 3), in the circuit of which the neon light will light up, will close earlier. Another neon will not be able to light up due to its reduced potential. In the same way, you can check the order of operation of the relay contacts when switching to reception by connecting them to the test circuit.

Summarize

When using lamps connected according to a common cathode circuit and operating without grid currents, such as GU-43B, GU-74B, etc., it is advisable to install a powerful 50 Ohm non-induction resistor with a power of 30...50 W at the input (R4 in Fig. 4).

  • Firstly, this resistor will be the optimal load for the transceiver on all bands
  • Secondly, it contributes to exceptionally stable operation of the amplifier without the use of additional measures.

To fully drive the transceiver, a power of several or tens of watts is required, which will be dissipated by this resistor.

Fig.4.

Safety precautions

It is useful to remind you about observing safety precautions when working with high-power amplifiers. Do not carry out any work or measurements inside the housing when the supply voltage is turned on or without making sure that the filter and blocking capacitors are completely discharged. If, if accidentally exposed to a voltage of 1000...1200V, there is still a chance to miraculously survive, then when exposed to a voltage of 3000V and above, there is practically no such chance.

Whether you like it or not, you should definitely provide for automatic blocking of all supply voltages when opening the amplifier case. When performing any work with a powerful amplifier, you must always remember that you are working with a high-risk device!

S. Safonov, (4Х1IM)

L. Evteeva
"Radio" No. 2 1981

The output P-circuit of the transmitter requires careful adjustment, regardless of whether its parameters were obtained by calculation or it was manufactured according to the description in the magazine. It must be remembered that the purpose of such an operation is not only to actually tune the P-circuit to a given frequency, but also to match it with the output impedance of the final stage of the transmitter and the characteristic impedance of the antenna feed line.

Some inexperienced radio amateurs believe that it is enough to tune the circuit to a given frequency only by changing the capacitances of the input and output variable capacitors. But in this way it is not always possible to obtain optimal matching of the circuit with the lamp and antenna.

The correct setting of the P-circuit can only be obtained by selecting the optimal parameters of all three of its elements.

It is convenient to configure the P-circuit in a “cold” state (without connecting power to the transmitter), using its ability to transform resistance in any direction. To do this, connect a load resistance R1 parallel to the input of the circuit, equal to the equivalent output resistance of the final stage Roе, and a high-frequency voltmeter P1 with a small input capacitance, and a signal generator G1 is connected to the output of the P-circuit - for example, in the antenna socket X1. Resistor R2 with a resistance of 75 Ohms simulates the characteristic impedance of the feeder line.

The load resistance value is determined by the formula

Roe = 0.53Upit/Io

where Upit is the supply voltage of the anode circuit of the final stage of the transmitter, V;

Iо is the constant component of the anode current of the final stage, A.

The load resistance can be made up of BC type resistors. It is not recommended to use MLT resistors, since at frequencies above 10 MHz high-resistance resistors of this type exhibit a noticeable dependence of their resistance on frequency.

The process of “cold” tuning of the P-circuit is as follows. Having set the given frequency on the generator scale and introduced the capacitances of capacitors C1 and C2 to approximately one-third of their maximum values, according to the voltmeter readings, the P-circuit is tuned to resonance by changing the inductance, for example, by selecting the tap location on the coil. After this, by rotating the knobs of capacitor C1 and then capacitor C2, you need to achieve a further increase in the voltmeter reading and again adjust the circuit by changing the inductance. These operations must be repeated several times.

As you approach the optimal setting, changes in capacitor capacitances will affect the voltmeter readings to a lesser extent. When further changes in capacitances C1 and C2 will reduce the voltmeter readings, the adjustment of the capacitances should be stopped and the P-circuit should be adjusted as accurately as possible to resonance by changing the inductance. At this point, setting up the P-circuit can be considered complete. In this case, the capacitance of capacitor C2 should be used by approximately half, which will make it possible to correct the circuit settings when connecting a real antenna. The fact is that often antennas made according to the descriptions will not be tuned accurately. In this case, the conditions for mounting the antenna may differ markedly from those given in the description. In such cases, resonance will occur at a random frequency, a standing wave will appear in the antenna feeder, and a reactive component will be present at the end of the feeder connected to the P-circuit. It is for these reasons that it is necessary to have a reserve for adjusting the elements of the P-circuit, mainly capacitance C2 and inductance L1. Therefore, when connecting a real antenna to the P-circuit, additional adjustments should be made with capacitor C2 and inductance L1.

