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 fan DC. 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 one. Capacitor C4 must have a gap between the plates of at least 0.5 mm.

The oscillatory 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 - 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.

Let's sum it up

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 worth reminding about compliance with 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 Roe, 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 a further change 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 to the feeder, no additional adjustment was required.

Automatic adjustment of the anode capacitor of the P-circuit of the HF power amplifier

Operating principle.

The theoretical basis for the development and manufacture of this device is the principle of comparing the voltage phases on the grid and on the anode of the lamp. It is known that at the moment of full resonance of the P-circuit, the phase difference between the voltages on the grid and the anode is strictly 180 degrees and the resistance of the anode load is purely active. A P-circuit that is not tuned to resonance has a complex resistance and, accordingly, a phase shift of the grid and anode voltages different from 180 degrees. The nature of the reactive component of the complex resistance depends on whether the natural resonance of the P-circuit is higher or lower in frequency relative to the operating frequency. Those. the capacitance of the capacitor on the anode side is greater or less relative to the capacitance at resonance.

Of course, the setting of the P-circuit is affected not only by the capacitance of the capacitor on the anode side, but this device and does not pretend to full automation settings. That. the task is to rotate the capacitor axis to a position in which the reactive component of the complex resistance will be minimized in the event of a P-circuit detuning.

A similar problem was solved by Yu. Dailidov EW2AAA, using in his design a phase detector made according to a ring balanced circuit on diodes. The disadvantage of this scheme is the low accuracy of tuning, the need to select parts for a balanced mixer, the need for careful shielding, and as a result, a very strong frequency dependence and complexity of tuning.

That. this design can be considered as a modernization of the EW2AAA circuit design.

Design feature.

In this design, the phase detector is made on digital chip DD2 type KR1531TM2. The operating principle is very simple and is based on the D-trigger operating algorithm, i.e. recording the state at input D along the leading edge of the pulse at input C. The logic elements NOT of the DD1 microcircuit act as shapers of rectangular pulses from the sinusoidal voltage on the grid and anode. That. A sequence of pulses is received at the inputs D and C of the flip-flops and their edges are compared.

For example, the voltage at the anode is ahead of the voltage at the grid, the front of the positive pulse at input D of element DD3:1 appears earlier than the front at input C, a unit is written and output 5 is set to “1”. At inputs D and C of element DD3:2, pulses appear exactly the opposite and, accordingly, zero “0” is recorded at output 9. If the phase of the voltage on the anode lags behind the phase of the voltage on the grid, the state of outputs 5 and 9 of the DD3 microcircuit changes to the opposite.

It should be noted that the moment of switching triggers from one state to another when the phase difference passes through 180 degrees is not ideal and has a certain “fork”, the width of which is determined by the delay time of the logic element and for 1531 series microcircuits is several nanoseconds. This “fork” mainly determines the maximum accuracy of tuning the P-circuit to resonance. Looking ahead, I note that the maximum accuracy of tuning tracking on the 14 MHz range is +- 5 KHz. What actually looks like rotating the anode capacitor tuning knob following the rotation of the transceiver frequency tuning knob.

Purpose of some elements of the circuit.

Capacitors C1 and C2 constitute a capacitive RF voltage divider of the anode. Capacitors C3 and C4 constitute a capacitive divider of the RF grid voltage.

The RF voltage taken from the dividers should be about 6 V in amplitude in operating mode. C1 – type KVI-1. C2 and C4 are passable.

Microcircuits DD2 and DD4 are integrated stabilizers; they may be absent if there is a separate +5V power supply.

DD5 - logical elements 3I - prevent the simultaneous appearance of logical ones at the output of the phase detector (which is unacceptable), and also block the operation of automatic tuning, if necessary, when closing the “Control” contacts.

The analog part of the circuit on transistors VT1-VT8 acts as a current amplifier with motor control switches and changes the polarity on the motor depending on the state of the logical one and zero at the output of the phase detector.

Transistors must have the letter B or G.

The “To LEDs” outputs can be used as a visual indication of the state of the phase detector (setting) when manually tuning to resonance.

Features of setup and installation.

All elements of the circuit are located on a printed circuit board in the basement of the chassis with the exception of C1, C2, C3, C4, R1, R2. Additional shielding printed circuit board not required.

