Making an anode choke for a HF amplifier. Anode choke of the output stage of a low-power AM radio broadcast transmitter. Purpose and design of chokes
Rice. 17
A KPI with a divided stator can be used as an anode capacitor in the P-circuit and ensures its optimal setting, provided that there is a sufficient distance between the plates (so that the RF voltage does not break through. There is another method for reducing the initial capacitance of the anode KPI. By connecting this capacitor to the tap from the P-circuit coil, we achieve a reduction in the capacitance introduced into the circuit and a decrease in the influence of the KPI on its tuning frequency - UA9LAQ).
Capacitors with air dielectric and vacuum: Capacitors with air dielectric are easier to find, they are cheaper, but they have some of the disadvantages outlined above. Vacuum KPIs are expensive, they are not so easy to find, but only they sometimes provide the P-circuit with everything we want to get from it without the use of additional switchable capacitors of constant capacity. Another advantage of these capacitors is their high operating voltage, insensitivity to pollution of the surrounding atmosphere and changes in its humidity and pressure, and can conduct large RF currents. I have never heard of any vacuum capacitor being shot or arced. An average vacuum-type capacitor used in an HF amplifier can pass through itself RF currents many times greater than those that a real RA is capable of producing. Most vacuum capacitors change the capacitance from minimum to maximum by turning the control axis (multi-turn). The design of the vacuum KPI allows the installation of various reading devices with reset and installation in a specific position required for individual ranges. Limiters at the beginning and end of the KPI capacity adjustment are also provided to avoid its damage. Installing vacuum KPIs may or may not be a problem, since most of these KPIs also contain mounting devices; if they are not provided, they are easy to manufacture. Vacuum control units can be mounted in any position: vertically, horizontally, in a suspended position.
For a truly powerful amplifier, the best choice would be to use vacuum control units, which do not flash even with very high powers supplied to them. Yes, they are not cheap, but the stingy pays twice... (The entry of a small part of air during storage, transportation or operation makes such KPIs absolutely unsuitable due to the occurrence of discharges in them. Before operation, it is necessary to check the KPIs for leaks using a high-voltage tester and protect them from deformation and shock during operation - UA9LAQ).
One moment: The higher the anode voltage used in the amplifier, the more difficult it is to find a suitable KPI with an air dielectric that would withstand a constant anode voltage plus RF and would not cause arcs or problems with capacitance overlap. When the voltage at the anode of the RA lamp(s) is 3 kV, it is still possible to use CPE with an air dielectric; the problems of using them at an anode voltage of 4 kV or more increase exponentially. (The author apparently means the direct connection of the KPI to the anode of the lamp without a separating capacitor, but also, being connected after the separating capacitor, the anode capacitor with an air dielectric in the P-circuit must have an increased distance between the plates: with an increase in the anode voltage, the output resistance increases lamps, which means the RF voltage also increases, which means the risk of breakdown of the gap between the KPI plates increases - UA9LAQ).
When purchasing vacuum control units, pay attention to the condition of the electrodes (plates) inside the glass case. If they have lost their shiny copper appearance, it means that the vacuum in the KPI is most likely broken. If, when the adjusting screw is completely unscrewed, there is no resistance when moving the plates apart, then, most likely, the KPI is broken. In general, the movement of the plates inside the KPI should be accompanied by resistance (force is required), and the insides of the KPI should shine, as if they had just been cleaned. Otherwise, better avoid this KPI!
Range switch: Don't skimp on this important part of RA. Buy yourself the best one you can get. Otherwise, you will simply regret it! Very decent switches are made by Radio Switch Corp. Their Model 86 switch is good, however, the best is the top model 88 switch. This switch is rated at 13 kV and 30 A. Even a 5 kW transmitter will not be able to “arc” this switch. For P- or L- circuits in this switch will require at least two sets of contacts, but three is better. A set of contacts must be provided for each range used. A special adapter must be used to connect the switch axis in the P-circuit to the switch axis of the input circuits ( i.e., when switching PA ranges with one knob). If resistors are used at the PA input (non-adjustable input), then, naturally, there is no need for an adapter. There is also the possibility of using separate switches at the input and output of the amplifier, but to eliminate installation switches to the wrong inappropriate position, it is necessary to apply some kind of interlock: mechanical or electronic.
