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Build your own audio valve amplifiers

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Rainer zur Linde,
Build your own audio valve amplifiers
Circuits for hi-fi and musical instruments
Elector Electronics (Publishing) P.O. Box 1414 Dorchester England DT2 8YH,
First Published in the United Kingdom 1995, This Edition September, 1997

Contents

Introduction

To most people working in electronics, the thermionic valve or electron tube is history. Indeed, few of us would be content nowadays without the ease of use, the technical quality, the low weight and the small dimensions of transistorized or integrated electronic equipment. None the less, for many hi-fi enthusiasts and musicians, a valve amplifier still forms the nucleus of their audio equipment. The combination of modern high-quality peripheral equipment (signal sources, loudspeakers) and a classical valve amplifier to come into their own as far as tonal quality is concerned. In modern hi-fi valve amplifiers much attention is paid to obtaining a signal transfer that is as nearly linear as possible. This obviates the need of complex tone control, which is the past were necessary to mask the deficiencies of signal sources then available. For instance, a CD player does not produce stylus noise which, when the audio was young, was combatted by chopping off the higher frequencies so as to obtain an acceptable sound quality (which modern listeners would not tolerate).

Whether it is nostalgia, interest in the technical parameters, the appeal of a gleaming amplifier chassis with softly glowing valves, respect for the technical know-how of earlier generation, or perhaps the firm conviction that the sound of a valve amplifier can not be bettered, it is a fact that this technical tradition, which deserves a place of honour in any science museum, is still in demand. It is particularly gratifying that many of the younger generation admire valve amplifiers. Perhaps this is due to the popularity of the electric guitar.

The field of the thermionic valve is extensive, shown by the enormous amount of literature it attracted in the 1960s.

This book is intended for a broad cross-secion of the public: apart from projects for preamplifiers, power amplifiers, and two amplifiers for musical instruments, aimed at the practical audio/hi-fi enthusiast, it offers much information on the operation od electron tubes, while the first chapter gives a short history of the thermionic valve.

Part 1. GENERAL
- Chapter 1 - History of the thermionic valve (1)
- Chapter 2 - Operation of the electron tube (39)
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Marantz 40/20-watt Power Amplifier - Marantz Audio Consolette

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Marantz 40/20-watt Power Amplifier - Marantz Audio Consolette
(Equipment Report)
AUDIO, AUGUST, 1956, VOL. 40, No. 8 (Successor to RADIO, Est. 1917).

If the average audiofan were to start building a preamplifier-control unit exactly fo suit his fondest dreams as to performance, absence of hum and noise, flexibility of control, and over-all appearance, it is quite likely that he would come quite close to dublicating the Marantz Audio Consolette - if he had the necessary ezperience, ability, and perseverance. And that is just about what Saul Marantz did, and over many months he worked out the design. The rezult was sufficiently "commercial" to warrant putting the unit on the marked. The performance curves in Fig. 1 show why.

Hi-Fi 20W amplifier with psophometric volume control

Details: Category: Radioamator i Krótkofalowiec; Hits: 5451

Hi-Fi 20W amplifier with psophometric volume control
Author: Stanisław Głowacki
Radioamator i Krótkofalowiec, Year 15, December 1965r., No 12

The human ear is not equally sensitive over the entire range of audible frequencies. It shows maximum sensitivity for frequencies ranging from about 1kHz to 3kHz, and this property occurs the stronger, the weaker the intensity of the sound perceived by the ear. This non-linear frequency response of the ear reduces the perceived sound experience when listening to music at low volume levels. At high sound levels the differences in ear sensitivity decrease and the reception is more accurate. It follows from the above that the adjustment of the volume of the reproduced broadcasts should be related to the adjustment of the frequency characteristics of the loudspeaker amplifier.