From the capacitive dividers to the board, the signal is supplied via a shielded wire (cable). It is very important that the length of the cable from the divider C3, C4 must be greater than the length of the cable from the divider C1, C2. This is determined by the need to compensate for the signal delay in the lamp from the grid to the anode. In practice, the difference in length for the GU-43B lamp is 10 cm. In your particular case, the difference may be different.

It is interesting to note that the “fork” of tuning accuracy depends on the bias voltage on the DD1 elements. The bias voltage is selected using potentiometers R4 and R6 and in my case has the following dependence.

U bias on inputs 1 and 13 (V)	Operation accuracy +-(KHz)

That. it is necessary to set the voltage at the inputs of the microcircuits to 1.4 V, which ensures maximum adjustment accuracy.

Placing the motor and connecting it to the axis of the tuning capacitor in in this case is not considered because it is very individual and depends primarily on the capabilities of the designer. In my case, I use a motor with a gearbox from a money counting machine with an operating voltage of 6V. Therefore, it was necessary to install a limiting resistor with a nominal value of 62 Ohms in series with the motor. A vacuum capacitor KP1-8 5-250 pF is used as a tuning capacitor. The transmission of rotation is carried out through plastic gears.

It is advisable to use resistors of type C2-10 (non-inductive) as resistors R1 and R2, but this is not necessary.

• Download the complete set of files.

If you carefully look at the photograph of the printed circuit board, you will notice that instead of the KR1531LI3 microcircuit there is KR1531LI1. It’s just that the same logic can be performed on different elements; it’s easier on LI3, but I had LI1 on hand.

I am ready to provide all possible advisory assistance only by email: rv3fn()mail.ru

Mashukov Alexander Yurievich (RV3FN).

Automatic adjustment of the coupling capacitor of the P-circuit of the HF power amplifier
(addition to the article about automatic configuration anode capacitor P-circuit)

Introduction

The P-circuit is a matching device between the active amplifying element (lamp or transistor) and the radiating device (antenna-feeder system). With rare exceptions, the resistances of these elements are different. In addition, their resistance is complex in nature, i.e. In addition to the active, it has a reactive (capacitive or inductive) component.

Strictly speaking, both capacitances of the P-circuit affect both the tuning of the P-circuit to resonance and the degree of connection with the load (antenna). In case tube amplifier, i.e. when the output resistance of the amplifying element is significantly greater than the resistance of the antenna, the influence of the capacitance of capacitor C1 has a greater effect on the resonance, and the influence of the capacitance of capacitor C2 on the level of communication with the antenna. We assume that C1 tunes the P-circuit to resonance, and C2 establishes the optimal level of communication with the antenna.

The indicator of the optimal communication level for a tetrode is the value of the screen grid current. This value is different for different lamps. Without going deeply into theory, I will only note that with an optimal screen grid current, the optimal level of unwanted harmonics in the spectrum of the emitted signal of a given power is ensured. In practice, during the setup process, by rotating the knob of capacitor C2, we set the desired screen grid current. So, it is necessary to automate this process.

Block diagram

The current control unit of the second grid produces a signal when the current drops to a level of less than 20 mA and when the current is more than 40 mA. When the current is in the range of 20-40 mA, no signals are issued. Of course, the levels can change as desired during setup.

The control unit performs two functions. The first is to form a logic level for digital control of logic elements, the second is permission for motor control. That is, the motor can rotate (be controlled) only if there is a resonance condition in the P-circuit. This signal comes from the control unit for capacitor C1. And only if there is a required level of RF voltage at the anode. This is done in order to eliminate false rotation of the motor in the absence of a drive signal, when the screen grid current is zero, or when the current is too low due to insufficient drive.

The DC amplifier doesn't need much explanation. It is similar to the amplifier in the control circuit for capacitor C1, only it is made with different elements.

Schematic diagram

It should be noted here that in the previous article on setting up an anode capacitor, output to this circuit was not yet provided. Therefore, I present an upgraded anode capacitor control circuit. There are no fundamental changes in it. Only some parts have been replaced, signals for resonance control (A, B) have been removed, and a “Receive-Transmit” control signal has been added to prevent rotation of the motors in the (Receive) mode. This is the same control signal that comes from the transceiver to put the amplifier into transmit mode. In practice, with the correct setup of the circuit, such rotations do not occur, but during the setup process they are possible. This is like an additional guarantee. But let's return to our diagram.

