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When building a compact power amplifier (PA) for a radio station, there is no alternative to blowing lamps. This is also confirmed by foreign practice, since tubes are used in most modern branded amplifiers.

One of the important structural elements of the amplifier can be called a lamp cooling system. There is practically no information on the design of such systems in the literature, and this is probably the biggest "blank spot" in the "amplifier industry". Meanwhile, this information is important, since the layout of the PA depends on the design of the cooling system, and in case of an erroneous decision, a laborious rework will be required. The cooling system must be done right immediately.

The proposed article presents practical justifications for the design parameters of air-cooled systems for generator lamps.

Selection of evaluation parameters for testing cooling systems and measurement technique

In the passport of powerful generator lamps, the manufacturer indicates the cooling conditions and the maximum allowable temperature of its structural elements [1]. Therefore, the maximum temperature of the anode heat sink \a max-

Cooling of the lamp depends on the supply (consumption) of air by the fan [1]. Therefore, for the most efficient use of the air flow, the air path of the amplifier must have a minimum aerodynamic drag (hereinafter drag). It is, in general, due to the location of the fan, the shape of the radio tube, its panel and the configuration of the air duct.

The flow moving in the duct is characterized by the speed v, m/s, and the flow rate V=vs, m³/s, where s is the cross-sectional area of ​​the air duct at the place where the velocity is measured, m² [2]. Any resistance in the path of the air flow causes a decrease in speed, and therefore a loss of supply.

These values ​​can be used to estimate the resistance of the air path. Therefore, the second evaluation parameter in the comparative tests of cooling systems is the value of the reduction in the supply of AV, expressed in% AV = [(Vb-V) / Vb] -100%,

where V - fan supply in the blowing system, m³/ h;

Vb - fan supply in the basic version, with which the comparison is made, m³/ H.

For example, the supply of a fan installed in an empty duct, Vb = 120 m³/h When placing a panel with a radio tube in the duct, the flow decreased to 53 m³/h Feed reduction due to their resistance will be

AV = [(120-53)/120]-100% = 56%.

The second auxiliary parameter can be used when comparing cooling systems without a working radio tube.

For the experiments, we tested the GU-84B lamp blowing system, which consisted of a standard panel, air ducts with an inner diameter of 112 mm, and a fan.

It allowed testing various cooling systems and their individual elements. During the tests, the radio tube worked as a heat generator, i.e. all the RA power supplied to the anode was converted into heat.

The air supply was determined by a vane anemometer (designed for testing ventilation systems) [2], located directly behind the air duct.

The temperature was measured with an M838 digital multimeter with a thermocouple. The measurement error was ±3° at t < 150°C and ±3% at t > 150°C. The temperature was determined after ten minutes of lamp operation in the measured mode.

Axial Fan Cooling Systems

In practice, there are four options for blowing the radio lamp: side, axial supply, axial exhaust and axial two-fan supply and exhaust. The optimal one was determined practically by the cooling efficiency.

For testing, an axial all-metal fan TYP 4658N with an impeller diameter of 110 mm and n = 2200 rpm was used. Fan supply in an empty duct - 120 m³/ H.

With side blowing (Fig. 1), the cooling air passes only through a part of the heat sink fins of the lamp and the cooling surface is reduced by 9...21 times (Table 1). You can improve cooling by increasing the air speed, but this will increase the size and noise of the fan. The inefficiency of the scheme is obvious. The manufacturer also does not recommend using side airflow for lamps designed for axial airflow [1].

The results of testing the exhaust (Fig. 2) and supply (Fig. 3) blowing systems are presented in Table. 2.

The measurements showed that the fan flow in the exhaust system (53 m3/h) is 2,4 times higher than in the supply system (22 mXNUMX/h).³/h). If a comparison is made by the heat sink temperature, which can be measured more accurately, then tAmax = 130 °C is achieved in the supply circuit at RA = 240 W, and in the exhaust circuit tAmax = 126 °C at RA = 460 W. Therefore, the exhaust fan removes about twice as much heat as the supply fan.

For a person accustomed to dealing with electrical circuits, this result may seem unexpected. Indeed, any resistor causes the same voltage drop regardless of which side of the power source it is located. The laws of air movement differ from Ohm's law, and the aerodynamic resistance of a lamp with a panel in this case depends on the location of the fan. The result obtained is explained as follows.

