Buy a powerful transmitter for the 3 MHz range. Second category transmitter
The transmitter consists of the following blocks: master oscillator; buffer stage; output stage; modulator.
Master oscillator.
The master oscillator is assembled according to a capacitive three-point circuit using a 6P44S lamp. The contour coil is wound on a frame with a diameter of 20 mm, with a wire of 0.8 mm diameter, 40 turns. To achieve frequency stabilization in the control grid, it is necessary to use KSO capacitors of group G + -5%.
Buffer cascade
The buffer stage is designed to decouple the master oscillator from subsequent stages, which contributes to the stability of the generation frequency. In the same cascade, amplitude modulation of the carrier frequency occurs. The modulator must be a tube modulator that provides 200 volts or more at the output of the modulation transformer. 
Output stage
The Dr1 inductor is wound with 0.23-0.35 mm wire on a ceramic frame with a diameter of 10-15 mm, four sections of 80 turns per pile. Choke Dr2 is wound with three 0.5 mm wires on a thick ferrite rod. The chokes in the filament circuit are also wound on ferrite rods with 1.0-1.5 mm wire. The chokes are wound until the rod is completely filled, leaving room for its attachment. The contour coil is wound on a frame with a diameter of 50 mm with a 2.0 mm wire, the number of turns is 35-38
Modulator for AM transmitter
The modulator is a 4-stage low frequency amplifier. The microphone amplifier is made on one half of the 6N2P. The microphone used is an electret (tablet). C1 limits it at high frequencies to avoid excitation. Resistances R1 and R2 determine the voltage on the microphone (affects sensitivity); it should be within 1.5...3.0 V (depending on the type of microphone). Capacitor C3 prevents high DC voltage from reaching subsequent stages. Next comes a two-stage voltage amplifier. The signal comes to it from resistance R4 “volume”. Resistor R9 is a volume control for the line input (tape recorder, CD player, computer, etc.), and it is also a tone control for the microphone input. The audio power amplifier is assembled on a 6P3S. The amplifier is loaded onto a transformer, which you can wind yourself, the data is shown in the diagram. The power transformer from old Record and Vesna TVs (TS-180) also works well. When connecting to a transmitter, you may need to change the polarity of the secondary winding connection.
Antenna
The transmitter was loaded onto an American type antenna. Antenna length 48m made of 1.6mm wire. The transmitter was connected with a 1.0mm wire. The reduction is connected at a distance of 1/3 of the entire length.
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Transcript
1 Manufacturing of a 2.8-3.3 MHz transmitter with amplitude modulation on a protective grid. To drive three GU 50 lamps into the control grid, you need from 50 to 100 V RF voltage, with a power of no more than 1 W. And for pumping “to the cathode” - already tens of watts. It was necessary to decide on the “pathogen” scheme. The prototype of the “pathogen” was made according to scheme diagram 1. It produced an “honest” 10W without much effort. But this power is clearly in excess to drive three GU 50 lamps into the control grid. When the supply voltage was reduced to 12V, the power dropped to 5W. During the experiment, a generator was also tested according to scheme diagrams 2 and 3. On the emitter of the generator transistor in this version, the voltage diagram was somewhat more beautiful, but this did not affect the final result in any way.
