Mullard-circuit Ultralinear HomeBrew Audio Amp In addition to boatanchor receivers and transmitters, I occasionally enjoy working on tube audio amps. Here is one that had been gathering dust and dirt for some years before I picked it up at an auction. The construction quality and clean underside appealed to me as well as its top-of-the-line Acrosound TO ultralinear output transformer. HomeBrew amp as purchased Output tubes and rectifier were missing. I did not know the identity of the output tubes.
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History[ edit ] Before the commercial introduction of transistors in the s, electronic amplifiers used vacuum tubes known in the United Kingdom as "valves". By the s, solid state transistorized amplification had become more common because of its smaller size, lighter weight, lower heat production, and improved reliability. Tube amplifiers have retained a loyal following amongst some audiophiles and musicians.
Some tube designs command very high prices, and tube amplifiers have been going through a revival since Chinese and Russian markets have opened to global trade—tube production never went out of vogue in these countries. In this case, generating deliberate and in the case of electric guitars often considerable audible distortion or overdrive is usually the goal.
The term can also be used to describe the sound created by specially-designed transistor amplifiers or digital modeling devices that try to closely emulate the characteristics of the tube sound. The tube sound is often subjectively described as having a "warmth" and "richness", but the source of this is by no means agreed on. Possible explanations mention non-linear clipping, or the higher levels of second-order harmonic distortion in single-ended designs, resulting from the tube interacting with the inductance of the output transformer.
Audible differences[ edit ] The sound of a tube amplifier is partly a function of the circuit topologies typically used with tubes versus the topologies typically used with transistors, as much as the gain devices themselves. Beyond circuit design, there are other differences, such as the differing electronic characteristics of triode , tetrode , and pentode vacuum tubes, along with their solid-state counterparts such as bipolar transistor , FET , MOSFET , IGBT , etc.
These can be further divided into differences among various models of the said device type e. EL34 vs.
In many cases circuit topologies need to account for these differences to either homogenize their widely varying characteristics or to establish a certain operating point required by the device. The low frequency roll-off can be explained by many tube amplifiers having high output impedance compared to transistor designs.
The roll-off is due to higher device impedance and reduced feedback margins. More feedback results in lower output impedance. Some tube amplifier designs use minimal feedback while others use quite a bit more of it. How much feedback is optimal for tube amplifiers remains a matter of debate. Low frequency roll-off in tube amplifiers is also largely due to AC coupling used almost exclusively, particularly at the speaker.
Output transformers require a change in voltage in the primary to produce any voltage at all in the secondary. Very slow changes in primary voltage will produce reduced voltage changes in the secondary. Also, the speaker itself cannot easily reproduce frequencies below its cone resonance when AC coupled to the amplifier. With DC coupling, as is found at the output of most transistor amplifiers, a voltage of any given amplitude or polarity can be continuously applied to the voice coil, and subsequently lock the voice coil in any respective position for any length of time.
A psychoacoustic analysis tells us that high-order harmonics are more offensive than low. For this reason, distortion measurements should weight audible high-order harmonics more than low. The importance of high-order harmonics suggests that distortion should be regarded in terms of the complete series or of the composite wave-form that this series represents. It has been shown that weighting the harmonics by the square of the order correlates well with subjective listening tests.
Weighting the distortion wave-form proportionally to the square of the frequency gives a measure of the reciprocal of the radius of curvature of the wave-form, and is therefore related to the sharpness of any corners on it. Especially in case of designing or reviewing instrument amplifiers this is a considerable issue because design goals of such differ widely from design goals of likes of HiFi amplifiers.
HiFi design largely concentrates on improving performance of objectively measurable variables. Instrument amplifier design largely concentrates on subjective issues, such as "pleasantness" of certain type of tone. Fine examples are cases of distortion or frequency response: HiFi design tries to minimize distortion and focuses on eliminating "offensive" harmonics. It also aims for ideally flat response. Musical instrument amplifier design deliberately introduces distortion and great non-linearities in frequency response.
Former "offensiveness" of certain types of harmonics becomes a highly subjective topic, along with preferences towards certain types of frequency responses whether flat or un-flat. Push—pull amplifiers use two nominally identical gain devices in tandem. One consequence of this is that all even-order harmonic products cancel, allowing only odd-order distortion.
Power amplifiers are of the push-pull type to avoid the inefficiency of Class A amplifiers. A single-ended amplifier will generally produce even as well as odd harmonics. Push—pull tube amplifiers can be run in class A rarely , AB, or B. Also, a class-B amplifier may have crossover distortion that will be typically high order and thus sonically very undesirable indeed. Class-A amplifiers measure best at low power. Class-AB and B amplifiers measure best just below max rated power.
Loudspeakers present a reactive load to an amplifier capacitance , inductance and resistance. This impedance may vary in value with signal frequency and amplitude. The influence of the speaker impedance is different between tube amplifiers and transistor amplifiers.
The reason is that tube amplifiers normally use output transformers, and cannot use much negative feedback due to phase problems in transformer circuits. Notable exceptions are various "OTL" output-transformerless tube amplifiers, pioneered by Julius Futterman in the s, or somewhat rarer tube amplifiers that replace the impedance matching transformer with additional often, though not necessarily, transistorized circuitry in order to eliminate parasitics and musically unrelated magnetic distortions.
An amplifier with little or no negative feedback will always perform poorly when faced with a speaker where little attention was paid to the impedance curve. Design comparison[ edit ] There has been considerable debate over the characteristics of tubes versus bipolar junction transistors.
