science

Ultra-linear

Ultra-linear is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ultra-linear rather than just read about it. In short: 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. In 1938 he applied for the US patent 2218902.

Key takeaways

Ultra-linear belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ultra-linear to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ultra-linear from memory before moving on to harder problems.

Reference excerpt

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. 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. 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), 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, although it is possible that this is an impedance ratio, which case it is the same as a 43% turns ratio, because the impedance ratio is the square of the turns ratio. Mullard circuits such as the 5-20 specified either 20% or 43% distributed loading, while LEAK amplifiers after 1954 used variously 25% or 49%, although again it is unclear whether these are turns ratios or impedance ratios. 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".

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.

References

Worked examples

Example 1 — a first encounter with Ultra-linear

Start with the simplest possible case. Write down what Ultra-linear claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ultra-linear before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ultra-linear ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ultra-linear

In research: Ultra-linear appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ultra-linear in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ultra-linear is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electronic amplifiers, Vacuum tubes, so understanding it makes those chapters shorter.
In everyday life: Look for Ultra-linear outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Ultra-linear in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ultra-linear means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ultra-linear out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ultra-linear in simple terms?

Why does Ultra-linear matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ultra-linear?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ultra-linear.