1 Introduction

The vacuum tube was an essential part of early radios, used to create and amplify the electrical signals needed for the radio to work. The vacuum tube radio was a new stuff of the early 20th century, and immediately became the "new favorite" of that era with the launch of the radio show. Due to the continuous development of technology and the emergence of transistors, vacuum tubes were swept away by the powerful transistors in the 1960s and 1970s. With the improvement of people's lives, they have a new understanding of sound quality, and people are once again interested in the beautiful sound of vacuum tube radios.

Compared with the semiconductor radio, the biggest advantage of the tube radio is that the sound quality is obviously better than that of the semiconductor radio. The second is that it is stable and elegant, with classical temperament. In addition, the history of tube radios is far greater than that of semiconductor radios, which are more valuable for collection.

2 Vacuum Tube Radio Circuit

2.1 Input Circuit

The circuit from the antenna of the radio to the input of the first-stage tube is called the input circuit. It has two tasks to complete: One is to transmit the high-frequency signal voltage induced on the antenna to the grid of the first electron tube (usually the frequency conversion stage). To accomplish this task, there should be a certain coupling between the radio antenna and the input circuit to facilitate signal transmission. Second, because the antenna induces a lot of signals, it is necessary to select the radio broadcast signal you want to listen to  suppress unnecessary signals. To accomplish this task, the input circuit should be composed of a selective resonant circuit, so the input circuit should compose of coupling elements and the resonant circuit.

Depending on the coupling form of the antenna and the resonance circuit. The input circuit can be divided into three types: inductive coupling (transformer coupling) circuit, capacitive coupling circuit, and inductive capacitive coupling circuit. Since the voltage transmission coefficient of the inductive coupling circuit is relatively high and uniform, tube radio usually adopts this type. In order to improve the suppressing ability of mid-frequency interference (because the intermediate-frequency detuning is smaller for medium wave, and it is larger for short wave.). An intermediate-frequency trap circuit is also applied in the antenna loop.

2.2 Converter Stage

The converter stage is located between the input circuit and the intermediate amplifier stage. Its function is to change the high-frequency amplitude modulation signal into a fixed intermediate-frequency amplitude modulation signal. In order to complete the task, the frequency converter should be composed of four parts: oscillator (generating a high frequency constant-amplitude oscillating voltage with an intermediate-frequency different from the frequency of the external radio signal), a high frequency circuit (used to select the radio signal, usually it is the input circuit), non-linear components (usually pentagrid converter, used to change the frequency), the intermediate-frequency loop.

The converter stage of a tube radio is generally composed of a pentagrid converter tube. The high-frequency circuit is connected to the third grid (signal grid). The oscillator is composed of the screen grid (oscillation anode), the first grid (oscillation grid) and the cathode. The anode load is an intermediate frequency resonance circuit. Therefore, the main mark that distinguishes the converter stage from the tube amplifier is the pentagrid tube and three resonant circuits. The frequency converter can be divided into single-grid converter and double-grid converter according to the different ways of inputting signal voltage and local oscillation voltage. The single-grid inverter applies the signal voltage and the local oscillation voltage to the same electrode of the inverter tube, so the traction effect is large, and the operation is unstable. The double-gate inverter adds the signal voltage and the local oscillation voltage to the different electrodes of the inverter tube separately, the traction effect is greatly reduced, thus double-bar frequency converters are commonly used in tube radios.

The structural feature of pentagrid tubes is that there are two metal sheets on the second grid, called the collection screen. It blocks the electrons moving to the anode. There are two metal rods on the third grid in the gap of the collecting screen, which carry negative potential. When the electrons passing through the second grid hit the negatively charged metal rod while advancing, the electrons flying to the anode is divided into two streams, one flying to the anode, and the other being collected by the collecting screen without returning to the cathode. In this way, when the signal grid voltage changes, it will only affect the distribution of the two currents, and has little effect on the total current. Because the local oscillator adopts the total current feedback, the oscillator is basically not affected, so the oscillation frequency is stable. There are many types of frequency converter circuits with different performances, but their principle difference lies in the way of local oscillation feedback.

2.3 Detection Stage

The detection stage is located between the intermediate amplifier stage and the audio voltage amplifier stage. Its task is to transform the intermediate-frequency signal into the original modulated audio signal. To complete the task of frequency conversion, the detector must consist of an intermediate-frequency signal circuit (it is the output circuit of an intermediate-frequency amplifier), a non-linear elements (usually a diode), and a load (resistor).

