Valve amplifier

A tube amplifier is a device used in the electron tubes for amplifying the electrical signal. Also in the audio field, the electron tubes but have now been largely replaced by semiconductor devices (transistors ). Electron tubes are still used frequently in guitar amps and some high-fidelity amplifiers of the high-end class.

Circuit principles of audio tube amplifiers

Two basic examples of the low-frequency amplifier circuit technology with tubes provide an insight into how they work.

Single ended amplifier operating mode A

The circuit diagram shows a single-ended amplifier with a pentode in cathode - base circuit, both the positive and the negative half wave of the input signal will be processed by said one tube. The only way an approximately linear amplification of the signal portions is the choice of the operating point A on the Ug / Ia characteristic ( input characteristic ) of the intensifier tube which is located in the middle of the linear characteristic portion, resulting in a poor high quiescent current through the tube, and a few advantageous efficiency of the amplifier has the result - the classification of the mode of operation of the amplifier is related to the position of this operation point.

The coupling capacitor C1 separates from DC components of the signal to be amplified, thereby preventing a work shift. The very high-value resistor R1 is used to hold the control grid equal in voltage to ground potential. The cathode resistor R2 is responsible for Gittervorspannungserzeugung, its value determines the operating point of the tube. The cathode is positive because of the current flowing through R2, the cathode current and the associated voltage drop over the grid, the resulting negative bias automatically regulates itself depending on the cathode current (static negative feedback for bias stabilization). The resistor R2 should, for example, have a value of 135 ohms for A amplifier with EL84 pentode, which generates a bias voltage of -7.2 volts. An incorrectly sized cathode resistor R2 has an asymmetric operation of the device, in which a half wave of the output signal comes much earlier in the boundary than the other. Thus, the usable linear dynamic range is reduced, it will be distortions generated.

The capacitor C2 is used for AC- moderate bridging of resistor R2. The cutoff angular frequency 1 / (R2 * C2 ) of the negative feedback sets the limit below which the gain is reduced. If you let off C2, the amplifier is also coupled to alternating voltage, which reduces both the gain and the distortion.

The output transformer isolates the speaker from the high anode voltage and transforms the high output impedance of the output tube (with an EL84 output pentode in single-ended class-A operation 5.2 kOhm ) to the low impedance value of a dynamic speaker.

In contrast to the above diagram, the screen grid of the final tube to limit the screen grid current is usually connected via a resistor to the anode voltage. It serves to increase the control range and the efficiency by maintaining the electric field by the anode current even at low anode voltages.

Advantages of the tube Eintaktprinzips Class A:

• The simplest circuit design with few components in the signal path.
• No phase splitting as in the push-pull output stage necessary.
• No current crossover distortion at low volume.

Cons:

• Lack of linearity when (as in the above circuit ) of no feedback is used and the output transformer is biased on one side by the constantly flowing anode current. The desire for reduced distortion led to the invention of the negative feedback.
• Low efficiency and high power dissipation.
• High demands on the ripple of the anode voltage especially in headphone amplifiers (hum sensitivity).
• For hi-fi applications a time-consuming and expensive output transformer is required.

Push-pull amplifier AB mode

The example on the right shows the circuit diagram of a high-performance and typical tube audio amplifier, the two Endpentoden EL34 operate on the push-pull principle - in contrast to the highly efficiency weak single-ended amplifier sharing in the output stage two tubes, the reinforcement work by a tube for the positive half-wave, while the other tube is responsible for the negative half cycle, which results in improved power output, if the output tube directs a, blocks the other and vice versa, the push-pull amplifier can therefore assume different operating points as a class. The far-reaching principle of operation was developed in 1912 by the Canadian electrical engineer Edwin H. Colpitts. In addition to the possibilities of a Class A operation (higher quiescent current) and the class B operation (crossover distortion at the zero crossing of the signal) has the audio push-pull amplifier primarily the mode with the favorable position AB of the operating point on the above Ug / Ia tubes enforced characteristic: a low quiescent current makes the tubes at small signal amplitudes in class a operation work, with increasing modulation of the amplifier goes gradually into the class B operation on the tubes operate fully at full load in mode B, leading to much higher output power and improved efficiency leads.

Are controlled by the output tubes with a tube circuit design of British specialists DTN Williamson, which has become known as split load phase inverter with driver stage: there is no complementary tube types in analogy to semiconductor devices, this part of the circuit for the phase reversal of the input signal has to make - the two control grids of the output tubes need two reinforced equal amplitude, mirrored about 180 ° phase-rotated signals, which are symmetrical to the ground.