In Fig. Figure 17 shows the switch configuration, which will help the novice designer understand the requirements for the P-circuit for the ranges of 160...10 meters. Look for similar switches at fairs, markets, and also search on the Internet; you will also find serviceable used ones.
Filament chokes: A choke in the filament circuit of a lamp with a direct filament cathode is absolutely necessary; with heated cathodes, like those of lamps of type 8877, such a choke can be dispensed with. The direct filament cathode can be found in almost all old high-power glass bulb lamps, using thoriated tungsten as the filament and cathode. At such a cathode there is both a large current and a large RF voltage, which must be isolated from penetration into other circuits, so this is where powerful chokes are installed. Such a choke is usually bulky, it is wound with double wire, turn to turn on a ferrite rod and contains a number of turns sufficient to completely remove the RF after the choke. Decoupling capacitors are usually placed immediately after the inductor on the side of the filament voltage supply from the power supply, on the housing. This type of inductor has a very large inductance value, and at the same time, it ensures the passage of large currents through itself. I also tried the use of a toroidal inductor and was pleased with it, especially since this inductor also had small dimensions.
In lamps with heated cathodes, such a cathode is an oxidized “sleeve” dressed on a filament, which heats it to produce electron emission. Cathodes of this type require lower filament currents than the first ones discussed above and do not allow RF propagation, since the cathode “sleeve” has a constant shielding effect (the outer side, in accordance with the skin effect, emits and is drawn into the functioning circuit of RF currents, the lower side is not subject to RF currents and serves as a closed screen, here you can also remember about Foucault currents - UA9LAQ). However, chokes must be included in the filament circuit to prevent even an accidental RF surge from entering the power supply complex. The filament choke in circuits with lamps with heated cathodes should no longer be large, bulky, or have a high inductance, since the RF currents acting in the filament circuit are small. The inductor has small dimensions, is wound with a double wire of sufficient cross-section to pass filament current in rubber or Teflon insulation, winding is carried out on a small ring or rod ferrite core. The inductance of the choke for operation on the ranges of 160...10 meters should be 30...300 µH. Decoupling capacitors are connected from both filament wires to the amplifier body at the point of connection to the inductor on the power supply side. Also place capacitors between the filament wires on the side of the lamp base and the cathode. The HF connection of the filament with the cathode will help equalize the HF potentials on both. This will prevent various kinds of inhomogeneities in the signals: flashes, lumbagoes, crunches, breakdowns on the filament, and will equalize both edges of the filament along the RF, which will eliminate fluctuations in the filament voltage.
Rice. 18
In Fig. Figure 18 shows a typical circuit diagram for switching on a lamp with a heated cathode with a conventional incandescent choke.
ALC: This scheme is a must. You can do without it only if you use a lamp that can be driven by the full power of the available exciter. An example is the 3CX1200A7 lamp, which can swing with a power of up to 120 W, inclusive. However, regardless of whether you use an 8877 or a 3CX800A7, 120 W is enough power to systematically destroy the grids. The ALC system prevents this, but if you "like" changing tubes more often than necessary, don't do any ALC. The best point for connecting the exciter to the amplifier is the point between the input/receive relay and the input tuning device.
The ALC circuit detects a small portion of the exciter RF input signal in the amplifier. This rectified signal is of negative polarity and can vary from -1 to -12 V. The negatively changing signal is fed back to the exciter, which biases the power amplifier in the exciter, which in turn reduces the output power of the exciter and thereby prevents pumping of the final RA.