The air flow leaving the axial fan is not straight-through, but swirled (twisted like threads in a twisted rope), and it enters the annular slot of the panel not perpendicularly, but at an angle (Fig. 3). The swirling air entering the panel behaves like a stone thrown into water at an angle; repeatedly bouncing off it before sinking. Therefore, 82% of the fan flow is lost to friction between the individual flow layers. This significantly impairs heat dissipation.

When the exhaust fan is operating under the action of a vacuum, a straight-through flow passes through the lamp, so the amount of supply reduction is much less. In this case, it is mainly due to a head-on collision with the cathode.

Insufficient air supply can be increased in two ways: use a more powerful fan or install a second fan coaxially with the first one. Dual-fan blower systems were tested to determine the best method.

It has been established that the efficiency of supply of coupled fans depends on the distance between them. At a distance of 30 mm, the increase in feed was 5%. The reason, obviously, is that the swirling air flow from the first fan hits the blades of the second at a non-optimal angle, is not captured by these blades, but is reflected from them. With an increase in the distance to 100 mm, the flow increases by 30%, since the air flow from the first fan becomes axial and is more successfully captured by the blades of the second fan. Obviously, with increasing distance, the efficiency of the second fan will increase. But a long duct will increase the size and complicate the layout. Therefore, the use of dual fans is not justified.

The joint operation of two sources (converters) of energy has always been a difficult task and required the use of special technical solutions. Obviously, for the coordinated operation of the fans, it is necessary to select the distance between them, the shape and relative position of the blades, and also to install the “rectifying” air flow of the plate. In any case, this task is already beyond the scope of "amplifier building".

Axial two-fan supply and exhaust airflow is shown in fig. four.

According to the measurement results given in table. 3, it can be seen that after connecting the second supply fan to the exhaust circuit, the air supply increased only by 20%, and tAmax decreased by 8%. Therefore, the use of a second, supply, fan is inefficient. The reasons for this phenomenon have already been discussed above.

According to the test results of various blowing options with axial fans, the following conclusions can be drawn:

1. An exhaust cooling system with a single fan providing the necessary air supply is optimal.

2. The use of a second fan to increase the flow is unjustified for any cooling system.

Justification of the design parameters of the exhaust cooling system with an axial fan

At PA = 460 W and a gap B between the lamp heat sink and the air duct equal to 7 mm, the distance A between the fan and the anode heat sink was set equal to 50, 80, 115, 150 and 210 mm. The measurement results are shown in the graph (Fig. 5).

With a decrease in distance A to 50 mm, the heat sink of the lamp enters the zone of turbulence in front of the fan and tAmax increases by 10% due to the deterioration of cooling. With a significant removal of the fan, cooling also deteriorates due to an increase in the loss of air kinetic energy due to friction against the walls of a long duct. The best cooling conditions are provided at A equal to 1,0...1,2 fan diameters.

The air temperature in front of the fan decreases from 97 to 49 °C as it moves away from the anode due to cooling through the walls of the air duct. For better heat transfer, they should have a minimum thickness.

The temperature of the blades is less than that of the air flow entering the fan. This is due to the fact that the hot air leaving the fan is intensively mixed with the outside, quickly cools itself and cools the outer sides of the fan blades. For the same reason, with decreasing A, the temperature of the blades rises more slowly than the temperature of the hot air in front of the fan.

The results of measurements are given in table. 4 show the dependence of tAmax on the size of the gap B at PA = 770 W and A = 115 mm.

With a gap B = 0, the side surface of the heat sink does not participate in heat transfer and the anode temperature is maximum. At B = 7 mm, tAmax decreased by 15°C, since the side surface of the heat sink began to participate in cooling. With an increase in gap B to 17 mm, tAmax decreased by another 5 °C. As the gap increases, the air speed on the outside of the heat sink increases, so cooling improvement is possible, but the difference with previous experience does not exceed the measurement error. Therefore, for effective cooling of the outer surface of the heat sink of the lamp, a gap of 5 ... 10 mm is sufficient.

Taking into account the above results, an exhaust cooling system for the GU-84B lamp was manufactured and tested (Fig. 6).

The measurements showed that tAmax is achieved at RA = 770 W. The temperature of the fan blades in this case is 73 ° C, so an all-metal fan at maximum power will provide greater reliability.

