ULTRA - LINEAR
AUDIO AMPLIFIERS




Observations: from published literature relating to the so called "ULTRA LINEAR" type of audio amplifier: as used in valve based HiFi, Public address, Modulator amplifiers etc.



In a push-pull amplifier (compared to a parallel pair of valves): 2nd Harmonic distortion produced by either valve is cancelled by equal and opposite distortion from the other.

Two triodes in Push-pull therefore give greater undistorted output than if connected in parallel. 3rd harmonic distortion does not cancel in this way; pentodes whose output is limited by 3rd harmonics therefore do not share in this particular advantage.

They can be made to do so by connecting their screen grids to suitable tapping’s on the primary of the output transformer, in what is rather absurdly called the "Ultra-Linear" system.

FOUNDATIONS OF WIRELESS 7TH EDITION: SCROGGIE (1958) pp319




Ultra-Linear operation provides a compromise between the high efficiency of tetrodes and the low distortion and uncritical load impedance of triodes.

There is an undoubted advantage in modulator service because the matching of the modulator to the RF stage is not so critical and the screen (DC) supply is obtained "automatically" from the transformer.

Screen grid tappings: 43% gives minimum distortion 20% gives maximum power.

RSGB handbook 4th Edition (1968) pp9.19




Patent 496,883 dated 5 June 1937 is one which is considered to be among the most important, which later became known as the "Ultra - Linear" amplifier.
With so many of Blumlein’s circuits the design is deceptively simple.

It shows a pentode output stage of a single ended audio frequency amplifier with a tap on the primary winding of the output transformer.
This tap provided feedback to the second grid to improve the linearity of the amplifier.
Blumlein realized that if the tap was placed at the Anode end of the primary winding, the valve would then be connected as a triode, and if the tap was at the supply end, it would be a pure pentode.

It was well known that when a pentode was connected as a triode it would be far less efficient and therefore provide a lower power output. If however the tap was placed at a distance 15-20% down from the supply end of the supply end of the transformer, the valve would combine positive features of both the triode and the pentode.

The Ultra-Linear circuit in its push-pull form became widely used in high-fidelity valve amplifiers during the second world war, though it was a good many years before general practice caught up with Blumlein’s thinking.

While Blumlein did not consider the problems associated with distortion resulting from feedback to the screen, he regarded it mainly as a convenient alternative to the control-grid feedback for reducing the undesirably high impedance of pentodes while retaining their efficiency.

He did suggest lowering the output impedance of the valve to approximately equal the optimum load. This would have been done in order to prevent damage to the valve or the output transformer if, the load became disconnected.

THE LIFE AND WORKS OF ALAN DOWER BLUMLEIN:
ROBERT CHARLES ALEXANDER (2000) pp213




ADDITIONALLY:



The EF86 pentode has a heater structure as a helix and an electrostatic shield around the anode (mesh) both designed to reduce hum in audio circuits.
The input stage; an EF86 pentode, is responsible for the high sensitivity but poor noise performance of these amplifiers.

VALVE AMPLIFIERS: MORGAN JONES. NEWNES (1995) pp224