Ultra-Linear

Ultra-linear is a term used to describe a type of electronic circuit that is used to couple a tetrode or pentode vacuum-tube (also called "electron-valve") to a load (e.g. to a loudspeaker).

The circuit was developed by Alan Blumlein and patented in 1937 ("Improvements in or relating to Thermionic Valve Amplifying Circuits", Patent No. 496,883). It was popularised by David Hafler and Herbert Keroes in the early 1950's through articles in the magazine "Audio Engineering" from the USA.

Operation

A pentode or tetrode 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 circuits such as the 5-20, while LEAK 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 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 have also use a similar technique.

External links

* [http://www.doramusic.com/index_blumlein.htm Web site about Alan Blumlein]