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 falls under the more general principle of 'distributed-loading'.
Operation
A pentode or tetrode vacuum tube (electron 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 ultra-linear circuit, and is usually achieved by incorporating a suitable "tap" on the primary winding of the output transformer that the vacuum tube (electron valve) is connected to.
The impression of any portion of the output signal onto the screen grid (load distribution) can be seen as a form of feedback, which alters the behavior of the electron stream passing from cathode to anode. Among other benefits, this feedback helps to more accurately reflect the instantaneous reactance of the load (secondary circuit) back to the valves (primary circuit), thus offering better load-control and responsiveness (i.e. improved loudspeaker damping and power stability as load impedance varies across frequency band).
Advantages
By judicious choice of the screen tap percentage, the benefits of both triode and pentode vacuum tubes can be realised. Over a very narrow range of tap percentage, distortion is found to fall to an unusually low value—sometimes less than for either triode or pentode operation[2]—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 two to three times that achieved with a triode.
- Distortion is lowered to approach that achieved in triode operation, but may be even less in 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".[3]
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. 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.