2.4 Intermediate Amplifier Stage

The intermediate amplifier stage is located between the frequency converter stage and the detection stage, and specifically amplifies the 465kHz intermediate-frequency signal. The main feature of the intermediate amplifier stage is: in order to achieve automatic gain control, the intermediate amplifier tube generally uses a remote cut-off tube. The intermediate-frequency transformer is the main component of the intermediate-frequency amplifier, and its quality directly affects the quality of the radio. In addition, a shielding cover is usually used to avoid the parasitic coupling between the intermediate-frequency loop and the eternal to improve the working stability. The shielding cover is made of good conductors such as aluminum and copper, and is attached to the case (ground potential), to isolate the entry of high frequency electromagnetic fields.

2.5 Audio Voltage Amplifier

To make the power amplifier stage output a certain amount of power, its anode current must have a certain amount of AC component, which requires that the grid audio signal voltage used to control the anode current must have a sufficiently large amplitude (voltage). The output voltage of the detector stage is usually very low. Therefore, between the power amplifier stage and the detector stage, there is always one or several audio voltage amplifier stages. Its function is to amplify the audio signal voltage to meet the input signal requirements.

The audio voltage amplifier stage can be divided into a resistance-capacitance coupling amplifier and a transformer-coupled amplifier according to the different anode load forms. In the vacuum tube radio, the resistance-capacitance coupling amplifier is used mostly. The audio voltage amplifier stage and the detection stage usually share an electron tube. The input signal is generally controlled by a volume control potentiometer to adapt to different broadcasts and listening situations. Some radios have a tone control circuit connected to the output end of the audio voltage amplifier stage to flexibly change the frequency characteristics of the amplifier to make the sound pleasant. Due to the different forms of volume and tone control circuits. There are many types of audio voltage amplifier stages.

2.6 Audio Power Amplifier Stage

The audio power amplifier stage is the last stage of the tube radio, and its function is to output a certain amount of audio power to make the speaker work normally. Someone may wonder, can a voltage amplifier be used instead of a power amplifier? This will not work. Although there is no difference between the basic circuits of a power amplifier and a voltage amplifier, due to the different tasks of the two, there are significant differences in the selection of electron tubes, the circuit components, and the nature of the load. It should be pointed out that the so-called power amplifier does not amplify the input power itself, but uses the control function of the electron tube grid voltage to convert the power supplied by the anode circuit DC power supply into the required audio power. From the viewpoint of energy conversion, the power amplifier is an energy converter.

2.7 Rectifier

The rectifier stage of the tube radio is composed of three parts: power transformer, rectifier tube and filter. There are many forms of rectifier circuits, and the most common is a full-wave rectifier circuit.

3 Classification

3.1 AM Radio

In the era when tube radios were popular. AM radios were the mainstream products. Amplitude modulation is to modulate the high frequency carrier with audio signal. Its waveform is symmetrical up and down, and the amplitude is the same as the modulated signal. After detection, the highfrequency component is filtered out to obtain the audio signal. The frequency of the carrier signal (the frequency of the broadcasting station) is the carrier frequency.

AM radios can receive medium wave and short wave broadcasts, and some can receive long wave broadcasts. Since the mid-band frequency interval has been unified to 9KHz, its highest audio frequency is only 4KHz. The sound quality is affected, and electromagnetic interference is relatively large.

There are two main types of AM radios: direct-amplifier type and heterodyne type.

1)Direct-amplified radio, also known as high-amplification radio, its typical circuit structure is such as: 

high-amplification→detection→low-amplification→power amplifier

A circuit that uses a grid detector circuit and a high-frequency positive feedback, called a regenerative radio, can achieve better sensitivity and selectivity. It can receive AM telegraph signals with high-amplifier and short-wave. Old Japanese-made radios mostly have this circuit. High-frequency signals of direct-amp radios are prone to self-excitation, and the high-end and low-end gains are uneven, and there is no high-amplifier regeneration. Widespread use of reed speakers with poor sound quality made it obsolete. Out of nostalgia, some enthusiasts are still keen on getting recycled radios. The regenerative circuit can not only be applied to direct-amplified radios, but also in simple heterodyne radios, because proper positive feedback can also be used to improve sensitivity.

Simple regenerative radios often use reed speakers, which have high impedance (about 10K) and high sensitivity, and can be directly used as the load of the power amplifier tube. Its frequency range is only 350~3000Hz, so the sound quality is poor. Later regenerative radios used moving coil speakers, and the sound quality was better. However, due to the low impedance, an output transformer is needed, and its primary impedance must match the load impedance of the power amplifier tube. Moving coil speakers are divided into permanent magnet, constant magnet and excitation type. In addition, excitation horns are used in AC tube radios, and their excitation coils can also be used as filter chokes.