Using the described method, the P-circuits of several transmitters operating on different antennas were configured. When using antennas that were sufficiently well tuned to resonance and matched with the feeder, no additional adjustment was required.

Output P-circuit and its features

The P-circuit must meet the following requirements:

    Tune to any frequency of a given range.

    Filter signal harmonics to the required extent.

    Transform, i.e. ensure optimal load resistances are obtained.

    Have sufficient electrical strength and reliability.

    Have good efficiency and a simple, convenient design.

The limits of the real possibility of a P-circuit to transform resistances are quite high and directly depend on the loaded quality factor of this P-circuit. With an increase in which (therefore an increase in C1 and C2), the transformation coefficient increases. With an increase in the loaded quality factor of the P-circuit, the harmonic components of the signal are better suppressed, but due to the increased currents, the efficiency of the circuit decreases. As the loaded quality factor decreases, the efficiency of the P-circuit increases. Often circuits with such a low loaded quality factor (“squeezing power”) fail to suppress harmonics. It happens that with solid power, a station operating on the 160-meter band can also be heard on the
80 meters or operating on the 40 meter band is heard on the 20 meter band.
It should be remembered that “splatters” are not filtered by the P-circuit, since they are in its passband; only harmonics are filtered.

The influence of Roe on amplifier parameters

How does resonant impedance (Roe) affect amplifier parameters? The lower the Roe, the more resistant the amplifier is to self-excitation, but the cascade gain is lower. Conversely, the higher the Roe, the greater the gain, but the amplifier's resistance to self-excitation decreases.
What we see in practice: let’s take, for example, a cascade on a GU78B lamp, made according to a circuit with a common cathode. The resonant impedance of the cascade is low, but the slope of the lamp is high. And therefore, with this slope of the lamp, we have a high gain of the cascade and good resistance to self-excitation, due to the low Roe.
The amplifier's resistance to self-excitation is also facilitated by the low resistance in the control grid circuit.
Increasing Roe reduces the stability of the cascade in a quadratic manner. The greater the resonant resistance, the greater the positive feedback through the pass-through capacitance of the lamp, which contributes to the self-excitation of the cascade. Further, the lower the Roe, the greater the currents flow in the circuit, and hence the increased requirements for the manufacture of the output circuit system.

P-loop inversion

Many radio amateurs encountered this phenomenon when setting up an amplifier. This usually happens on the bands 160 and 80 meters. Contrary to common sense, the capacitance of the variable coupling capacitor with the antenna (C2) is prohibitively small, less than the capacitance of the tuning capacitor (C1).
if you tune the P-circuit to maximum efficiency with the highest possible inductance, then a second resonance appears at this boundary. The P-circuit with the same inductance has two solutions, that is, two settings. The second setting is the so-called “inverse” P-circuit. It is named so because the capacities C1 and C2 have swapped places, i.e. the “antenna” capacity is very small.
This phenomenon was described and calculated by a very old equipment developer from Moscow. In the forum under the tick REAL, Igor-2 (UA3FDS). By the way, he was very helpful to Igor Goncharenko in creating his calculator for calculating the P-circuit.

Methods for turning on the output P-circuit

Circuit solutions used in professional communications

Now about some circuit solutions used in professional communications. Serial power supply of the transmitter output stage is widely used. Variable vacuum capacitors are used as C1 and C2. They can be either with a glass bulb or made of radio-porcelain. Such variable capacitors have a number of advantages. They do not have a sliding rotor current collector, and the inductance of the leads is minimal, since they are ring-type. Very low initial capacitance, which is very important for high-frequency ranges. Impressive quality factor (vacuum) and minimal dimensions. Let's not talk about two liter “cans” for a power of 50 kW. About reliability, i.e. about the number of guaranteed rotation cycles (back and forth). Two years ago, the old RA “gone” was made on a GU43B lamp, which used a vacuum KPE type KP 1-8
5-25 Pf. This amplifier has worked for 40 years and will continue to work.
In professional transmitters, vacuum capacitors of variable capacity (C1 and C2) are not separated by a separating capacitor; this imposes certain requirements on the operating voltage of the vacuum KPI, because they use a series cascade power supply circuit and therefore the operating voltage of the KPI is selected with a threefold margin.

