Ultra-linear is a term used to describe a type of electronic circuit that is used to couple a tetrode

Tetrode

A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...

 or pentode

Pentode

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...

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

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

).

The circuit was developed by Alan Blumlein

Alan Blumlein

Alan Dower Blumlein was a British electronics engineer, notable for his many inventions in telecommunications, sound recording, stereo, television and radar...

 and patented in 1937 ("Improvements in or relating to Thermionic Valve Amplifying Circuits", Patent No. 496,883). In 1938 he applied for the US patent 2218902 "Thermionic Valve Amplifying Circuits". It was popularised by David Hafler

David Hafler

David Hafler 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....

 and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA. ("An Ultra-Linear Amplifier", Audio Engineering, NOV 1951, pp. 15-17)

Operation

A pentode

Pentode

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid vacuum tube , which was invented by the Dutchman Bernhard D.H. Tellegen in 1926...

 or tetrode

Tetrode

A tetrode is an electronic device having four active electrodes. The term most commonly applies to a two-grid vacuum tube. It has the three electrodes of a triode and an additional screen grid which significantly changes its behaviour.-Control grid:...

 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)

• 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)

• 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 "ultra-linear" 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. A commonly seen percentage for the tap is 43% (of the number of transformer primary turns on the plate-circuit), although a value of 20% was used for some Mullard

Mullard

Mullard Limited was a British manufacturer of electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed valves for the Admiralty before becoming managing director of the Z Electric Lamp Co. The...

 circuits such as the 5-20

Mullard 5-10

The Mullard 5-10 was a circuit for a valve amplifier designed by the British vacuum tube company, Mullard in 1954 at the Mullard Applications Research Laboratory in Mitcham Surrey UK, part of the New Road factory complex, to take advantage of their particular products...

, while LEAK

LEAK

LEAK is the brand name for high-fidelity audio equipment made by H. J. Leak & Co. Ltd, of London, England. The company was founded in 1934 by Harold Joseph Leak and was sold to the Rank Organisation in January 1969. During the 1950s and 60s, the company produced high-quality amplifiers, radio...

 amplifiers used 50%).

The characteristics of the circuit which make it suitable for audio power amplifiers are:

• the output impedance is lowered to be about twice that achieved with a triode

• distortion is lowered to approach that achieved with a triode tube

• the power output is increased to approach that achieved with pentodes

The ultra-linear circuit may be applied to either push-pull or single-ended amplifier circuits.

Related Circuits

The "QUAD II" amplifier from QUAD

Quad

Quad may refer to:-Architecture:*Quadrangle in architecture, e.g., on a university campus*Quad, a dormitory room or suite housing four residents...

 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. Audio Research Corp

Audio Research Corp

Audio Research Corporation is one of the oldest manufacturers of high-end audio equipment still in operation. They were pioneers at advancing the state-of-the-art for audio reproduction in the seventies, re-introducing the vacuum tube as the primary active amplification device. Audio Research...

have also use a similar technique.