R 6 and R 8 are shunt resistors through which the current of the second grid passes and on which the necessary voltage is actually released to open the diodes of the optocoupler DD 2. At a low current of the second grid (0-20mA), both LEDs are closed and the resistance of the output transistors of the optocoupler is high. At outputs 6 and 7 of the optocoupler there is a high voltage “1”. At normal current (20-40mA), one optocoupler opens, at a current of more than 40mA, the second optocoupler opens. Thus we have three modes. Up to 20mA, the motor should rotate in one direction, increasing the current of the second grid. The motor should be running in the current range of 20-40 mA. When the current is more than 40 mA, rotate in the other direction, reducing the current of the second grid. All this should work only at resonance, for which elements DD 1.2 and DD 1.1 are responsible, and only if there is a sufficient level of RF voltage at the anode of the lamp, for which the circuit on diodes VD 1, VD 2 and transistor VT 1 is responsible. Resistor R 1 sets the required the level of this voltage. At output 13 of element DD 1.4, the enabling logical “1” is set with “zeros” at inputs 11 and 12, i.e. subject to the above conditions. Elements DD 1.3 and DD 3.5 form the necessary coordination with the setting indication LEDs VD 4 and VD 5. Elements DD 4.1 and DD 4.2 generate control signals for the DC amplifier and analyze the presence of enabling signals, including the “manual - automatic” mode. DD 3.4 in manual mode supplies the required voltage to the manual engine rotation buttons KN 1 and KN 2, in automatic mode the buttons don't work. Buttons KN 3 and KN 4 limit switches are located on capacitor C2 in order to prevent its breakdown and protect the motor and circuit from excessive current in the event of the motor jamming at the edges of the capacitor rotation. The current amplifier is made on an opto-relay DD 5 and DD 6. Unlike the previous UPT circuit on transistors, this circuit provides greater reliability (the voltage drop on field-effect transistors is much lower) and of course is much simpler. The guarantee that the transistors will not be open at the same time is provided by the back-to-back connection of the control diodes. Transistor VT 2 protects the optocoupler LEDs from excessive current. With a resistance of resistor R 11 of 8.2 Ohms, VT 2 opens at a current of about 65 mA. Diode VD3 protects the circuit from reverse currents.

Schematic diagram of anode capacitor control

Conclusion

The setup process can be sequential, i.e. with a smooth increase in the level of build-up or fast. I use fast. This is when the capacitor handles are placed in an approximate position for a given range, the transceiver output power regulator is set to the operating level, the transceiver is switched to AM mode and the pedal is pressed. First, the handle of capacitor C1 begins to rotate until resonance is established, then the motor of capacitor C2 is turned on and the desired current of the second grid is set. In this case, capacitor C2 sometimes stops and the resonance is corrected by capacitor C1. Sometimes you have to adjust the drive level to get the required power.

That's it. We switch the transceiver to SSB mode and do not forget to switch the switches to manual mode settings to avoid “yaw” of the capacitors during operation.

I wish you good luck! Constructive comments are welcome.

R 3FN ex RV 3FN Alexander Mashukov.

Format: jpg, txt.
Archive: rar.
Size: 163 kb.

The correct choice of the minimum required wire diameter for the coils of P-circuits (PL-circuits) of tube power amplifiers is a rather urgent task. Tables that provided information about the diameter of the P-circuit wire depending on the operating range and output power of the final stage of the transmitter were published a long time ago, around the late 50s. XX century.
Moreover, the information provided in them was not very detailed, and the calculations considered the power supplied to the final stage. Apparently, the need for a detailed and accurate table containing complete data for choosing the minimum required wire diameter for P-circuit coils has been long overdue.
According to the empirical formulas of Evteev and Panov, the diameter of the wire for coils with frameless winding is equal to:

(1), where:
Ik - circuit current in amperes;
F - frequency in megahertz;
- permissible overheating of the circuit wire in relation to the ambient temperature during natural cooling during long-term operation of the power amplifier.