3.2 Heterodyne Radio

The heterodyne radio uses a frequency conversion circuit. The signal generated by the high-frequency oscillation circuit is different from the input signal at a certain frequency. After the two are combined, a fixed intermediate frequency signal (455~465KHz) is generated. The oscillation circuit needs independent vacuum tube as a part before the emergence of the dedicated frequency conversion tube. Some people call the oscillation frequency higher than the signal frequency by a heterodyne type.

Heterodyne plus intermediate-frequency amplifier circuit is called superheterodyne. This type of circuit requires a single electron tube to oscillate. Later a multi-pole or composite tube dedicated to frequency conversion appears, such as 1A2, 6A2, 6SA7GT, 6U1, 6K8, etc. The superheterodyne type is the most common circuit of commercial radios. It has an automatic volume control circuit and can add tuning instructions. The superheterodyne radio can obtain more stable and higher gain due to the fixed frequency amplified. The disadvantage is that there is image frequency interference.

The circuit structure of a typical superheterodyne radio is as follows:

Frequency conversion→middle amplifier→detection→low amplifier→power amplifier

3.3 FM Radio

FM is to use audio signal to modulate the frequency of high-frequency carrier. Its advantages are strong anti-interference ability, high signal-to-noise ratio, good frequency bandwidth and sound quality, and the audio frequency can reach 20~15000Hz. Because the FM audio work in the ultra-high frequency band, it can accommodate many radio stations. Because of its linear propagation characteristics, the same frequency can be reused at a distance of hundreds of kilometers, which can effectively solve the problem of congestion of medium and short-wave radio stations.

Modern FM broadcasting is compatible with stereo and mono (in the early days of stereo broadcasting, two frequencies were used and received by two radios). Amateurs like to use a simple super-regenerative circuit to receive FM broadcasts. Because it works in a self-oscillation state, it is unstable and has strong super noise.

3.4 Service Life

The general design life of the valves is 2000 hours (special tube has 5000~10000 hours), but in reality, many electron tubes break after using 2000 hours, for example, there is no sound. Its performance is not up to the design standard. When there is not any sound from the actual tube, it may have reached 3000 hours or more. The actual service life of the amplifier tube of the front stage is longer. To cite the simplest example: the old picture tube (picture tube is also a kind of vacuum tube) of a TV set, the actual life span usually excesses 10 years.

If use it for 5 hours a day, you need to change the tube once a year to meet the actual design performance, and the replaced tube can also be used for other purposes.

4 Audio Maintenance Principle

How to repair the failure of the vacuum tube radio depends on the difficulty of the failure. Under normal circumstances, the following steps are used.

The first point is to seek clues. What is the difference in the use of the radio before and after the failure, thinking about whether it has been repaired, or what components have been replaced, etc.

The second point is the fault display. For faults such as flashover, smoking, burning, etc. Turn off the power immediately and find the factor to avoid damage to the audio. According to the displayed failure phenomenon, rotate audio button to compress the failure to clarify where the failure may occur, and to provide necessary information for analysis and judgment.

The third point is analysis and judgment. Make comprehensive analysis and judgment based on the information obtained by self-examination inference and fault display. List all possible direct causes of the failure, and develop a scientific inspection procedure. This not only prevents blind movement, but also accumulates maintenance experience and improves efficiency.

The fourth point is troubleshooting. Gradually narrow the scope of the fault, and accurately detect the fault point. Sometimes it refers to (such as electron tubes, capacitors, resistors, coils, transformers, etc.) connecting wires, welding points of a certain component. Determine where the failure occurred, which is important for fault repair. Tube radios are usually divided into high-frequency part, intermediate frequency part, audio part, power supply part and auxiliary circuit according to their working frequency and circuit function. The fault should be compressed to a specific circuit, such as DC circuit, AC circuit, or the anode circuit, the screen grid circuit, grid circuit, cathode circuit, etc. Since each loop in the electronic tube circuit affects each other, a failure of a component may affect several loops at the same time. Therefore, in the inspection process, comprehensive analysis should be performed, that is, each circuit should not be isolated.

The fifth point is repairing practice. The repair test can be performed after determining the cause of the failure.

The sixth point is repair inspection. In the actual maintenance process, if a failure point is detected, it must be immediately eliminated and repaired. After all the faults are investigated, the radio should be inspected as necessary to ensure the quality of the repair. The specific content of the inspection depends on the specific fault and the type and quantity of the instrument. The simplest one should also be auditioned and take general technical observations.