2 I present diagrams of stress at point A. Diagram “a” refers to diagram 1. Diagram “b” and “c” refers to diagram 2. Diagram “b” was obtained by reducing C5 to 180Pf. It was decided to make “EXITITOR” according to diagram 3. Transistors can be used at any RF of low and medium power. Tr1 and Tr2 are wound on ferrite rings with an outer diameter of 10-12 mm with a permeability of 1000 or more. The windings contain turns of homemade twisted “three” and “five”. Transformers are made in the usual way: we wind a twisted (lightly, 1 turn per cm) bundle of PEL wire turn to turn, evenly distributing the winding around the circumference of the ring. Then in Tr1 the primary winding is made of two “lines” connected in series, the secondary is single, in Tr2 the primary is single, and the secondary winding is made of four (for a purely AM transmitter of two or three) serial “lines”. On the secondary winding (when all four lines are turned on) of the output stage, an RF voltage amplitude of up to 120V develops (the varnish insulation of the wires must be “correct”) at a load of 820 Ohms at a local oscillator consumption current of 1A. This is clearly a lot of power. Therefore, the output stage must be configured for a load of approximately 2.7..3K. By adjusting the current consumption of T3 with resistor R8, it is necessary to obtain the amplitude of the output voltage V. My resistance of resistor R8 was 1 1.3K. With a circuit supply voltage of 9 to 12V, the TOTAL current consumption was 150-
3 250mA. Below are oscillograms of voltages across the load. In the final version, elements numbered R8, D4, C12 (sch.2) were removed, and the beginning of the secondary winding TP1 was connected to “MACE”.
4 From them it is clear that it is quite possible to “start” the lamps both in class “B” for an AM transmitter (two (three) serial lines in Tr2 are used in the secondary winding) and in class “C” (all four serial lines in Tr2 are used in the secondary winding). Due to the fact that the output stage provides excess power, there was a temptation to use only the pre-final stage on T2 with transformer Tr2. But it was not possible to obtain more than 20V amplitude at a 2K load. Those who are not satisfied with the shape of the signal from the generator driver should make an “exciter” according to a scheme where the second and third stages operate in economical class C, and the output has a sinusoid, but the amplitude is already thirty percent less. I ended up using it so as not to force the lamp modes. Power supply The power supply of the transmitter is without any special features, it is made on a TS-270 transformer. It is installed on the chassis through shock-absorbing rubber washers. The chokes are used from old tube TVs. The diodes in the rectifiers are any rectifier type, for a current of 1-3A and a reverse voltage of 600V. All of them must be bypassed with capacitors. Transmitter output stage. The output stage of the transmitter is built on three GU50 lamps operating in class “B” and one 6P15P as a modulator with an inductive load. You don’t have to “unsolder” the limiter if you are not in the habit of shouting very loudly into a microphone, or you can adjust it to suit your speech characteristics by adding one more - two cells of back-to-back diodes (any low-power rectifier). Modulation is carried out on the protective grid GU50. There are no special features in such a circuit solution; therefore, a detailed explanatory text is not required. It can also be added that the anode choke can have any design, as long as the inductance is at least 1200 μH, this is due to the fact that the π circuit is designed for a high-resistance load, approximately 4.6K, since it is supposed to “power” the antenna at “half a wavelength” in one of its ends (started). Grid choke no less than 500 mcg. The whole “vegetable garden”, with fixed biases and chokes, was done on the assumption that the quiescent current would be set for each lamp separately, but in practice it turned out that this gives little. Therefore, a fixed Negative offset may not be
5 do, but combine all the control grids and ground them through a 30K..40K auto bias resistor. The π circuit data is calculated independently, depending on the frequency range and the antenna used. (The equivalent output resistance of one GU50 lamp is 4600 Ohms. Three, respectively, 1533 Ohms).