At a low volume level, i.e. with lower amplification of the amplifier, the low tones and high tones should be emphasized in relation to the medium tones, or the medium tones from 1 ÷ 3kHz should be muffled in relation to the low and high tones, and so much more, the smaller it is. power output from the amplifier. As a result, it is possible to achieve such changes in the characteristics of the amplifier that the ear will perceive the emission with the full balance of sounds, regardless of the output power.

Fig. 1 shows a schematic of a high-quality amplifier that meets the above conditions. The 20W output power allows you to amplify large rooms or drive a large sound column.

Fig. 1. Schematic diagram of the 20W power amplifier

The first stage of the amplifier with the ECC85 electron tube is a voltage amplifier with a cathode follower that controls the low-resistance quadruple of the negative feedback circuit. The frequency response of the quadruple has a flat maximum in the range of 1 to 5kHz, so that the negative feedback at these frequencies is the strongest. The reverse voltage Uzw is fed to the opposite end of the volume control potentiometer in relation to the voltage U_o controlling the amplifier. The feedback voltage is deposited on the resistance of the potentiometer and the internal resistance R_o of the signal source U_o, as shown in Fig. 2.

How to make a push-pull transformer for a low frequency amplifier

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How to make a push-pull transformer for a low frequency amplifier
Author: Ryszard Zarzecki
Radioamator i Krótkofalowiec, Rok 19, Sierpień 1969r., Nr 8
(Radio amateur, Year 19, August 1969, No. 8)

When designing push-pull amplifiers, it is often difficult to acquire or manufacture a suitable output transformer. The Soviet monthly "Radio" no. 2/1967 presents a simple method of making such a transformer. For this purpose, two identical, "normal" output transformers are needed (for example from a "Pionier" type receiver). Metal clamps and a package of simple plates closing the core should be removed from these transformers, and plates of the "E" type should be left together with the bodies with windings placed on them.

Transformer cores with windings should be put together as shown in Fig. 1.

Fig. 1.

In this way, the assembled transformers are connected and squeezed with a new metal clamp, and then - the beginning of the anode winding of one transformer is connected to the end of the anode winding of the other transformer (Fig. 2).

Amateur acoustic amplifier "Melodia"

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Amateur acoustic amplifier "Melodia"
Lech Krzymowski
Radioamator i Krótkofalowiec, Rok 20, Wrzesień 1970r., Nr 9
(Radio amateur, Year 20, September 1970, No. 9)

The amplifier, the diagram of which is shown in Figure 1, is not a revelation or a novelty, but due to the results obtained, it may be of interest to amateurs of good music and enthusiasts of small music bands.

The use of such units as a 3-channel mixer system and a key switch for changing the amplifier's frequency characteristics with simultaneous smooth adjustment in the range of extreme frequencies of the acoustic band gives good results, which was found using the ZK-120 tape recorder and a turntable. The results were incomparable with the capabilities of average systems, even factory production. Also the attempt to use the amplifier by the small music band was positive.

Fig. 1. Schematic diagram of the "Melodia" acoustic amplifier
(The drawing is large so you can copy it, for example to a graphics program and view the details)

The block diagram of the amplifier in Figure 2 explains the purpose of each stage of the circuit.

We are building a stereo amplifier

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We are building a stereo amplifier
Eng. Zbigniew Faust
Radioamator i Krótkofalowiec, Rok 22, Maj 1972r., Nr 5
(Radio amateur and amateur radio operator, Year 22, May 1972, No. 5)
(Prepared on the basis of "Funktechnik" no. 23, 24/1965 and no. 2/1966)

Here is a description of the construction of a stereo amplifier intended for cooperation with a stereo turntable. The amplifier has two channels: left and right. Each channel consists of an input stage, volume and balance controls, and an output stage. In the input stage, the weak signals from the turntable are pre-amplified, as well as the correction of the frequency characteristics of the reproduced recordings from gramophone records by appropriate raising or lowering of bass and treble. The volume control system allows you to continuously change the sound strength of recordings, while the balance control allows you to equalize the playback volume of both channels. The output stage is no different from a similar mono amplifier stage.

