Ultra-linear
Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube (also called "electron-valve") to a load (e.g. to a loudspeaker).
'Ultra-linear' is a special case of 'distributed loading'; a circuit technique patented by Alan Blumlein in 1937 (Patent No. 496,883), although the name 'distributed loading' is probably due to Mullard.[1] 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 and Herbert Keroes in the early 1950s through articles in the magazine "Audio Engineering" from the USA.[2] The special case of 'ultra linear' operation is sometimes confused with the more general principle of distributed loading.
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 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-[3] 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. Mullard circuits such as the 5-20 also used 20% distributed loading (but did not achieve ultra-linear operation), while LEAK amplifiers used 50%).
The characteristics of the circuit which make distributed loading suitable for audio power amplifiers, when compared to a tetrode or pentode amplifier, 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, 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 loadng 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.
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 used a similar technique.
References
- ↑ Mullard Ltd. (1959) Mullard Circuits for Audio Amplifiers
- ↑ Hafler, D., Keroes, H. (1951) An Ultra-Linear Amplifier. Audio Engineering. November, pp15-17
- ↑ Hafler, D., Keroes, H. (1951) An Ultra-Linear Amplifier. Audio Engineering. November, pp15-17