Valve or Vacuum Tube Amplifiers Demystified - a layman's guide. For those new to the world of valve audio or hi-fi amplifiers it can be tricky to decipher some of the terminology you may read about in magazines, on the web or in manufacturer's brochures. This article will give you a whistle-stop high level overview of some of the terms you are likely to come across when researching a potential tube amplifier purchase. So, if you've ever wondered what Class A operation is, about single ended or push-pull output stages and even what on earth ultra-linear means in an amplifier design, read on. What is a Valve or Tube Amplifier? A valve amplifier is a device for amplifying the power of electrical signals. These signals are most often at audio (sound - as in a guitar or Hi-Fi amplifier) or radio frequencies (in radio and radar transmission). It uses the ability of the thermionic valve to control the flow of current through the device in response to a signal applied, amplifying this signal in the process. The first valve to possess the ability to electronically amplify a signal was the triode valve invented by Lee De Forest in 1907. A triode valve has three electrodes - a cathode, an anode (frequently referred to as the plate) and the control grid located between the two. The cathode is heated electrically (directly or via a separate adjacent heater element) until thermionic emission begins when electrons are emitted. The Anode is positively charged with respect to the cathode and attracts these electrons, leading to an electron current flowing between the cathode and anode. A 'conventional' current is said to flow the other way. The control grid is biased negatively with respect to the cathode (i.e. it is 'more' negatively charged when compared to the cathode). This creates an electrostatic repulsion effect of the electrons emitted from the cathode and shields the plate from them, reducing the current flow. The effect of this negative bias is thus to control the flow of current through the device - the more negatively biased the control grid is, the less current flows. In broad terms, apply a signal to the grid and the current flow through the device will be regulated in proportion to the signal - bingo! - an device whose output is regulated in proportion to a signal applied, otherwise known as an amplifier.Lets now have a look at some of the terms you may come across when reading about tube amplifiers. Output Transformer Although a very good amplifying device, the valve or tube has one problem - it has a high voltage / high impedance / low current output unsuitable for directly driving many real world loads. In the specific case of audio amplifiers, the output is unsuitable for directly driving the typical low impedance load presented by a loudspeaker, which is obviously a bit of a problem in an audio amplification device. Two main techniques are used to overcome this - negative feedback to reduce the output impedance and matching transformers to match the output of the valve power stages to the load supplied by the loudspeaker. The transformers are very specialised, heavy and expensive - significantly adding to the weight and cost of a valve amp. The quality of these transformers is also very important in the sound quality produced by the amp. OTL Amplifier OTL - or output transformerless amplifier designs do away with the bulky output transformers in favour of running many tubes in parallel. In this design, as many as 12 or more tubes are connected in parallel (electrically connected side-by-side, rather than one after another) to reduce the output plate (anode) impedance to more closely match that of a typical loudspeaker load(4 to 16 ohms). The design was pioneered by Julius Futterman and such designs are often referred to as Futterman OTL amplifiers. OTL designs are renowned for running hot, having lots of tubes and a rather unique sound quality. What is Amplifier Output Class and what is meant by 'Push-pull' operation or 'Single Ended' operation? You will often see reference to an amplifier being a class A design - indeed one of several classes (Class A, B, C, AB, AB1, AB2). What does this all mean? The amplifier class refers to its mode of operation. As nearly all active electronic devices (e.g. valves or tubes) pass current in only 1 direction (a direct current) but audio signals are in alternating current form, there is a need to resolve this difficulty to allow an audio signal to be amplified - to make 1 way devices amplify 2 way signals. Two basic circuit topologies are used to achieve this, push-pull and single ended. Push-Pull This makes use of one or more pairs of amplifying devices (valves or tubes in our case - but it applies equally to solid state designs), one of which deals with the signal when it swings positive, the other with the negative swings of the signal. Single Ended In this method, a direct current of suitable size is superimposed on the AC signal to ensure that the overall signal remains positive even at the negative signal peaks - hence there is never any actual reversal of the current flow, so a single output device (valve) may be used rather than the pair of the push-pull topology. The superimposed direct current can then be filtered out of the output using a blocking capacitor or transformer. Amplifer Class Where does this leave us with regard to class of operation? If you consider the audio signal as a regular sine wave with positive and negative peaks, due to