For fans with plastic parts, the maximum allowable operating temperature is up to 60 °C [3,4].

With an increase in PA from 0 to 770 W, tAmax increased from 36 to 207 °C, and for the cathode, from 120 to 145 °C. Therefore, to cool the cathode part of the lamp, even at its maximum thermal regime, an exhaust fan is sufficient.

On fig. Figure 7 shows the dependence of tAmax on the heating time at RA = 770 W and cooling time at RA = 0. The time for the lamp to fully warm up after applying all voltages is 10 minutes. Cooling time to 36 °C - 11 min. The anode cooling graph allows you to calculate the temperature correction for measuring the anode temperature not in transmission mode, but after a period of time necessary to disconnect dangerous voltages.

Dependence in fig. 7 explains why, even with an inefficient cooling system, the amplifiers are capable of operating in CW and SSB modes.

In everyday work, the transmission time does not exceed, as a rule, 1 ... 2 minutes and the lamp simply does not have time to warm up, and during reception it quickly cools down. Therefore, the blowing intensity in CW and SSB modes can be several times lower than with continuous radiation.

Cooling systems with centrifugal fan

Three blowing systems with a centrifugal fan were tested: supply air with coaxial flow (Fig. 8), exhaust air (Fig. 9); supply air with side flow (Fig. 10).

For testing, a centrifugal fan with an impeller 30 mm wide and 92 mm in diameter was used, which was rotated by an electric motor KD-3,5As n = 1400 rpm. Fan supply in an empty air duct - 90 m3/hour.

The test results showed (Table 5) that the supply centrifugal fan with coaxial flow is the most efficient. Its air flow is straight-through and has a higher speed v than that of an axial fan. With the same air supply, its kinetic energy is much greater, since it is proportional to v². High-speed straight-through air flow better overcomes the resistance of the air path, and in contact with the lamp, provides greater heat transfer. The fan works in the best conditions. Cold air is supplied here, therefore, a light plastic impeller can be used, thereby reducing the load on the bearings and extending their life. The electric motor is shielded from RF radiation by the walls of the input compartment. The use of an electric motor with bearings made of porous bronze made it possible to minimize the noise level.

The inefficiency of blowing the supply system with side flow (Fig. 10) is visible without testing, since the air, hitting the wall, loses most of its kinetic energy and only then, ricochets, goes to the lamp. Measurements were taken to compare the performance of this and other systems. The test results (Table 6) showed that the smallest losses are achieved with the minimum dimensions of the inlet compartment, i.e. when it is actually a continuation of the duct with a side outlet. In this case, the flow, in comparison with the coaxial flow (Fig. 8, Table 6), is 2,8 times less, and tA max is 70°C higher or 1,7 times.

The advantage of a side flow system is that it simplifies the installation of the ventilation unit. It can be placed on either side of the lamp and keep a small height of the PA body. The disadvantage is the worst heat dissipation due to a significant loss of fan supply (80 ... 85%) when turning the air flow.

This system is used in branded UM. It is efficient when using small-sized lamps (GU-74B, GU-91B), which require a small air flow [5].

Influence of anode mounting on lamp cooling

There is no significant difference in the cooling of the lamp with and without the "anode mount". With repeated comparison of tA max for a lamp fixed in a proprietary anode ring and without such mounting, the difference was within the measurement error (ceteris paribus).

Fastening by the anode ring is necessary for reliable fixation of the lamp. But if the user has a panel without an anode ring, it can also be used. The instruction allows for fixing the lamp in the panel to focus on the ring of the second grid with the lamp pressed from the anode side [1]. To implement such a fastening, instead of the missing branded anode ring, an air duct is installed, in which a stop is placed on the insulators to press the lamp from the anode side. This method is especially convenient when using an exhaust cooling circuit with an axial fan.

Determination of fan flow in SSB and CW modes

All the above measurement results were obtained after 10 minutes of lamp operation, which corresponds to the simulation of the continuous radiation mode. For SSB and CW, the average heat generation at the anode will be much less. In this case, the fan speed (and hence the noise) can be significantly reduced.