Circuit solutions used in imported amplifiers

In the circuit systems of imported amplifiers, made with GU74B lamps, one or two GU84B, GU78B, the power is solid and the FCC requirements are very stringent. Therefore, as a rule, a PL circuit is used in these amplifiers. A two-section variable capacitor capacitor is used as C1. One, small capacity, for high-frequency ranges. This section has a small initial capacity, and the maximum capacity is not large, sufficient for tuning in high-frequency ranges. Another section, with a larger capacity, is connected by a biscuit switch in parallel to the first section, for operation on low-frequency ranges.
The same biscuit switch switches the anode choke. In high-frequency ranges there is low inductance, and in the rest it is full. The circuit system consists of three to four coils. The loaded quality factor is relatively low, therefore, the efficiency is high. The use of a PL-contour results in minimal losses in the loop system and good filtering of harmonics. In low-frequency ranges, contour coils are made on AMIDON rings.
Quite often I communicate via Skipe with my childhood friend Christo, who works at ACOM. Here's what he says: tubes installed in amplifiers are first bench trained, then tested. If the amplifier uses two tubes (ACOM-2000), then pairs of tubes are selected. Non-paired lamps are installed in the ACOM-1000, which uses one lamp. The circuit is configured only once during the prototyping stage, since all amplifier components are identical. Chassis, component placement, anode voltage, chokes and coil data - nothing changes. When producing amplifiers, it is enough to slightly compress or expand only the coil of the 10-meter range; the remaining ranges are obtained automatically. The taps on the coils are sealed immediately during manufacture.

Features of calculations of output loop systems

At the moment, on the Internet, there are many “counting” calculators, thanks to which we are able to quickly and relatively accurately calculate the elements of the contour system. The main condition is to enter the correct data into the program. And this is where problems arise. For example: in the program, respected by me and not only, Igor Goncharenko (DL2KQ), there is a formula for determining the input impedance of an amplifier using a circuit with a grounded grid. It looks like this: Rin=R1/S, where S is the slope of the lamp. This formula is given when the lamp is operating in a characteristic section with a variable slope, and we have an amplifier with a grounded grid at an anode current cut-off angle of approximately 90 degrees with grid currents at the same time. And therefore the formula 1/0.5S is more suitable here. Comparing empirical calculation formulas both in ours and in foreign literature, it is clear that it will most correctly look like this: the input impedance of an amplifier operating with grid currents and with a cutoff angle of approximately 90 degrees R = 1800/S, R- in ohms.

Example: Let's take the GK71 lamp, its slope is about 5, then 1800/5 = 360 Ohm. Or GI7B, with a slope of 23, then 1800/23=78 Ohm.
It would seem, what is the problem? After all, the input resistance can be measured, and the formula is: R=U 2 /2P. There is a formula, but there is no amplifier yet, it is just being designed! It should be added to the above material that the value of the input resistance is frequency dependent and varies with the level of the input signal. Therefore, we have a purely rough calculation, because behind the input circuits we have another element, a filament or cathode choke, and its reactance also depends on the frequency and makes its own adjustments. In a word, an SWR meter connected to the input will reflect our efforts to match the transceiver with the amplifier.

Practice is the criterion of truth!

Now about the “counter”, only based on VKS calculations (or, more simply, the output P-circuit). There are also nuances here; the calculation formula given in the “counting book” is also relatively incorrect. It does not take into account either the class of operation of the amplifier (AB 1, V, C), or the type of lamp used (triode, tetrode, pentode) - they have different CIAN (anode voltage utilization factor). You can calculate Roe (resonant impedance) in the classical way.
Calculation for GU81M: Ua=3000V, Ia=0.5A, Uс2=800V, then the amplitude value of the voltage on the circuit is equal to (Uacont=Ua-Uс2) 3000-800=2200 volts. The anode current in the pulse (Iaimp = Ia *π) will be 0.5 * 3.14 = 1.57 A, the first harmonic current (I1 = Iaimp * Ia) will be 1.57 * 0.5 = 0.785 A. Then the resonant resistance (Roe=Ucont/I1) will be 2200/0.785=2802 Ohm. Hence the power supplied by the lamp (Pl=I1*Uacont) will be 0.785*2200=1727W - this is the peak power. The oscillatory power equal to the product of half the first harmonic of the anode current and the amplitude of the voltage on the circuit (Pk = I1/2* Uacont) will be 0.785/2*2200 = 863.5 W, or simpler (Pk = Pl/2). You should also subtract the losses in the loop system, about 10%, and you will get an output of approximately 777 watts.
In this example, we only needed the equivalent resistance (Roе), and it is equal to 2802 Ohms. But you can also use empirical formulas: Roе = Ua/Ia*k (we take k from the table).

