Ultra-linear electronic circuits are those used to couple a

tetrode A tetrode is a vacuum tube (called ''valve'' in British English) having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids, and a plate electrode, plate (called ''anode'' in Bri ...

pentode A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a ''tri ...

vacuum-tube (also called "electron-valve") to a load (e.g. to a

loudspeaker A loudspeaker (commonly referred to as a speaker or, more fully, a speaker system) is a combination of one or more speaker drivers, an enclosure, and electrical connections (possibly including a crossover network). The speaker driver is an ...

). 'Ultra-linear' is a special case of 'distributed loading'; a circuit technique patented by

Alan Blumlein Alan Dower Blumlein (; 29 June 1903 – 7 June 1942) was an English electronics engineer, notable for his many inventions in telecommunications, sound recording, stereophonic sound, television and radar. He received 128 patents and was considere ...

in 1937 (Patent No. 496,883), although the name 'distributed loading' is probably due to

Mullard Mullard Limited was a British manufacturer of electronics, electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed thermionic valves (US ...

. In 1938 he applied for the US patent 2218902. The particular advantages of ultra-linear operation, and the name itself, were published by

David Hafler David Hafler (February 7, 1919 – May 25, 2003) was an American audio engineer. He was best known for his work on an improved version of the Williamson amplifier using the ultra-linear circuit of Alan Blumlein. Biography In 1950, Hafler foun ...

and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA.Alt URL
The special case of 'ultra linear' operation is sometimes confused with the more general principle of distributed loading.

Operation

vacuum-tube (valve) configured as a common-cathode amplifier (where the output signal appears on the plate) may be operated as: * a pentode or tetrode, in which the screen-grid is connected to a stable DC voltage so there are no signal variations on the screen-grid (i.e. the screen-grid has 0% of the plate's output signal impressed on it), or * a triode, in which the screen-grid is connected to the plate (i.e. the screen-grid has 100% of the plate's output signal voltage impressed on it), or * a blend of triode and pentode, in which the screen-grid has a percentage (between 0% and 100%) of the plate's output signal impressed on it. This is the basis of the distributed load circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum-tube (valve) is connected to. The impression of any portion of the output signal onto the screen-grid can be seen as a form of feedback, which alters the behaviour of the electron stream passing from cathode to anode.

Advantages

By judicious choice of the screen-grid percentage-tap, the benefits of both triode and pentode vacuum-tubes can be realised. Over a very narrow range of percentage-tapping, distortion is found to fall to an unusually low value—sometimes less than for either triode or pentode operation—while power efficiency is only slightly reduced compared with full pentode operation. The optimum percentage-tap to achieve ultra-linear operation depends mainly on the type of valve used; a commonly seen percentage is 43% (of the number of transformer primary turns on the plate-circuit) which applies to the KT88, although many other valve types have optimum values close to this. A value of 20% was recommended for 6V6GTs.

circuits such as the 5-20 also used 20% distributed loading (but did not achieve ultra-linear operation), while

LEAK A leak is a way (usually an opening) for fluid to escape a container or fluid-containing system, such as a Water tank, tank or a Ship, ship's Hull (watercraft), hull, through which the contents of the container can escape or outside matter can e ...

amplifiers used 50%. The characteristics of the circuit which make distributed loading suitable for audio power amplifiers, when compared to a triode, beam tetrode or pentode based amplifier, are: * The output impedance is lowered to be about half that achieved with a triode. * Distortion is lowered to approach that achieved with a triode tube, but may be even less for ultra-linear operation. * The power output is higher than from a triode, approaching that delivered by a pentode. * The power output is more constant as distributed loading is a combination of a transconductance amplifier and a voltage amplifier. The distributed load circuit may be applied to either push-pull or single-ended amplifier circuits. Note that the term 'ultra linear' was expressly reserved only for the condition of optimum tapping point. As Hafler and Keroes wrote: "''Our patent claims cover the use of any primary tap in this circuit arrangement. However, we have restricted the use of the term "Ultra Linear" to the conditions where the dynamic plate characteristic curves are most linear''".{{Citation , last1=Hafler , first1=David , last2=Keroes , first2=Herbert I. , date=September 1952 , title=Ungilding the Lily (letter) , journal=Audio Engineering , page=8 , url=http://dalmura.com.au/projects/Gilding%20the%20Lily%20-%20July%201952%20-%20Audio.pdf , access-date=2014-07-17

Related circuits

The "QUAD II" amplifier from

QUAD QUaD, an acronym for QUEST at DASI, was a ground-based cosmic microwave background (CMB) polarization experiment at the South Pole. QUEST (Q and U Extragalactic Sub-mm Telescope) was the original name attributed to the bolometer detector instrume ...

uses a circuit in which the cathode has a portion of the output signal applied to it, and was referred to as "distributed load" by Peter Walker of QUAD. In the United States, McIntosh Laboratories used this technique extensively in their vacuum-tube power amplifiers. Audio Research Corp have also used a similar circuit.

References

Vacuum tubes Electronic amplifiers