The first of the triode ECC83 accomplished the required voltage gain of the input signal, the second of the triode is the actual ECC83 concertina (or Kathodyn ) phase splitter. At the cathode and at the anode of the triode, the concertina two antiphase signals are coupled out and passed to the two Treibertrioden ECC85 of which drive the two of EL34 output tubes in push-pull.

For the combination of the signals and the power adaptation to low impedance loudspeakers in turn provides an output transformer, which must have a center because of the push-pull method on the primary side. An adjustable - negative feedback loop from the secondary side of the transformer to the cathode of the first triode linearized frequency response and reduced distortion.

Advantages of the tube push-pull class AB concept:

• Higher output power with good efficiency.
• Relatively simple and inexpensive output transformers.
• This is not biased one-sided and therefore produces less distortion.
• Cheaper tolerance to residual ripples of the anode voltage.

Cons:

• The more complex circuit design, in which a signal is split into positive and negative half-waves, which are then separately amplified and summed to the overall signal when the power supply again.
• The AB- working leads to crossover distortion.

Special form Coreless amplifier

A modern permanent- dynamic speaker can easily be adapted to its low impedance to the low impedance output of a transistor amplifier. The high-impedance output stage of an audio amplifier tube, however, almost always requires a power adjustment with a low-frequency transformer ( transformer).

An exception to this were concepts of " coreless " OTL audio tube amplifiers ( OTL stands for OutputTransformerLess ) in which the output transformer was saved for reasons of cost, but were not successful for various reasons later: In the well-known folk receiver, the low-frequency output tube worked directly on a cantilever speakers with its high impedance of 2000 ohms, continue the company Philips has used later a circuit with different tube types and the necessary 600-800 ohm speakers in the 1950s and 1960s in a number of tube radios and televisions. The output signal is taken via a coupling capacitor and directly fed to the speakers. Philips was the concept after a few years on: The high-impedance voice coils often suffered damage ( break), and it was no external second speaker to be connected.

Since the defects of the transmitter to signal degradation, developed mid-1950s, the American Julius Futterman the first hi-fi tube amp without output transformer for use with regular low-impedance speakers, whose conception later in the OTL amplifier series the company nyal (New York audio Labs) was continued.

More modern concepts where in the output stage is usually a large number of relatively low- flow control tubes are connected in parallel, continue to occupy a niche in the market. These amplifiers are very inefficient in terms of their available power, the tubular consumption ( life of the power tubes used), and the power loss.

The reason - the too large impedance difference - be explained on the basis of a 50 W amplifier: A matching 4 Ω speaker takes the current

On. Corresponding to a maximum current of 5 A, the power transistor can handle without problems. The voltage at the loudspeaker

The voltage at the speaker must therefore vary between the maximum values ​​of -20 V and 20 V. This is ideal for the direct connection of a transistor amplifier with complementary transistors (combination of NPN and PNP ). These numerical values ​​indicate the basic weakness of electron tubes for " iron-free " output stages:

• The maximum cathode current of the power tubes is 1 A, with the exception of large-diameter pipes for radio stations. The required total current 5 A can only be achieved by parallel connection of a sufficient number of copies.
• To this cathode current to "pull " the tube with at least 150 V anode voltage must be supplied. More would be better. Of these, only 20 V are used for the speaker, the rest contributes to the considerable loss of performance of the anodes and the poor efficiency at:

Avoid unwanted oscillations

Electron tubes operate in a very wide frequency range from DC to approximately 2000 MHz and can therefore unsuitable for construction to unwanted swings in the high frequency range tend. It is characterized by a tendency to oscillate on the type of Huth - Kühn- circuit at very high frequencies. Whether unwanted oscillations occur, is particularly dependent on the lengths of wire at the control grid and the anode (see line theory ): the shorter, the higher the resonant frequency. Since the tube with increasing frequency from bad to worse reinforced, the minimum gain is sometime below and it can be no more vibrations.

The operating point or the modulation influence the slope: it is frequently observed at -15 V ( AB operation without signal ) no oscillation, because the gain is too low. At -1 V characteristic curve steeper and the circuit oscillates. This means that from a certain volume produce the rhythm of NF such audible distortion that do not exist at low modulation.

The proof is difficult without a spectrometer or an oscilloscope, because the frequency of the wild oscillations is not even known about normally. As an indicator, a series circuit of a glow lamp be placed with a 50 pF capacitor between anode and ground. The capacitance of the capacitor is too large for NF and the glow lamp is not lit. But at frequencies above a few megahertz it flickers.