Later forms of the tube, the tetrode and pentode , have quite different characteristics that are in some ways similar to the bipolar transistor. But there are exceptions, for example designs such as the Zen series by Nelson Pass. Output impedance[ edit ] Loudspeakers usually load audio amplifiers. In audio history, nearly all loudspeakers have been electrodynamic loudspeakers. There exists also a minority of electrostatic loudspeakers and some other more exotic loudspeakers.
Electrodynamic loudspeakers transform electric current to force and force to acceleration of the diaphragm which causes sound pressure.
Due to the principle of an electrodynamic speaker, most loudspeaker drivers ought to be driven by an electric current signal. The current signal drives the electrodynamic speaker more accurately, causing less distortion than a voltage signal. Practically all commercial audio amplifiers are voltage amplifiers. Due to the nature of vacuum tubes and audio transformers, the output impedance of an average tube amplifier is usually considerably higher than the modern audio amplifiers produced completely without vacuum tubes or audio transformers.
Most tube amplifiers with their higher output impedance are less ideal voltage amplifiers than the solid state voltage amplifiers with their smaller output impedance. Soft clipping[ edit ] Soft clipping is a very important aspect of tube sound especially for guitar amplifiers. A hi-fi amplifier should not normally ever be driven into clipping.
The harmonics added to the signal are of lower energy with soft clipping than hard clipping. However, soft clipping is not exclusive to tubes. It can be simulated in transistor circuits below the point that real hard clipping would occur.
See "Intentional distortion" section. Large amounts of global negative feedback are not available in tube circuits, due to phase shift in the output transformer, and lack of sufficient gain without large numbers of tubes. With lower feedback, distortion is higher and predominantly of low order.
The onset of clipping is also gradual. Large amounts of feedback, allowed by transformerless circuits with many active devices, leads to numerically lower distortion but with more high harmonics, and harder transition to clipping.
As input increases, the feedback uses the extra gain to ensure that the output follows it accurately until the amplifier has no more gain to give and the output saturates.
However, phase shift is largely an issue only with global feedback loops. Design architectures with local feedback can be used to compensate the lack of global negative feedback magnitude.
Design "selectivism" is again a trend to observe: designers of sound producing devices may find the lack of feedback and resulting higher distortion beneficial, designers of sound reproducing devices with low distortion have often employed local feedback loops. Soft clipping is also not a product of lack of feedback alone: Tubes have different characteristic curves. Factors such as bias affect the load line and clipping characteristics.
Fixed and cathode-biased amplifiers behave and clip differently under overdrive. The type of phase inverter circuitry can also affect greatly on softness or lack of it of clipping: long-tailed pair circuit, for example, has softer transition to clipping than a cathodyne.
The coupling of the phase inverter and power tubes is also important, since certain types of coupling arrangements e. In the recording industry and especially with microphone amplifiers it has been shown that amplifiers are often overloaded by signal transients. Russell O. Monteith Jr and Richard R. Flowers in their article "Transistors Sound Better Than Tubes", which presented transistor mic preamplifier design that actually reacted to transient overloading similarly as the limited selection of tube preamplifiers tested by Hamm.
In practice the clipping characteristics are largely dictated by the entire circuitry and as so they can range from very soft to very hard, depending on circuitry. Same applies to both vacuum tube and solid-state -based circuitry. For example, solid-state circuitry such as operational transconductance amplifiers operated open loop, or MOSFET cascades of CMOS inverters, are frequently used in commercial applications to generate softer clipping than what is provided by generic triode gain stages.
In fact, the generic triode gain stages can be observed to clip rather "hard" if their output is scrutinized with an oscilloscope. Bandwidth[ edit ] Early tube amplifiers often had limited response bandwidth , in part due to the characteristics of the inexpensive passive components then available. In power amplifiers most limitations come from the output transformer; low frequencies are limited by primary inductance and high frequencies by leakage inductance and capacitance.
Another limitation is in the combination of high output impedance, decoupling capacitor and grid resistor, which acts as a high-pass filter.
If interconnections are made from long cables for example guitar to amp input , a high source impedance with high cable capacitance will act as a low-pass filter. Negative feedback[ edit ] Typical non-OTL tube power amplifiers could not use as much negative feedback NFB as transistor amplifiers due to the large phase shifts caused by the output transformers and their lower stage gains.
While the absence of NFB greatly increases harmonic distortion, it avoids instability, as well as slew rate and bandwidth limitations imposed by dominant-pole compensation in transistor amplifiers. However, the effects of using low feedback principally apply only to circuits where significant phase shifts are an issue e. In preamplifier stages, high amounts of negative feedback can easily be employed. Such designs are commonly found from many tube-based applications aiming to higher fidelity.
On the other hand, the dominant pole compensation in transistor amplifiers is precisely controlled: exactly as much of it can be applied as needed to strike a good compromise for the given application. The effect of dominant pole compensation is that gain is reduced at higher frequencies.
Mullard Tube Circuits For Audio Amplifiers
In subsequent years, a succession of designs for high-quality circuits has issued from the Mullard Applications Research Laboratory, and these designs include circuits for power amplifiers and pre-amplifiers, circuits suitable for tape- recording equipment and, most recently of all, circuits for stereophonic reproduction. Many of these circuits have been described in articles in various magazines or have appeared in the form of booklets or leaflets. The others have only recently been released by the laboratory, and have not yet been published. The object of this book is to present the most up-to-date versions of the published circuits together with the new stereophonic circuits in a way which will be useful and convenient to equipment manu- facturers, service engineers and home constructors. A list is given on page vi of the earlier Mullard publications which have been incorporated in this book. These publications may be useful for purposes of comparison, but it should be appreciated that the information given here is the latest available and consequently, supersedes that issued elsewhere.