Depending on the duration of the transmission operation, the RX / TX time ratio, the type of radiation, the quiescent current and the peak factor of the SSB signal, the average power dissipated at the anode can decrease several times. For example, when operating CW, taking into account pauses, the average power will be 60 ... 70% of the "tuning" mode. During reception, the lamp cools rapidly (see Fig. 7). Assuming an RX/TX ratio of 1:1 and a transmission time of 1...2 min, the receive time can be included in the calculation of the average heat dissipation on the lamp. In CW mode, it will be about 3 times less than with continuous radiation.

Using the coefficient found and the efficiency of the amplifier, it is easy to calculate the output power at which the tested system will be able to cool the lamp. But this is an approximate calculation based on a number of assumptions.

Accurate calculations of heat release at the anode in CW and SSB modes are complex and unjustified. It is easier to determine the required flow (turns) of the fan from the anode temperature in real operating conditions.

For example, in the UM cooling system at GU-43B [6], the fan speed was reduced so that during SSB operation, the thermal protection of the lamp would work after 15 minutes. This is more than enough for any practical work. As a result of the adjustment, the fan noise is less than the noise from the speaker at medium volume.

A well-executed airflow system will provide the operator with comfortable radio communication to the speaker, and the radio tube will fully work out the planned resource.

Noise reduction during operation of the cooling system

The operation of the cooling system is accompanied by two main sources of sound - an electric motor and fan blades. The flow in the duct creates a slight noise.

Bearings are the main source of sound in an electric motor. Therefore, special low-noise plain bearings made of porous bronze should be used. In commutator motors, noise occurs when the brushes rub against the commutator.

Particular attention should be paid to the method of mounting the centrifugal fan motor. The sound of a motor attached to the body of the "snail" is amplified by sound resonance. Therefore, it should be attached to the body of the UM. For a massive chassis, the motor is not a strong vibration exciter, and the resonant frequency of the body due to its dimensions and weight is much lower than the disturbing frequency. To reduce the vibration of the motor, a reduced voltage should be applied to it. These measures, plus vibration isolation, made it possible to completely get rid of the sound resonances of the electric motor.

A strong sound is generated when the impeller rotates. Therefore, the next task is to reduce the speed at which the blades meet the air. This problem is successfully solved by using a centrifugal fan. The sound of the axial fan installed at the outlet of the cooling system spreads freely in the surrounding space. In a centrifugal fan, the impeller operating area, where sound waves are generated, is separated from the operator by a double acoustic screen. The first is the fan case ("snail"), the second is the walls of the PA case. In addition, in a centrifugal fan, the air is accelerated by repeated action of the impeller blades on it. Each blade gradually increases the movement of the flow, so the speed of its collision with the air and the noise is less than in an axial fan. As the impact speed decreases, the frequency of the sound decreases and shifts to the region of minimum sensitivity of our ear.

When using an axial fan, noise is reduced by optimizing the blower system. The use of an exhaust cooling system with optimal parameters, in comparison with the supply one, will allow to reduce the fan flow and the speed of the blades by 2,5...3 times. Some noise attenuation can be obtained by placing a fan on the back of the amplifier [6]. In this case, for the oneator, the amplifier housing is an acoustic shield.

The next way is to use an axial fan of the largest possible diameter, but reduce the speed of rotation of the impeller. (At the same time, the speed of air passage through the lamp remains unchanged).

Completely sound interference during blowing cannot be eliminated, but in a well-made PA they are extremely insignificant. The above methods will achieve good results with any lamps.

Conclusions from the test results

1. It is most efficient to use a single fan with sufficient power to cool the lamp. The use of a dual fan system is unjustified.

2. Due to the peculiarities in the organization of the air flow, the axial fan creates a direct flow and works more efficiently in the exhaust cooling system, and the centrifugal fan - in the supply cooling system.

3. According to the results of tests of cooling systems, the two most effective designs were determined.

In the aggregate of all parameters, the supply cooling system with a coaxial flow from a centrifugal fan is the best. This ensures maximum efficiency of the ventilation unit, minimum noise, as well as reliable operation of the fan, as it supplies cold air. Disadvantages - the complexity of installation in the input compartment, the low prevalence of the necessary fans and electric motors on the market for components and their high cost.

The second option is an exhaust cooling system with an axial fan. Its disadvantages are increased noise level and heating of the fan. And the advantage is the minimum dimensions and multiple simplification of installation. In addition, axial fans are much cheaper than centrifugal fans, and the required sizes can be easily found on the component market.