6 Transmitter automation Switching the transmitter to the “RECEIVING” mode occurs simultaneously by removing the excitation, that is, turning off the local oscillator power supply and de-energizing the power supply rectifiers of the transmitter power section. Microphone amplifier The microphone amplifier-compressor is made on a microcircuit “torn out” from a DVD set-top box (from the “karaoke” microphone path) and two transistors. It “gives out” the “positioned” ones into the 6P15P grid
7 2..2.5V LF amplitude. For fans of modulation “in the foreground”, the amplitude level can be raised to 5V using trimming resistor R10. There is also a control button in the microphone body, through which voltage is supplied to the power circuit of the transmitter control relay. This button is also duplicated by the “right-right” toggle switch. on the front panel of the transmitter. I used both electret and dynamic microphones, they work well, naturally each with its own frequency spectrum. Another version of the MU with a dynamic microphone. and my most “favorite” MU option: The design of the transmitter must meet the usual requirements for the layout and installation of powerful RF devices. The circuit design of the transmitter has the right to its own life, but the practice of its implementation
8 showed that it is much simpler and clearer to build such a transmitter entirely using tubes, well, maybe with the exception of a microphone amplifier. Then the power supply will be simpler and there will be fewer ambiguities in understanding the setup process. I would also like to note that the “protective grid” modulation method is good, correspondents note a “clean, neat signal,” but in terms of “assertiveness” and “arrogance” it is still inferior to the proven modulation to the screen grid via a cathode follower. The simplicity of the solution - to “power” a high-resistance antenna directly from the output of the pi circuit, is fraught with unpredictable “HF interference” to the low-signal paths of the transmitter. Therefore, if you want such “simplicity”, then you need to take care of the normal shielding of the low-signal path of the transmitter and eliminating the paths for the formation of a multiplicative background. This is due to the fact that the antenna has a very high input resistance, and the output stage, trying to “push” the “RF power” out of itself, pushes it anywhere, and not only into the antenna. Any design that has a small capacitive (5-10 pF) connection with the Pi circuit and the initial section of the antenna fabric already successfully absorbs almost a quarter of the transmitter’s output power. And if RF interference gets into, say, the circuit of diode rectifiers not shunted by capacitors, then the diodes will work as mixers of the frequency of the RF signal and the frequency of the alternating mains voltage. From the above, we can conclude that it is more correct to “connect” half-wave antennas to the Pi circuit of the transmitter through a low-resistance feeder, “powering” them at the corresponding points of the antenna fabric.
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Class D tube modulator: allows you to increase the efficiency of the radio transmitter in AM mode to 85-90%.The tetrode is used as a key element. A tetrode requires less power to excite in the control grid circuit than a triode.
During operation: a significant part of the tetrode switching frequency period is in saturation, while the residual voltage at the anode is small, therefore, the shielding mesh current increases sharply. To eliminate the shortcoming, a mode is selected so that the power losses on the shielding mesh do not exceed the permissible level.
Uadditional is connected to the anode L1 through the Diode (D2). constant voltage source. It fixes the residual U of the anode in the open state, and reduces i current of the shielding grid, reduces static losses on the screening grid L1 (not related to switching processes). The power loss on the shielding grid turns out to be limited and will not exceed the permissible level, since the current of the shielding grid cannot increase more than the value determined by the voltage Uadd., and the power loss at the anode will be several times less than the permissible value.
The voltage value Uadm should be selected based on the permissible level of losses in the screen grid circuit while maintaining a sufficiently high efficiency. Calculations show that good results can be obtained by choosing Uadd ≈0.1 Ea. In this case, the output power of a radio transmitter with a class D modulator almost doubles, while the modulator efficiency decreases by -10%.
Fig.1
The modulating signal Uin is supplied to the input of the PWM signal shaper, which generates voltage pulses on the control grid, the duration of which is proportional to the magnitude of the modulating signal. Accordingly, the voltage on the anode L1 also has the form of PWM pulses. The component of this voltage, varying in accordance with the modulating signal, is isolated by a low-pass filter consisting of (Dr and C). Fig.1
The calculation shows the nominal output power of the radio transmitter in a single-cycle class D modulator on a GU-81m tetrode with 200 watts. up to 600W with a slight decrease in modulator efficiency (from 95 to 85%). In this case, the power dissipated on the shielding mesh will not exceed the permissible level (0.4 kW), and the increasing power losses at the anode will be several times less than the permissible value (600 W).
In order to increase the efficiency in push-pull anode modulators, instead of a class B amplifier, a class D modulator can be used.