For example, if we take the temperature inside the power amplifier case to be +60oC, and the maximum heating temperature of the coils to be +100oC, then t = + 40oC.
In the table, numbers 1, 2 and 3 for each range indicate the method of manufacturing the coil:
frameless winding;
winding on a ribbed frame (wire diameter increases by 28%);
winding into the grooves of the frame (the diameter of the wire doubles). An increase in the diameter of the coil wire is associated with a deterioration in the cooling conditions of the wire with which they are wound.
However, to determine the wire diameter using formula (1), the current Ik flowing in the circuit should be calculated. To do this you can use the formula:

(2) where:
Rant - amplifier output power (antenna power, W);
Q is the loaded quality factor of the circuit, usually equal to 8...25; accepted value for calculations Q=12;
h pc - efficiency factor of the P-circuit (PL-circuit), the accepted value h pc = 0.9;
x is the anode voltage utilization factor for tetrodes operating in class B.
In the calculations, the average value x = 0.8 was adopted. For other operating modes of tetrodes, as well as triodes and pentodes, the corresponding averaged values ​​of Ј are accepted, taken into account in the correction factors given in the notes to the table; Ea is the voltage of the anode power supply, V.

Formula (2) is obtained from the relations published in, through algebraic transformations. Calculating the value of the current flowing in the circuit is not only an intermediate result of calculating the diameter of the circuit wire, but also allows you to correctly select the circuit switching elements - biscuit switches, relays, vacuum contactors, etc.
The diameter of the wire, as follows from formulas (1) and (2), is directly proportional to the value of the loaded quality factor Q, which in practice is not necessarily 12 (as is customary in the table). There are several reasons for this.
Firstly, the P-loop (PL-loop) calculation may have been done for Q = 10.
Secondly, this is due to the design of the P-circuit (PL-circuit). So, if the power amplifier operates with a high anode load resistance Roe (high anode voltage Ea and low anode current), then the anode capacitance of the P-circuit should be small.

It follows from this that:
Qact = Qtable · k, (3)
Dact = Dtable k, (4)
Ik act = Ik table · k. (5)
Qact, Dact, Ik act are really the required values ​​of the quality factor, wire diameter and current in the circuit, and Qtable, Dtable, Ik tab. - tabular (calculated) values.
Coefficient k is calculated using the formula:

Let's look at an example.
Let the output power of the tetrode amplifier (Roe = 4000 Ohm, Ea = 1000V, Rant. = 75 Ohm), operating at a frequency of 28 MHz, be equal to 200 W. From the table we determine that to manufacture a frameless coil it is necessary to use wire Dtable = 3.1 mm; at the same time Ik table. = 6.67 A. For Roe = 4000 Ohm, the capacity of the anode capacitor Sant.table = 15 pF.
Minimum structurally achievable San capacity. RMS = 35 pF.
Hence,
k = 35:15 = 2.33;
Qact = 12-2.33 = 28;
Ik actual = 6.67-2.23 = 15.5(V);
Dactual = 3.1-2.23 = 7.23.
In addition, when switching a P-circuit it is often necessary to connect inductors in parallel.

To correctly select switching elements, it is necessary to know the currents in parallel-connected coils. Figure 1 shows a connection diagram in which Ik is the total current in the circuit, IL1 is the current through inductor L1, IL2 is the current through inductor L2. The ratio of currents flowing in the coils is inversely proportional to the ratio of the inductances of the coils

Since Ik and inductances are known,
reactive currents through coils L1 and L2 are determined by the formulas:

For example, if Ik = 10 A, L1 = 10 µH, L2 = 5 µH, then

Notes to the table:1. Coil diameters and loop current are specified for tetrodes operating in class B.
2. For tetrodes operating in class AB, the wire diameter and loop current should be multiplied by 1.053, in class C - by 0.95.
3. For triodes and pentodes operating in class AB, the wire diameter and loop current should be multiplied by 0.936, those operating in class B by 0.889, and those operating in class C by 0.85.
4. The table data is calculated for Q=12.
5. Material for coils - enameled copper wire. If the diameter of the coils is more than 3 mm, it is recommended to make them from a copper tube. It is advisable to wind all coils with silver-plated copper wire, which is especially important for frequencies of 14...30 MHz.
6. The diameter of the wire is taken from the nearest larger one from the standard range of winding wires.
A. Kuzmenko (RV4LK)
Literature:
1. Melnikov. Radio amateur's directory. - Sverdlovsk - 1961.
2. Radio, 1960, N1.
3. A. Kuzmenko. Calculation of the load of tube power amplifiers. - Radio amateur. KB and UKV, 1999, N6.