the nature of single ended operation (converting this to larger and smaller positive only peaks) it can be seen that the output device is always 'on' and passing a signal. This is known as class A operation, where the output device passes (conducts) during 100% of the signal cycle. Looking at the push-pull circuit, it can be seen that one of the pair of devices is 'on' for positive swings of the signal, the other for negative swings - with our sine wave again they each conduct for precisely 50% of the signal cycle. This is Class B operation.Class C operation refers to circuits where the output devices conduct for less than 50% of the signal cycle - but these designs are generally not used in audio amplification and won't be considered further. Of course things are never quite that simple. When looking at the class B push-pull amplifier and considering our sine wave again, it can be seen that there is a point when the positive signal crosses over to the negative part of signal - here the positive output device will be switching off as the negative is starting to switch on. At the precise crossover point, there should be no current flowing in either half of the circuit. Now amplifying devices tend to be 'nonlinear' in their operation at these low current cut off points - this introduces distortion at the cross over point referred to as crossover distortion (surprise!). The solution to this is to introduce a bias current that flows when the amplifier is idling so the output devices don't actually reach their low current cut off points and introduce crossover distortion. Perhaps you can spot where this is going? As the bias current increases it increases the conduction time for each half of the circuit to more than 50%. When the bias current reaches a value of half of the maximum output current, the devices in both halves of the circuit are conducting 100% of the time. So, in this case the amplifier is operating fully in Class A on both halves of the signal cycle. The downside to this is that the bias current flowing when there is no signal leads to the output devices dissipating this energy in the form of heat. This is why a Class A amp will run hot, even if just idling with no input signal - Class A operation in this case is not as efficient as class B. You may be able to guess now what a class AB amplifier is - this is one that is biased partly between Class A and Class B in its operation using the method described above. Class AB1 and AB2 are only relevant in valve (tube) amplifiers and relate to the output impedance of the driver stage of the amp. In class AB1, this has a high output impedance, leading to clipping on the onset of a current flowing in output tube grid. In class AB2, it has a low output impedance allowing greater output power for a given bias current. In the real world amplifier, all this tends to mean that Class A amplifiers have lower output due to the efficiency penalty, but many audiophiles feel that they produce a sweeter and smoother sound and are better at driving typical loudspeaker loads than class AB designs. Ultralinear Operation You may see the term ultralinear bandied about when discussing valve amplifiers - what does it mean? In essence, the screen grid in the valve is connected to a tap on the primary windings of the output transformer, so that the voltage at the screen grid varies in proportion to the signal at the plate (or anode). Why is this done? It is in effect a local negative feedback loop and gives characteristics somewhere between those of a triode and a pentode. Some audiophiles and designers believe this gives an amp combining the best features of the triode and the pentode tubes. This being Hi-Fi however, others believe it has the abilities of neither. Why use ultralinear mode? Simply because it represents the cheapest way to get both good performance and high power output from a pentode amplifier design Characteristics Of Triode and Pentode Amplifiers. That last section may make more sense if we consider what the characteristics of the two tube types are (to find out more about the types of tube, there is another article on my site explaining the different types of valve). The triode is a low power, low efficiency design contrasted with the higher power and efficiency of the pentode. The triode uses more power at idle and its sonic characteristics are usually described using words like sweet, pure and rich - having a very natural sound conveying vocals and instrument timbre very well. It also has very benign clipping charcteristics when over driven. Of course, to others it is coloured with low output power, a restricted bandwidth - and in many cases - overpriced. This is especially true of the 'classic' single ended triode designs featuring triode tubes like the 845, 300B and 2A3. These tubes are expensive but very simple, having directly heated cathodes -where the cathode is both the heater and electron emissive element. You will often see amps using these being referred to as DH-SET amps - Directly Heated Singe Ended Triode. For aficionados, these DH-SET amps represent audio nirvana - frequently with equally enlightening prices...Cheaper triode amps actually use pentodes like the EL-34 in a triode configuration. The pentode in contrast is usually described as being analytic in its sound - or harsh if you are a triode lover! It can also clip more sharply than a triode. At the end of the day - only your ears can decide! vacoum valve 2a3 diy amp,