Unlike a single-acting amplifier, a push-pull amplifier operates with a duty cycle of two pulses (periods of initial oscillations); there is no voltage at the output of the modulator, since the total average value of these pulses is zero. The voltage, audio frequency Usv.h (Fig. 3) from the PWM unit (Fig. 2) is converted into two sequences of width-modulated pulses G1 and G2 of opposite polarity with a duty cycle of the pulses equal to two initial cycles of oscillations (Fig. 3) for lamps L1 and L2 operating in key mode.
Encoded audio pulses from the PWM modulator are fed to the input of the optocoupler 6N137. At the output of 6N137: the signal is inverted. Therefore, two additional buffer inverting elements D1.1 and D1.3 are used. - (D1-74HC14) inverting Schmitt triggers. (Fig. 4) The signal for the lower key is inverted by inverter D1.2. The control signals of the upper and lower keys are sent to the dead-time generation nodes. They are made on logical elements “AND” D2.1 and D2.2. - (D2-74HC08) . As a result, only the leading edges of incoming pulses are delayed. The amount of delays and, therefore, dead-time is determined by the products of R3*C3 and R4*C4 and can be adjusted to the parameters of the power module. Further processing of the control signals of the upper and lower keys occurs in different ways:
The lower key signal is amplified on the MAX4420 chip and goes to the driver output.
The upper key signal is amplified on the MAX4420 chip and has a “floating” common wire potential. Therefore, galvanic isolation is necessary. In this case, transformer isolation with DC component correction is used.
For a frequency range of 100-300 kHz and a duty cycle from 0 to 0.5, this solution is quite satisfactory.
Transformer parameters: T1 (core M 2500 NMS 16*10*8) winding 2*13 vit. These values are focused on the frequency range 100-300 kHz. If it is necessary to operate at lower frequencies, the number of turns must be increased, and at higher frequencies the number of turns must be reduced. Installation of the half-bridge driver in Fig. 5
Rice. 5 layout option and driver design.
Fig.3
Figure 3 shows the diagram: an alternating component (audio frequency voltage) is supplied to the load through a separating Cp and a constant component through a modulation choke Lg. In order to prevent current interruptions through the inductance Lf when switching lamps L1 and L2, diodes D1 and D2 and shunt lamps are used L1 and L2 and passing currents ivD1 and ivD2 at the required time intervals. In accordance with the direction of the current in the load and in the inductor, only L1 and D2 work in the positive half-cycle of the amplified voltage, and in the negative half-cycle, L2 and D1.
There is no voltage at the modulator output, since the total average value of these pulses is zero. Dependences of changes in the values of average currents through lamps and diodes, related to the peak value. Dependence of the power supplied by a push-pull modulator to the output stage of the transmitter on the AM coefficient, dependence and obtaining efficiency.
Anode modulators for broadcasting transmitters up to 500 kW are built using the sloping principle. Developed by Marconi.
Increasing the efficiency of high-power radio transmitting devices / Ed. A. D. Artyma: Communication 1987.
Foreign radio transmitting devices / Ed. G. A. Zeitlenka, A. E. Ryzhkova - M.: Radio and Communications, 1989.
US Patent N 4272737, class. H 03 F 3/217, 1981.
AM TRANSMITTER at 3 MHz
The transmitter consists of four stages. The author used almost all used parts, soldered at different timesfrom different techniques, and lay around in boxes for many years. The output power of the transmitter has not been measured, according to rough calculations it is about 5 Watts +/-, but most likely a plus. The master oscillator is assembled according to a classic three-point circuit, and despite its simplicity, it maintains a stable frequency. The buffer stage on VT2 is loaded on a broadband transformer, there was no desire to install circuits and then equalize the characteristic over the entire range, there are more brands and details extra , and here in one fell swoop, or rather with one transformer. The buffer stage is the load of a modulator assembled on an LM386 ULF chip. The author took the modulator circuit from Japanese radio amateurs, tested it and was satisfied. Well, the most important part is the final stage. It is assembled on a transistor taken from some Korean radio. The KT805BM in the first version did not live up to expectations and was disgracefully removed from the transmitter. As a result of the operation, the structure was not damaged, but the patriotic spirit of the author was tested. However, having inserted 2T921A into the design for testing, peace of mind was restored. Even more, there was pride in our defense industry. But it was decided to leave the “Korean” as the most optimal option, and it is easier to attach to the radiator. The operating mode of the cascade is set by resistor R12. Diode D4 serves to stabilize the quiescent current. It must be mounted on the radiator directly next to the output transistor. On the Korean transistor, the author slipped a diode directly under the transistor, since there was room there. It is advisable to coat the mounting location with heat-conducting paste.