5 Specific Troubleshooting

A radio is composed of many components, and component damage is the main cause of radio failure. When the fault location is gradually reduced, the quality of the components must be checked to determine the fault. Therefore, inspecting components is one of the main methods to find the cause of failure.

5.1 Tube inspection

The common faults of electronic tubes are mostly filament broken, poor contact of the electric board, broken pole, pole touching, leakage, breakdown between the cathode and the filament, aging, air leakage, and micro-sound effects. When the radio is turned on, if the filament of a certain tube does not light up, the filament may be broken or a serious leak. The tube shell is cold when the filament is broken, and the tube shell is often warm when the gas leaks. Unplug the tube for further inspection. Use a multimeter to measure the filament resistance at the "R×1"gear. If the meter indicates infinite, the filament is disconnected. If the indicator of the needle is zero, it means that the tube is short-circuit. When the indicator of the needle indicates a constant value, it means that there is leakage between the electrodes. The smaller the resistance, the more serious the leakage. At this time, the top of the tube is usually milky white.

The electron tube electrode is in poor contact, intermittently electrode touching (some are hot electrode touching), and the radio will sometimes have no sound or noise. This type of failure is characterized by being greatly affected by vibration. Therefore, you can tap the tube shell lightly. If there is no sound or noisy sound (sometimes there may be no sound or normal sound after tapping), it means that the tube electrode is in poor contact or intermittently electrode touching. When the fault is serious, the electrode is completely disconnected, which means a broken pole. The tube does not work, and the radio is silent.

When the electron tube ages, the positive current and the amplification will decrease. If it was frequency conversion tube, the local oscillator will oscillate abnormally, resulting in the radio cannot receive the radio signal, low sound, or the high frequency end of the band can receive one or two radio stations, and the low-frequency end cannot receive the radio signal. The micro-phonic effect of the vacuum tube is often caused by improperly fixing the electrode, and it can be judged by tapping the tube case lightly. Sometimes it is difficult to determine whether the valve is normal or not. The easiest way is to replace it with the same type of electron tube. If the failure disappears, it means that there is a problem with the vacuum tube.

5.2 Resistor Inspection

There are two types of resistors used in radios: fixed resistors and variable resistors (potentiometers). They are not easy to damage. Possible damages include: resistor burnout, resistor body broken or lead broken, internal open circuit, or poor lead contact, etc. Some of failure can be observed. For example, the increase of fault current in a certain part of the circuit, which means the resistor is burned out. At this time, the paint layer on the outside of the resistor is burnt and hot. If the resistor is internally disconnected or the resistance value changes, it can be checked by measuring the ohm gear of a multimeter. If the meter indicates infinity, it means that the resistor is internally open; if the measurement result is too far from the nominal value (the error exceeds 20%), it means that the resistance has deteriorated.

The lead of the resistor body is in poor contact and will be broken unbroken, which will cause noise or sound from the radio. This kind of failure is very susceptible to vibration and produces vibration noise. Therefore, you can gently shake the resistor body to see if the sound changes. It can also be measured with a multimeter. When measuring, shake the resistor body. If the pointer is unstable, it means that the contact is bad.

The potentiometer that controls the volume of the radio usually has a power switch. The common faults of the potentiometer are: dirty carbon film, wear of the carbon film, poor contact of the sliding contact, loose lead-out piece, burnt of the carbon film, broken bakelite frame of the sliding contact, leakage, and damage to the power switch. When the potentiometer is normal, the radio can only hear a very light sound after power on. If the potentiometer is closed, the broadcast voice should not be heard. After turning off the power, there is no sound. If you cannot control the power on and off, it indicates that the power switch is partially damaged; if the noise is very loud, especially when the volume is adjusted, the loudspeaker has a large sound or interrupted sound, it means that the potentiometer is in poor contact.

5.3 Capacitor Inspection

There are two types of capacitors used in radios: fixed capacitors and variable (semi-variable) capacitors. The faults of fixed capacitors are usually leakage, breakdown, internal open circuit, poor internal contact, broken lead, or capacity failure, etc. The common faults of variable capacitors are bump, short circuit, leakage and so on.

5.4 Transformer Inspection

Common faults in transformers are coil mold breakage, a partial short circuit, leakage and other faults. A multimeter can also check out the coil mold. When high-frequency coils and intermediate-frequency transformers are locally short-circuited, it is not easy to measure. The substitution method can be employed to test or adjust the loop frequency to figure problems out.