As a countermeasure, damping resistors connected in series have proven themselves in the leads:

• Directly at the control grid of a 1 k resistor, which damps the resonant circuit quality of the wire very effective. At high frequencies ( above 100 kHz), but this resistance is increased by the Miller effect input capacitance of an RC low-pass undesirable. This low-pass effect can be reduced by a parallel-connected inductor.
• In pentode directly on the screen grid, a 100 Ω resistor

Comparison of tube - Semiconductors

Even after the invention of the transistor, the electron tube was for many years as an active control in all fields of electronics without alternative. The very low transit frequencies, the noise and temperature problems of the early germanium transistor types limited their use. With the use of the semiconductor material silicon, a consistent further development of the silicon transistor and its numerous advantages, the semiconductor devices bipolar transistor, field effect transistor and MOSFET increasingly replaced the tube in virtually all electronic applications. Compared with the electron tube, the transistor has to offer distinct advantages, including particularly in the following areas: small size, light weight and low price levels, mechanical ruggedness, simple and undemanding power supply coupled with high efficiency, extremely long life, hardly characteristic changes over the entire cycle of use, extremely good electrical values ​​of current transistor types by permanent and ongoing research.

Among the most serious disadvantages in the use of electron tubes include the need for a complex high-voltage supply, since there are hardly any tubes at low supply voltage on the order of 50 V a significant anode current (and thus an appreciable output power) can supply and also still function distortion in this area. The considerable demand for electricity for heating the cathode and the comparatively large power loss will also affect the circuit environment. Especially in power tubes solve various chemical and physical processes, especially the cathode accelerated aging process, which is why the tube has to be replaced after a certain period of use (see electron tube ).

When tube amplifier not the tubes, but the output transformer determine the upper and lower cut-off frequency of the amplifier. Even with special wrapping techniques you can hardly exceed the range of 30 Hz to 15000 Hz. Transistor amplifier easily create a range of 0 Hz to 50000 Hz, because they do not require a transformer.

The output resistance should be as low as possible in order to dampen unwanted resonances of the speaker. Due to the much stronger negative feedback circuit topology and the other he is with transistor amplifiers basically significantly smaller than in amplifiers with output transformer. Because of this phase shift, at least on the borders of the negative feedback bandwidth must be much smaller in the case of tube amplifiers. In addition, the circuit topology plays an important role: tube amps are usually implemented in grid - based circuit, which naturally has a high output resistance. Transistor amplifiers are always realized as a collector circuit which, without external feedback itself has totally a low output resistance.

There is one important difference, even if the connection between the amplifier and speaker is disturbed:

• If this connection is accidentally disconnected a tube amp at high LF power, the resulting high induction voltage will destroy the output transformer and / or damage the power tubes.
• A short circuit in the speaker lead destroys the transistor amplifier by the resulting overcurrent possibly the output transistors. However, in the semiconductor can be implemented and installed relatively quickly reagiernde simply a protection circuit.

Effective protection of tube amplifiers against inductive surges, however, is very difficult and even impossible in pure tube technology, which is why, in practice, little use is made ​​of it ( a snubber network is by far not sufficient protection ). An over-current protection of transistor amplifiers, however, is very simple and cost-effective manner, which is why it is present in almost every transistor amplifier.

Compared to semiconductor devices whose active regions are located in a confined space inside a solid, electron tubes, because of their mechanical construction resistant to short-term electrical overload, radioactivity and electromagnetic pulse (EMP ). Both have nothing to do with a hi-fi amplifier.

Hi-fi amplifier

Using sophisticated tube amps such as the English Mullard 5-10 began in the 1950s, the hi-fi era, aiming at the most accurate possible electroacoustic reproduction of sound events. Some years later, pointed Although increasingly the unstoppable rise of the semiconductor technology in the electronics industry, which up to a few niche applications from the market replaced the tube electronics - but it took some time until the first transistor-based hi-fi amplifier to the achieved high acoustic quality of the tube amplifier were able to continue.

It was not until the mid-nineties were quality audio tube amplifier in high-end circles again socially acceptable. Compared to the technical data cover its semiconductor-based competitors hopelessly in many areas, tube amplifiers often achieve better results in terms of subjective assessment of their tonal qualities. Some critics however, this is called the present high state of semiconductor technology as a euphemism, the listener or as a " pleasant distortion " of the sound, for the pleasant sound of the producers of music recordings, not the manufacturer of the players were responsible. The latter should be as neutral as possible.

On the other hand, are often only mixer with their qualified and appropriately trained hearing in a position to differentiate the subtle differences between very good and excellent amplifiers and reproducible exercise - they attest to high quality tube amplifiers often an outstanding sound quality. In a complex single project of the Technical University of Berlin was trying to give these results provide an objective basis. Metrological tests on specially built for the project Black Cat - tube amplifier showed an extremely low value of the Differenztonfaktors what a low tendency of the amplifier to nonlinear distortions group, their presence makes even the smallest signal components for an unpleasant listening experience.