Taking into account the design, the amplifier has been divided into 3 parts:

tone correction system (preamplifier),
two-stage power output amplifier,
power supply system.

The individual elements of the system are mounted on separate bakelite plates, thanks to which experimenting with the system is very easy.

Basic technical parameters:

Output power: max 2×4W
Sensitivity: about 100mV
Frequency response: 5Hz÷20kHz (-3dB)
Nonlinear distortions: less than 5%
Power supply: 220V/50Hz
Power consumption: about 80W.

SOUND TONE CORRECTION

The circuit includes two levels of voltage amplification in each channel and a tone control system, separately for low and high tones.

The schematic diagram of the system is presented in Fig. 1. Both amplifier channels are the same and therefore it is enough to describe only one of them. The input from the stereo turntable passes through the standardized input jack [We] and the coupling capacitor C₁ to the sound level controller R₁, and then through the capacitor C₂ to the tube control grid L_1a. The grid leakage resistor is 1MΩ. There is a resistor R₆ in the cathode circuit of the electron tube to generate the grid bias voltage, blocked by the capacitor C₄. The signal amplified in the anode circuit is fed through the resistor R₄ and the capacitor C₆ to the tone control system. So that the treble is not weakened too much, the R₄ resistor is bypassed by the C₅ capacitor. In the first amplification stage, there is furthermore a feedback between the anode and the grid of the tube L_1a (resistor R₅). Due to this coupling, a more linear transmission characteristic and a lower coefficient of non-linear distortion are obtained.

Fig. 1. Schematic diagram of the tone correction system

The tone control system consists of two RC circuits. The first circuit includes elements R₇R₈R₉C₇C₈ for bass adjustment, and the second circuit (C₉R₁₁C₁₀) allows the treble response to be changed. The R₁₀ resistor decouples the bass control circuit from the treble control circuit.

An Amplifier Using New 6CZ5's

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An Amplifier Using New 6CZ5's
Nathan Grossman
AUDIO, JULY, 1958, VOL. 42, No. 7 (Successor to RADIO, Est. 1917)

Most experimenters have sufficient equipment in the "surplus" department to construct this simple amplifier which gives good performance in a small package

The 6CZ5, a new RCA miniature power tube, holds much promise for the art of audio power amplification. This tube is not to be confused with the 6AQ5, or English 6BQ5, both of with it resembles in construction and purpose. It is not interchangeable with them.

It has the same filament, plate and screen, and load characteristics as the 6V6, of which the 6AQ5 is the miniature type, and costs about the same. However, the other characteristics are different and provide a considerable improvement over tha latter types. The negative bias on the signal grid and the transconductance are about 15 per cent greater and the power output about 20 per cent greater for a plate voltage of 250. In push-pull operation the 6CZ5 resembles the 6L6 in that it generates a low percentage of odd harmonics and can be operated as a pentode with a plate voltage of 350. Under this latter circumstance and with 280 volts on the screen, a bias of -23.5 volts on the signal grid, and a plate-to-plate load of 7500 ohms, two 6CZ5's are rated by the manufacturer to deliver 21.5 watts of audio power and with only 1 per cent of harmonic distortion.

This adds up to lower supply requirements, lower distortion, lower all around cost, and higher power output. To try a pair of 6CZ5's the writer built an amplifier from parts in the junk box including a husky output transformer which was manufactured about 20 years ago. To avoid expense in obtaining good voltage regulation, the writer used a large bleeder and worked the amplifier half way between the recommended pentode and the lower-plate voltage tetrode operation. With 325 volts from plate to cathode, the results proved better than expected. The plate voltage did not vary from minimum to maximum power output, and total variation of the screen voltage was only 3.5 per cent.

