Single-Ended Triode Amplifiers
Article By Dario De Souza Melo

Preliminary

  Audio amplifiers are divided into several types, with the main types being the following:

Tube single-ended triode (SET)
Tube single-ended OTL (Output transformer less)
Tube push pull OTL (Output transformer less)
Tube push pull
Transistorized single-ended
Transistorized push pull
Transistorized digital
Single-ended hybrids
Push pull hybrids

Most of these types are further offered in variants depending on the operation class ("Class A", "Class AB", "Class D", etc.), and with various output tubes or transistors types employed. With or without feedback, the number of stages and with or without interstage transformers. In the case of single-ended tube amplifiers, one output tube or more can be in parallel with being directly heated triode (DHT) or not. Join to this the fact amplifiers would be made in the versions: power only, integrated with active or passive pre-amplifier, and we have monaural, stereo, three, four or more channels for special applications like home theater for example.

At first you cannot say that any type is superior to another. Just look to the several articles that appears on the specialized magazines and internet. Also, personal reviews that justifies each one type and its variations with arguments that sometimes reach religious or fanatical status. You simply can not doubt those people as they can be honestly reporting what they hear with the type of music they enjoy and in their location with their loudspeaker systems, cables, pre-amplifier and front end (CD player, turntable, etc.).

Generally, the most famous audio amplifier designers in the world of tube amplifiers employs single-ended or push pull, with OTL appearing from time to time. Only recently the hybrid single-ended, the single-ended OTL and the transistorized single-ended have come to the surface. It was the pioneering efforts of Nelson Pass (of Pass Labs) that made the first commercial transistorized single-ended amplifier. It was, arguably, Gilbert Yeung from Blue Circle that made the first commercial hybrid single-ended amplifier. Furthermore, the output transformerless design (OTL) goes to New Yorker Julius Futtermann with his single-ended OTL amplifier and also to David Berning to some degree. The first tube single-ended triode amplifier was built by the American inventor (everyone must know that) Lee de Forest in 1912.

Not too many years before, in the more influential audio markets around the world, people only talked about tube push pull amplifiers and a few devoted audiophiles about the OTL of Futtermann fame being the tube single-ended triode amplifiers used only by a very small portion of the worldwide audiophiles. Possibly due to the fact that after World War II the economics in some countries made up for the rebuilding or purchasing at low costs of every SET amplifiers at disposal anywhere. First, because they love high efficiency exponential horn loudspeakers and secondly, due to the fact that the Americans and British audiophiles simply adopted the newly introduced transistorized gear and let go at very cheap prices his tube sets to others.

A Little Bit Of History
Around the year 1750 the British inventor Sir William Watson and others built the first crude tube-like device. A glass envelope containing both a cathode (emitter) and an anode (collector) but with only a portion of the air pumped out. Sophisticated vacuum pumps were available only after 1912. Before that, the flow of electricity through a tube was believed to be caused by ionization or gas discharge. Nonetheless, these early tubes facilitated the discovery of cathode rays (1858) and X-rays (1895).

After 1880 the American Thomas Alva Edison, working on his light bulb, noted a strange occurrence. He noted that a dark spot appeared on the inside glass wall of his bulbs when a carbon filament was used and correctly surmised that electrical particles drifting through the bulb and landed on the glass wall. This led him to insert into the bulb a metal plate biased with a positive potential relative do the filament. The effect of the positive plate was to divert the particles away from the glass wall. This was called the "Edison effect" and he could not explain why this occurred. He found a practical use for it anyway: he patented the device as an ammeter for measuring current flow!

Because Edison did not see the far-reaching implications of this device, it was left to others -- notably the British scientist John Ambrose Fleming -- to develop further applications for the "Edison effect". Fleming's experiments led him in 1904 to develop a vacuum tube diode capable of detecting wireless radio signals. Fleming surmised that by connecting an aerial to the positive plate he might thereby modulate the flow of current through the tube by means of wireless signals. This was the first practical example of a radio receiving tube.

On January 15^th 1907, Lee de Forest was attempting to improve upon the Fleming diode and patented his famous Audion (the first audio tube ever made). This was a three-electrode tube containing a third element known as grid. In 1908 De Forest improved the grid and moved it more directly into the path between cathode and anode. This innovation is widely considered to be one of the most valuable in the patent literature.

Like the Italian Marconi, Forest intended his new device to play a critical role in replacing the telegraph cable with wireless transmissions. The idea of instantly communicating across the continents fired De Forest's imagination like nothing else "I early resolved come hell or high water, to achieve an envied position in the well-nigh virgin field by inventing outstanding wireless transmitting and receiving devices," said De Forest. "I foresaw that wireless telephony would ultimately supplement, if not supplant, the telegraph; that the human voice, and possibly music, would replace the time-honored dots and dashes of the Morse code." These were prophetic words indeed!