Due to their own typical characteristics produce different tubes Klirrspektren (spectrum of harmonics ) as semiconductors: triode, pentode and field effect transistors have a square in the first approximation curve, while bipolar transistors exhibit an exponential to a first approximation characteristic. The often -heard claim, however, that triode and field effect transistors produce as undesirable distortion products predominantly even-numbered multiples of the fundamental frequency, bipolar transistors and power pentode primarily odd multiples of the fundamental frequency, is wrong, as demonstrated by measurements on basic circuits.

For all Basic amplifier circuits independent of the active component primarily arise harmonics at twice the fundamental frequency and a few higher-order harmonics. Even multiples of the fundamental frequency sound more "warm" in the perception for many listeners and " brightening ", whereas the odd-numbered partials a rather sharp sound is said.

Instrumental amplifier

In the classical stage amplifier for the typical in rock music instruments electric guitar and electric bass tube technology has for various reasons claims to this day: the particular behavior (soft clipping ) of the tube amplifier, be gradually increasing slip- in signal distortion in the Selective use of an override is to be regarded as an integral part of the instrument that gives its individual sound character to the e- instrumental sound and just does not serve a possible exact, undistorted amplification of the sounds produced by the instrument.

The American Aspen Pittman has published a fairly complete collection of schematics historic guitar tube amplifier in his book, The Tube Amp Book ( en. ). Here also many of the mentioned circuit details can be found in different variants.

See also: Guitar amplifier

Feedback

The electronic negative feedback ( NFB negative feedback ) is an important for the circuit design of hi-fi amplifiers development of American electronics engineer Harold Stephen Black from 1927: a part of the output signal is added in reverse phase with the input signal, which when properly dimensioned, the responsible for switching elements harmonic distortion and frequency linearity of the amplifier can be positively influenced - depending on the extent of the negative feedback, however, reduced by the anti-phase voltages, the voltage gain. In the design of tube- stage amplifiers for electric guitars, the designers do without, inter alia, also because often the corrective, but reinforcing -reducing effect of an over- all negative feedback from the secondary side of the output transformer over the whole amplifier path - is the inimitable sound of the special Klirreinsatzes of the amplifier electronics here downright desirable, the famous guitar amplifiers Vox AC30 is a famous example.

Through the cathode resistor in the amplifier stages created its own DC to AC negative feedback, which ensures a stable operating point, but also for a gain reduction - the first ECC83 triode of the above push-pull amplifier is a good example. Parallel alignment of the cathode resistor with a capacitor can be the negative feedback and thus the gain reduction reduce because its capacitive resistance, the AC voltage is derived to earth - an example of this variant of a DC negative feedback is the cathode R in the above single-ended amplifier.

The circuit design of high quality hi-fi amplifier provides for a gradual cross- voltage feedback from the secondary winding of the output transformer to the input tube, in many cases - as the transformer causes a frequency dependent phase shift of the signal occurs in case of excessive dimensioning of the feedback light, the risk of an undesired feedback.

With advent of Leistungspentoden and the increasing mass production of output transformers was experimenting with counter- couplings on the screen grid. Development objective was to set a triodenähnlichen operating characteristic curve while maintaining the pentodentypischen advantages such as high gain and reasonable efficiency.

This was achieved with the development of Ultra Linear or Distributed Load concept of the two Americans David Hafler and Herbert I. Keroes 1951, which dates back to a patent Alan Dower Blumlein the English engineer from 1938. With this configuration, the screen grids of the Endpentoden is supplied with a part of the anode alternating voltage via a respective tap of the primary coil of the output transformer - this are the optimum Ultralinearanzapfungen for the screen grid of a push-pull output stage at about 40 % of the number of turns of the primary winding with respect to the center tap ( V in the above diagram a push-pull amplifier ) of the transformer. Is the position of the tap in the direction of the anode terminal output valve, predominates Triodenbetriebsart - a displacement of the Schirmgitteranzapfung in the direction of the center tap of the primary coil causes a transition to the Pentodeneinstellung.

An alternative between traditional current or voltage feedback and the ultra- linear circuit described above is placed by the British company Quad Electroacoustics on the market cathode negative feedback, in which a secondary winding of the output transformer flows through the cathode current of the output tube and the cathode with the induced alternating voltage is applied, that it counteracts the control voltage.

All feedback types can be equally applied to single-ended as for push-pull output.