The procedure for setting the ALC threshold is as follows:
1. Set the amplifier to full output power.
2. Adjust the ALC threshold setting potentiometer to such a level that a barely noticeable decrease in its power appears in the output signal.
3. That's it. Installation is complete.
Once the ALC threshold is set, the RF boost level can be increased or decreased, but the amplifier's maximum output power set using the ALC control will not be exceeded.
The location of the ALC system adjuster can be either on the rear or on the front control panel, but, in any case, is well marked. Installation adjustment pays off in practice, since it cannot be accidentally knocked down (to adjust, you need to take a screwdriver and also crawl under the cover, removing a possible lock). Once set, the ALC threshold adjustment is rarely changed.
In Fig. Figure 19 shows a typical ALC system diagram, simple and effective.
Rice. 19
Adjustments: The most visible part of the amplifier is the control panel, and it is also the most complex. There are many ways to position and control the device. How simple the control panel will be depends on the developer and manufacturer.
There are ready-made boards that can be purchased and installed in an amplifier, but this is a little different, because creating an amplifier yourself from scratch is much more interesting, however, for a beginner it is a way out. Remember, the more complex the device, the more difficult it is to operate and repair. Simplicity and reliability are what you need to start from when developing an amplifier. If a designer wants to create a fully automated amplifier and feels that he can cope with the task, then the flag is in his hands... It will be difficult, and there will be problems, problems... For beginners, I advise you to build the simplest, most reliable amplifiers without any frills. After you build simpler ones, there will be more complex, more elegant devices.
Look at the problem like this: “You are a development engineer, you decided that you will make a device, no matter how much time and effort it requires!”
Afterword: In an age where it's easy to buy and use whatever hobby equipment you want, it's easy to forget the satisfaction that comes from making it yourself. Anyone who buys and then plays with an expensive toy will never experience this feeling. This article is dedicated to those who, after all, want to test it, put their own hands and head to work and make their own RF amplifier, as our colleagues and predecessors did in their time. It is impossible to describe in words that feeling of completion, fulfillment of duty, satisfaction from the experience gained. You’ll also get something new in the process...
If you have any questions, I will be happy to share my knowledge and experience with you if you sincerely wish to do so.
73 de Matt Erickson, KK5DR
Free translation from English: Victor Besedin (UA9LAQ) [email protected]
Tyumen November, 2003
Every fan of making electronic devices has more than once encountered the need to wind an inductor or inductor. In the diagrams, of course, they indicate the number of windings of the coil and with which wire, but what to do if the specified wire diameter is not available, but is much thicker or thinner??
I'll tell you how to do this using my example.
I wanted to make this diagram. The winding data of the coils is indicated in the diagram (6 turns of 0.4 wire on a 2mm frame), these winding data correspond to 47nH-nano Henry, everything would be fine, but my wire was 0.6mm. I found help in the Coil32 program.
Open the program
At the bottom we see that the program can calculate almost any coil. You just need to select the one you need from the list, select (single-layer coil turn to turn)
Go to settings and click Options
In the window that appears, select nGn
Let’s return to our diagram, for example, I didn’t tell you what the inductance of the coils is and you only have winding data, how can we now find out what their inductance is??
To do this, we insert the data of these coils known to us into the windows, select the winding length until the calculations coincide with our data.
And so calculations showed that the winding length is 3.1 mm with 6 turns of wire 0.4, on the mandrel 2 mm. and the inductance is 47nH.
Now we set the diameter of our wire to 0.6mm.
But now the inductance is small, which means we begin to increase the winding length, for example, it turned out to be 5.5mm
That's all, the coil is ready.
But if, for example, you have already etched the boards, but the size of the contacts for the coil remained the same, that is, for a coil with a winding length of 3 mm, but you got it at 5.5 mm (much more and soldering 3 such coils side by side will be problematic)
This means we need to reduce our coil, put the diameter of the frame in the window not 2mm, but 4mm. And our coil with a 0.6mm wire decreases in length from 5.5mm to 3mm and the number of turns is 3.5, +/- 1-2 nH will not play a big role, but we can easily solder our inductors.