Both cooling systems are justified. The final choice will depend on the availability of components, the layout of the amplifier and the opinion of the author of the design.

Lamp overheating protection

Metal and ceramics have different coefficients of thermal expansion. If the maximum allowable lamp temperature is exceeded, the mechanical stresses caused by the expansion can exceed the tensile strength of the ceramic. The resulting microcracks will lead to a rapid loss of vacuum.

Protection of the lamp in case of failure of the ventilation unit in professional PA is carried out using an air flow sensor. In the absence of airflow, its aerocontacts are triggered and the automation de-energizes the lamp. A reed switch is most often used as aerocontacts, and its operation is achieved by a miniature magnet mounted on a movable plate, which is rotated by the air flow.

This protection has two drawbacks: it does not protect the lamp from overheating when the P-circuit is detuned, and when small-sized lamps are blown, the air flow will be insufficient to trigger the mechanical sensor.

If it was not possible to achieve reliable operation of the aerocontacts, a relay protection circuit can be used (Fig. 11).

In the event of an open in the motor circuit, the control relay K1 is de-energized, contacts K1.1 close and turn on the executive relay K2, which turns off the lamp with contacts K2.1. The protection operation is signaled by the VD2 LED. After the break is eliminated, the current in the motor circuit causes K1 to operate, contacts K1.1 open and the protection circuit returns to its original state. When the current in the motor circuit is exceeded, the FU1 fuse blows, and then the protection circuit operates as if it were open.

An emergency stop of the fan may occur due to its failure or power outage.

In this case, a universal means of protection against overheating is the presence of a separate emergency fan, which is located in the same housing with batteries. When the standard fan stops, the operator installs an emergency fan on the amplifier housing above the air duct and cools the lamp for 5 minutes, as required by the instruction [1].

In case of excess heat release at the anode (for example, due to detuning of the P-loop), the nominal air supply will not be enough. To protect the lamp in this case, its maximum temperature should be constantly monitored. The hottest point is located in the upper inner part of the anode radiator. With a constant mode of operation of the ventilation unit, the temperature of the air behind the anode and the temperature of the anode are in a strictly defined relationship (see Fig. 6). Therefore, it is easier to control not the temperature of the anode, but the temperature of the air behind the anode.

After mounting the cooling system, it is necessary to experimentally obtain data on the temperature field behind the anode. Then the temperature sensor, the response temperature of which can be 70 ... 120 ° C, is placed at the corresponding point in the duct.

When the contacts of the temperature sensor SA2 are closed, the relay K2 is activated and the contacts K2.1 will turn off the lamp (Fig. 11). Contacts SA2 after operation remain closed for some time, while the heat is removed from the anode. The protection operation is signaled by the VD2 LED. After the lamp cools down, the protection circuit itself returns to its original state.

Placement of the cooling system in the amplifier case

Amplifiers traditionally use a horizontal case of the "DESK TOP" type. For this reason, the layout, which has historically developed and is rational for old glass lamps, has been "automatically" transferred to blowing lamps. To preserve the traditional design and simplify the installation of the ventilation unit, the parallel connection of small-sized GU-74B (or GU-91B) and the supply airflow with side flow were used. But due to large losses during air rotation, this circuit is not attractive for high-power lamps (see Table 6).

An amplifier of a given power is always easier and cheaper to make on one large lamp. Therefore, the layout of a powerful amplifier should ensure the installation of the most efficient cooling system.

To fulfill this requirement, it is necessary to abandon the traditional horizontal "DESK TOP" case, and use a vertical case of the "MINI-TOWER" type. It successfully hosts the most efficient coaxial flow centrifugal fan cooling system or the simplest axial fan exhaust cooling system (Figure 12).

Literature

1. Lamp GU-84B. The passport.
2. Kalinushkin MP Fan installations. - M.: Higher school, 1967.
3. Fan VVF - 112 - 2,5 - 12. Passport.
4. Fan VN-2. The passport.
5. Katsnelson B.V., Kalugin A.M., Larionov S.A. Electrovacuum, electronic and gas-discharge devices. Directory. - M.: Radio and communication, 1985.
6. Klyarovsky V. A. Power amplifier HF. - Radio, 2001, No. 8, 9.

Author: V. Klyarovsky (RA1WT), Velikie Luki

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