The 88-50 - a Low-Distortion 50-Watt Amplifier

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The 88-50 - a Low-Distortion 50-Watt Amplifier
Audio, January, 1958, Vol. 42, No. 1 (Successor to RADIO, Est. 1917).
W. I. HEATH and G. R. WOODVILLE

With harmonic distortion of less than 0.5 per cent throughout most of the audio spectrum, this 50-watt amplifier is comparatively simple in construction and requires only ordinary care in wiring.

For audio amplifiers of medium power, the KT66 output tube became well known with the Williamson amplifier, and its reputation for reliability has made it much sought after in "off - the - shelf" high - fidelity amplifiers, as well as in home - built kits.

From the same stable there now follows a new tube, the KT88, a pentode with a higher plate - to - screen dissipation of 40 watts, and a higher mutual conductance of 11 mA per volt (11,000 microhms).

The KT88 makes it possible to use familiar circuit techniques to build audio amplifiers giving the higher power output needed to handle the "peaks" in high - fidelity reproduction at home, or for public address equipment. This higher output is obtainable without using a plate voltage higher than that available from standard components. The KT88 achieves this by virtue of its lower plate impedance. For example with cathode bias, 30 watts of output power is obtainable with a plate supply of only 375 volts, compared with 425 volts required by the KT66. The maximum power obtainable with cathode - bias from a pair of KT88's is slightly over 50 watts with a supply voltage of 500 volts. This article describes the design and construction of such an amplifier; a second article will give similar details of a matching preamplifier. They are shown together in Fig. 1.

Fig. 1. External view of amplifier and preamplifier described by the author. This installment covers only the 50-watt power amplifier.

The complete amplifier, the "88-50" has been designed to give a high performance and a complete range of input and control facilities without complicated networks or unusual components. It is therefore reasonably economical to construct. With its preamplifier it will reproduce from any programme source such as radio tuner, magnetic or crystal phonograph pick-up, microphone, or direct from a magnetic tape replay-head. A rotary switch selects the required input circuit and at the same time adjusts sensitivity and frequency correction to the required playback characteristic. The preamplifier is separate from the power amplifier and is connected to it by a flexible cable. Its controls include a loudness control, a presence control, and a treble-slope control, all these being continuously variable with a flat position around half-way. A wafer switch preselects the frequency at witch the treble-slope control operates. To avoid one of the biggest gremlins of high-fi apparatus a rumble filter using an attractively simple circuit is incorporated in the preamplifier.

How an Output Transformer Causes Distortion Part 2

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How an Output Transformer Causes Distortion
In Two Parts - Part 2
Audio, March, 1957, Vol. 41, No. 3 (Successor to RADIO, Est. 1917).
Norman H. Crowhurst

The operation of audio transformers has long been surrounded with an aura of mystery. This article distinguishes the different forms of distortion an output transformer can produce, and gives some simple measurement methods.

As this distortion due to reactive loading is quite similar to the varietes that a transformer causes at high frequencies we will consider both together. (A) in Fig. 8 shows the practical circuit of an output transformer while (B), Fig 8 shows the load seen be the output tubes.

Fig. 8. Practical and equivalent circuit of output transformer for high frequency response: (A) actual circuit: (B) equivalent plate load for output tubes.

Directly shunting from plate to plate is the primary capacitance of the transformer. The load resistance gets stepped up by the ratio N2 but, due to leakage flux that gets between the primary and secondary windongs, there is an effective inductance between ths load and the tubes, shown in the equivalent circuit of (B), Fig 8 as leakage inductance.

The winding capacitance has the same properties as any other capacitance in a circuit. A leakage inductance is precisely similar to any air-cored inductance: it cannot introduce distortion of itself.

However, if leakage inductance is the dominant reactance at the high-frequency end, then the load resistance, referred back to the primary will look like a resistance with an inductance in series. If the output tubes cause distortion with series reactance added to the load resistance, then this kind of transformer will appear to cause distortion.

In other amplifiers, distortion may appear more rapidly when a reactance is added in parallel with the load resistance. In this case a transformer, in which the winding capacitance is the dominant reactance at the high frequency end, will show distortion more rapidly.