That's all, I hope my article will help you. In this program you can calculate different coils, choose from the list which one you need and everything will work out.
Purpose and design of chokes
What is a throttle?
An electric choke is a device that is an inductance coil and is designed to limit the alternating component of an electric current. In other words, if the current in an electrical circuit contains direct and alternating components, then a choke connected in series to this electrical circuit, due to its inductance and high resistance for alternating current, significantly reduces it, and has a minimal effect on the direct current component, due to its low DC resistance.
Rice. 1
Chokes allow you to store electrical energy in a magnetic field. Their typical applications are anti-aliasing filters and various selective circuits. Their electrical characteristics are determined by the design, properties of the magnetic circuit material, its configuration and the number of turns of the coil.
When choosing a throttle, consider the following characteristics:
- required inductance value (H, mH, μH, nH);
- maximum coil current;
- tolerance (the amount of deviation from the original value) of inductance;
- temperature coefficient of inductance (TCI);
- active resistance of the throttle coil wire;
- inductor quality factor, which is determined at the operating frequency as the ratio of inductive and active resistance;
- frequency range of the coil.
Depending on the frequency range, high-frequency and low-frequency chokes are technically distinguished
High frequency chokes are divided into two types:
- with a constant inductance value;
- with a variable inductance value, due to an adjustable ferromagnetic core.
The first type is used, as a rule, in the input circuits of telephone sets, in smoothing filters, and in the power supply circuits of RF equipment. The second type of coils is used in resonant circuits - HF, paths of receiving and transmitting devices.
In tube audio amplifiers, high-frequency chokes are used extremely rarely. As a rule, their use can be predetermined by the circuit design of the output stages, built on high-power high-frequency pentodes, prone to self-excitation at radio frequencies.
Structurally, high-frequency chokes are made in the form of single-layer or multilayer coils. The designs of high frequency chokes are shown in Fig. 2. For long chokes ( a, b) and average ( b, c) waves, sectioned multilayer winding is used. Chokes for short ( G) waves and for meter ( d) waves usually have a single-layer winding - continuous or with a forced pitch. Ceramic resistance rods VS-0.5 and VS-1.0 are often used as a frame.
Rice. 2
You can make a high-frequency choke yourself by winding the required number of turns to obtain the required inductance on a ceramic or fluoroplastic core. You can calculate the required number of turns using the formulas given in the section
It is better to use commercially produced RF chokes. They have clear, bright color markings and are characterized by high quality factor.
Rice. 2
Designed to suppress the low-frequency component of the alternating current of the supply network and its harmonics. Figure 3 shows a low-frequency choke with an inductance of 3 H at a bias current of 120 mA.
Rice. 3 Low frequency industrial choke
Chokes are better, and it’s easiest to use factory ones, preferably from old tube TVs Temp-6, Temp-6M, Temp-7, Rubin-102, Avangard, Belarus, or other old TVs with similar characteristics. But if the task is to make a tube amplifier of high quality and reliability with your own hands, then the inductor will have to be calculated using the method given below and made it yourself. A fundamentally new approach in modern tube circuitry may be the requirement for mandatory adjustment of the power filter chokes to resonance at a frequency of 100 Hz. This is necessary to increase the efficiency of filtering the rectified voltage.
Calculation of a low-frequency choke for an anode power supply
The inductor is an important element of the power supply of a tube amplifier. Together with electrolytic capacitors, it is part of a U-shaped low-frequency filter and becomes an indispensable element in the anode supply circuit of a Hi-End class amplifier. Depending on the power characteristics of the amplifier and its quality indicators, the dimensions of the inductor can vary greatly and reach up to half the size of the power transformer.