The extent of the negative feedback is used indirectly proportional to the internal resistance of the amplifier: the usually strong against coupled transistor amplifiers are characterized by a low internal resistance and thus a high damping factor.

In contrast, tube amplifiers behave exactly the reverse with a rather minor or nonexistent feedback - this results in the recommendation, as transducers hochbedämpfte yet to use efficient speakers.

Between transmitter

Interstage transformer in LF amplifier are transformers with very high numbers of turns and inductors. They are often used in older receivers tube before about 1933 to reduce the number of required tubes. The transducers were usually designed for a voltage transformation of 1:3, wherein the level tube had to face the primary side a low level of power available.

Push-pull intermediate transfer member can be replaced with a corresponding embodiment for the required push-pull output stage active phase inversion: the high-ohmic secondary winding has a center located at ground, at the two ends of the winding can be decreased in each case by 180 ° phase-shifted signal. This type of push-pull control was also found in many devices from about 1933.

Transformers are sometimes used today for electrical isolation and impedance matching and prevent -z. Example, when separately mounted Amplifiers - noise ground loop. However, they are not referred to as an intermediate transformer.

Output transformer

Electron tubes are high-impedance principle components, ie their output impedance is much higher ( in the low frequency range a few kilohms ) than that of speakers ( are usual 4 to 16 ohms). Therefore, the operation requires of usually low-impedance speakers to tube audio amplifiers, a special low- frequency transformer, the output transformer. The high bandwidth of hi-fi amplifiers and / or high output power can only be achieved by nested coils, which makes them accordingly expensive.

At AF output transformers following high demands are made:

• Impedance transformation ( turns ratio 1:20 to 1:50 )
• High relative bandwidth: the ratio of the upper to the lower cut-off frequency is about 200:1 ( low leakage inductance by nested windings, high number of primary, high permeability of the core)
• Linearity of the core ( high saturation induction, for amplifiers in class A mode air gap)
• Low copper losses: low ohmic resistances of the windings
• Low iron losses: little hysterisis and eddy current losses of the core material
• Good insulation (to protect against the anode voltage, in case of overload increases, the anode voltage periodically at a multiple of the operating voltage )

The impedance transformation is mathematically determined by the square of the turns ratio. Because the impedance of a speaker is not constant but depends on the frequency, the accuracy of the adjustment, in practical operation, however, are limited. The primary inductance must be so high that it causes only a small drop in level at the lower limit frequency. There are several Henry necessary.

The relative bandwidth is the leakage flux of the transformer is inversely proportional. A lower magnetic leakage flux is good coupling between the primary and secondary windings. The flux leakage can be minimized by interleaving the cut in part windings primary and secondary windings, the choice of a high-permeability core material affects that and also to the size of the primary inductance positive.

Core materials only at high magnetic flux densities ( above 1.5 Tesla) toward the onset of saturation have a good linearity. However, high-permeability magnetic materials often have a lower saturation field strength, therefore, an air gap is mandatory for any single -A amplifiers required.

A minimization of copper losses are about choosing a suitable core type and material, as so determined are the available cross -section for the winding and the specific winding inductance. Effective use of the window surface for insulation and shielding coils allows the use of large conductor cross-sections and thus low ohmic resistance.

The iron losses depend crucially on the choice of core type and material. To decrease very small sheet thicknesses although the relative iron content of the core volume, however, reduce the eddy currents, especially at high frequencies.

The arrangement of interleaved windings increases the capacitance between the primary and secondary winding. To avoid undesirable capacitive couplings, isolated, switched to ground metal foils between the winding parts are often inserted. These films form a capacitive screen.

Although they require a center tap, push-pull output transformers are simple in design and manufacture, since the quiescent currents of the two output tubes in opposite directions flow through the two primary windings and cancels out the DC magnetic flux generated by them.

More problematic are the output transformers for single- A output stages, as in this case, the quiescent current of the output tube, the primary winding of the output transformer flows through and those magnetized. In order to reduce the magnetic flux density of the transformer core and to avoid saturation of the core material, the core thus requires an air gap. The output transformer must be carefully assessed on the magnetic characteristic with respect to the operating point. The full scale of complex loads must be considered.

Another way to circumvent the constant field in the output transformer of a single-ended output stage, is the throttle linkage. Instead of the primary winding in the anode circuit a high inductance inductor is switched to supply the anode quiescent current in the anode circuit. This is provided with an air gap next to the throttle drops originating from the primary copper loss unavoidable DC voltage from the anode of the AC voltage. It is fed through a coupling capacitor to the output transformer. While another relatively large and heavy component is needed, but the output transformer of DC fields is kept free.