Lamp type

Amplifier operating class

Tetrodes

0,574

0,512

0,498

Triodes and pentodes

0,646

0,576

0,56

Therefore, in order to obtain correct data from the “reader”, you need to enter the correct initial data into it. When using a calculator, the question often arises: what value of the loaded quality factor should be entered? There are several points here. If the transmitter power is high, and we only have a P-circuit, then in order to “suppress” the harmonics, we have to increase the load quality factor of the circuit. And this means increased loop currents and, therefore, large losses, although there are also advantages. With a higher quality factor, the shape of the envelope is “more beautiful” and there are no depressions or flatness, the transformation coefficient of the P-circuit is higher. With a higher loaded Q, the signal is more linear, but the losses in such a circuit are significant and, therefore, the efficiency is lower. We are faced with a problem of a slightly different nature, namely the impossibility of creating a “full-fledged” circuit in the high-frequency range. There are several reasons - this is the large output capacity of the lamp and the large Roe. After all, with a large resonant resistance, the optimal calculated data does not fit into reality. It is almost impossible to produce such an “ideal” P-circuit (Fig. 1).

Since the calculated value of the “hot” capacitance of the P-circuit is small, and we have: the output capacitance of the lamp (10-30 Pf), plus the initial capacitance of the capacitor (3-15 Pf), plus the inductor capacitance (7-12 Pf), plus the mounting capacitance ( 3-5Pf) and as a result, it “runs up” so much that the normal contour is not realized. It is necessary to increase the loaded quality factor, and due to the sharply increased loop currents, a lot of problems arise - increased losses in the loop, requirements for capacitors, switching elements, and even for the coil itself, which must be more powerful. To a large extent, these problems can be solved by a cascade series power supply circuit (Fig. 2).

Which has a higher harmonic filtering coefficient than the P-circuit. In a PL circuit, the currents are not large, which means there are fewer losses.


Placement of coils of the output loop system

As a rule, there are two or three of them in the amplifier. They must be located perpendicular to each other so that the mutual inductance of the coils is minimal.
The taps to the switching elements should be as short as possible. The taps themselves are made with wide but flexible busbars with an appropriate perimeter, as, by the way, are the coils themselves. They need to be placed 1-2 diameters from the walls and screens, especially from the end of the coil. A good example of a rational arrangement of coils are powerful industrial imported amplifiers. The walls of the contour system, which are polished and have low resistivity, under the contour system is a sheet of polished copper. The body and walls are not heated by the coil, everything is reflected!

Cold tuning of the output P-circuit

Often at the “technical round table” in Lugansk the question is asked: how, without the appropriate devices “on cold”, can you configure the output P-circuit of the amplifier and select coil taps for amateur bands?
The method is quite old and is as follows. First you need to determine the resonant impedance (Roe) of your amplifier. The Roe value is taken from your amplifier calculations or use the formula described above.

Then you need to connect a non-inductive (or low-inductance) resistor, with a resistance equal to Roe and a power of 4-5 watts, between the lamp anode and the common wire (chassis). The connection leads for this resistor should be as short as possible. The output P-circuit is configured with a circuit system installed in the amplifier housing.

Attention! All amplifier supply voltages must be turned off!

The output of the transceiver is connected with a short piece of cable to the output of the amplifier. The “bypass” relay is switched to “transmit” mode. Set the transceiver frequency to the middle of the desired range, while the internal tuner of the transceiver must be turned off. A carrier (CW mode) with a power of 5 watts is supplied from the transceiver.
By manipulating the tuning knobs C1 and C2 and selecting the coil inductance or tap for the desired amateur radio range, we achieve a minimum SWR between the transceiver output and the amplifier output. You can use the SWR meter built into the transceiver, or connect an external one between the transceiver and the amplifier.
It is better to start tuning with low-frequency ranges, gradually moving to higher frequencies.
After setting up the output loop system, do not forget to remove the tuning resistor between the anode and the common wire (chassis)!

Not all radio amateurs are capable, including financially, of having an amplifier using tubes such as GU78B, GU84B, or even GU74B. Therefore, we have what we have - in the end we have to build an amplifier from what is available.

I hope this article will help you in choosing the right circuit solutions for building an amplifier.

Best regards, Vladimir (UR5MD).

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