Some options, found in the calculation formulas:
F- frequency Hz;
Sc- cross-sectional area of the core, sq. cm;
TOWith- coefficient of filling of the core with steel;
Sok- cross-sectional area of the window, sq. cm;
TOOK- window filling factor with copper;
INT- maximum induction in the core, T;
J- current density in wires, A/sq. mm.
I- direct current in the inductor winding wire, A.
The main parameter of the inductor is its time constant, the ratio of inductance to winding resistance L/R. The higher this value is required, the larger the dimensions of the magnetic core must be so that the wire of the required diameter and length fits in the core window.
It is calculated using the already known formula:
With a constant degree of permanent magnetization, the inductance is maximum at a certain length of the non-magnetic gap lz . The equivalent magnetic permeability of the core depends on the size of this gap:
In the presence of permanent magnetization lz is no longer an independent variable. The key quantity in the calculation of chokes and transformers is the degree of magnetization or the number of linear ampere turns ( aw0 ).
Formula for the relationship between magnetic field strength and engineering value aw0 , is given below:
The proposed calculation algorithm is based on an experimental graph of the dependence of magnetic permeability on aw0 Figure 4.
Rice. 4 Experimental graph of the dependence of the initial magnetic permeability on aw0
These graphs correspond to mass steel grades. High-quality steel has several times greater magnetic permeability, but in most cases you cannot count on this. The graph shows the dependence of the initial (i.e., in the absence of an alternating magnetic field) magnetic permeability on the magnetic field strength, expressed in ampere turns per centimeter. In the SI system, voltage is measured in amperes per meter. It should be remembered that the points on the graph correspond to different gaps. Higher tensions require larger gaps. At the beginning of calculating the value aw0 and correspondingly, μ z are not known. The number of turns in the windings can be obtained by the method of successive approximations using the formula:
To do this, the parameters of the transformer, the required inductance and the test value are substituted into the formula μ samples, Based on the obtained number of turns, the degree of magnetization is calculated aw0 . On schedule μ (aw0 ) located μ z , instead of graphs for machine calculations, you can use approximating equations:
For hot rolled steel
For cold rolled steel
Trial μ samples the number of turns is adjusted and calculated again. This procedure is repeated several times until the change in the number of turns from render to render is insignificant (several percent). In most cases, two or three passes are sufficient. If the new value is greater than the old one μ samples, That μ samples should be increased so that it becomes a little larger μ z and vice versa. At the end of the calculation, you need to make sure that the resulting L, N satisfy the requirement of constructive feasibility. To do this, calculate the maximum wire cross-section S, which can be placed in a window
The current density in the copper conductor of the inductor winding is calculated by the formula:
If the current density J does not exceed the usual 1.5-2 A/sq. mm, then the calculation can be considered complete, since the shell resistance does not require exact compliance with the given one. The number of turns should not exceed 3500-4000. If necessary, select a different standard size of the magnetic core and repeat the calculation. When assembling a wound inductor, it is necessary to place a non-magnetic gasket of the required thickness in the gap. Precise observance and selection of the gap size is necessary only for output transformers. For chokes, the accuracy of the empirical formula given below is quite sufficient. The gap size is calculated in mm:
The winding of choke coils has no special features. In most cases (for power supply chokes) there is no need even for interlayer insulation. The winding is usually at high potential, so it must be well insulated from the core. Impregnation of chokes is usually necessary to avoid humming. The results of calculating the inductor on a very common and cheap core from the output transformer of a W 16x25 tube TV with a window size of 16 x 40 mm are shown in Table No. 1:
Table No. 1
| Sc | 4 kb. cm |
| Sok | 3.84 kb. cm |
| Lc | 10.6 cm |
| L0 | 12.84 cm |
| Kok | 0,34 |
| I0 | 120 mA |
| aw | 29,4 |
| μz | 171,8 |
| N | 2600 vit |
| L | 5.51 Gn |
| D | 0.25 mm |
| R | 116.3 0m |
| P | 1.67 W |
| lz | 0.25 mm |