Design details: a variable capacitor was installed with an air dielectric from a tube receiver. You can install almost any KPI, the main thing is that it covers the range of 2.8 - 3.2 MHz.
Coil L1 of the master oscillator has 80 turns of PEL wire - 0.32 with a tap from 20 turns. Coils L2; L3 are the same and have 20 turns of PEL wire - 0.6.
All coils are wound on frames with a diameter of 12 mm.
The author used a polystyrene frame from a spool of thread as frames.
Tr1 is wound on a ferrite ring with a diameter of 10 mm and a height of 5 mm. Twenty turns of folded and slightly twisted PELSHO wire - 0.25. Winding is carried out evenly throughout the entire ring.
Tr2 is wound on the same ring and contains 18 turns of PEL wire folded in three - 0.32.
L4 - 30 turns PELSHO - 0.25 on the same ring as Tr 1;2. For L4, you can use a ring with smaller dimensions.
ATTENTION:
Before you start setting up, you need to connect a 50 - 75 Ohm load to the transmitter output. The author used two connected parallel 100 Ohm resistors, 2 W each.
SETUP:
The setup begins by checking the power supply, having previously set the variable resistor R12 to the position of maximum resistance. By connecting an ammeter (multimeter) set to maximum between the circuit and the power source, usually 10 A supplies power. If the readings have not changed much, then you can proceed to the actual setup. Disconnect pin Tr1, which goes to C24 so that power from the modulator does not flow to the cascade. Connect a milliammeter between power supply +24 and the right terminal of transformer Tr2. We connect the power, and with resistor R12 we set the quiescent current of the output stage to about 30 mA. Then we restore all connections, monitor the signal with a frequency meter or receiver for generation. Then we set the middle of the range and use capacitors C19 - C21 to adjust the output filter to the maximum indicator readings. We connect the antenna, adjust C21 again and the setup is complete.
Master oscillator.
To achieve frequency stabilization in the control grid, it is necessary to use KSO capacitors of group G + -5%. The circuit is wound on a frame with a diameter of 20 mm, with a wire of 0.8 mm diameter, 40 turns.
Buffer cascade
Everything is clear from the diagram. It can be simplified by removing Dr2 and everything else that goes with it. Place one 27k resistance from the control grid to ground. You can also apply modulation to one terminal of the transformer directly to the 3rd leg, and the other to ground, removing everything else. The modulator must be a tube one and produce 200 volts or more at the output of the modulation transformer; you can use the TC-180 from old tube TVs. 
Output stage
Dr1 is wound with 0.23-0.35 mm wire on a ceramic frame with a diameter of 10-15 mm, four sections of 80 turns per pile. Dr2 is wound with three wires on a thick ferite rod (from any receiver where there is a magnetic antenna) filament wire 1.0-1.5mm, cathode 0.5mm. It is wound until it is completely filled, leaving room for its attachment. The circuit is wound on a frame with a diameter of 50 mm with a 2.0 mm wire of 35-38 turns. For a more complete calculation of the P-contour, you can use the program: click here 
Antenna
The antenna used with this “American” transmitter has a length of 48 m with a 1.6 mm wire and a reduction of 12 m with a 1.0 mm wire. The reduction is connected at a distance of 1/3 from the hot end. 
But you can use any other antenna you like!