In 1912, De Forest concluded his early experiment with a three-element tube as an amplifier with 42dB of gain that attracted a lot of interest from the AT&T technical people for use as a telephone repeater device. In 1913 De Forest produced a nifty looking four-tube amplifier which, like his earlier one, was only a scientific curiosity due to the fact that the tubes were not fully operational.

Additional tube developments occurred at a rapid pace as more and more air -- and with it impurities that restricted performance, longevity, or both -- was removed from successive tube designs. Large commercial interests recognizing the tube's vast potential and soon added to their considerable capabilities. Harold Arnold at Western Electric (by then an AT&T subsidiary) and Irving Langmuir at General Electric headed the teams that would soon make tubes viable commercial products. By 1915 Western Electric had developed a tube with a very long 4,000 hour life span! Specifically, the type L which was later known as the 101B. It was not long before the race to place wireless reception in the home began in earnest.

The amplifiers developed at this time started to be used as telephone intercontinental line repeaters. Their first commercial application! The first book covering the theories about tubes appeared in 1920 and was published by the Western Electric senior research physicist H. J. Van der Bijl. Curiously, the first tube push pull amplifier circuit was developed in the same year of De Forest single ended triode amplifier, 1912, and was the work of E. H. Colpitts Western Electric researcher chief. The concept of distortion reduction by the application of feedback was established in 1922 by H. S. Black, also a W.E. employee. In 1925 Edward Kellogg, the co-inventor of the moving coil loudspeaker, published a circuit of a push pull tube amplifier to operate his invent and in 1928 the weekly British magazine Wireless World published an interesting analyses of the push pull operation.

By this time the American company Thordarson offered to sell several kits of more powerful single-ended and push-pull tube amplifiers to replace the built-in amplifiers of the home radio consoles sold at that time. The Western Electric model A7 single-ended direct heated triodes with interstages transformers amplifier (!!!) and a special horn was offered at that time for the same purpose. In the beginning of the 1930's the theaters and movie houses started to use amplifiers to increase the volume of the narrators voices to reach the public. Larger houses used the powerful ten watt push-pull tube amplifiers! The small ones preferred the more clean sound of the single-ended three watts tube amplifiers like the famous WE 91A with 300A and later, the now famous 300B output tube.

As theater and movie houses became larger, the demand for more powerful amplifiers led to the research of better push pull tube amplifiers and in 1934 W.T. Cocking published in Wireless World the seminal article "High Quality Amplification" with new ideas and the best tube push pull amplifier circuit until that date and that influenced D.T.N. Williamson to publish in 1947 his article "Design for a high quality amplifier" that paved the road for the actual tube P-P amplifiers.

A Rediscovery Of Single-Ended Triode Amplifiers
The Japanese audiophiles (known as audiomaniacs) have been using SET amplifiers for a long time. During the sixties Kei Ikeda gave good news about the Western Electric SETs. In 1970 Nobu Shishido published an article in a DIY magazine about a single-ended amplifier circuit using the 2A3 direct heating output tube that he called Loftin-White. Then later in 1972 Isamu Asano goes further by calling attention for the classic triode amplifiers built before World War II.

Jean Hiraga of France published an article in the DIY magazine L' Audiophile about another single-ended 2A3 output tube circuit and several articles on the subject that influenced the European audiophile community by the excellent sound results reported including the British Peter Qvortrup that, by his turn, influenced the Americans Dennis Had, Don Gerber, Ron Welborne, Herb Reichert, Nori Komuro, Ralph Karsten, J. C. Morrison, Jim Ricketts, Steve Berger, Lynn Olson and others to start projecting and building their own tube amplifiers.

Nostalgia Or Performance
In the vast majority of reviews concerning audio shows, many journalists considered vacuum tube designs as the best sound systems from such manufacturers as Cary Audio, Art Audio, Wavelength Audio, Wawac, etc. This confirms my suspicion that SETs has the most potential, depending on the loudspeaker they are driving, to be the bests amplifiers around. The exception being SET amplifiers with more than 60 Watts per channel that could driven medium to high efficiency loudspeakers with an impedance rating at 6 Ohms, the others must driven only high efficiency with more than 6 Ohms nominal impedance loudspeaker systems. The Lilliputian SET amplifiers (0.5 to 3 Watts per channel) must drive only 100dB/W/m loudspeakers with nominal impedances from 8 ohms upward (normally horn loaded loudspeaker systems) if we wish to get sound pressure levels up to 80dB in medium to average rooms (around 70 to 100 sq/ft).

There is a large quantity and variety of SET amplifiers being made and sold in several countries alongside with many projects published in DIY audio magazines and on the Internet (Glass Audio, Sound Practices, Mujjen to Jink, Construire HIFI, Audio Amateur, Musique et Technique, L'Audiophile, etc). Being impossible to list all SET amplifiers and circuits that are available in the world, I will not try to give a complete listing as this could become a full 30+ page catalog in and of itself!

Almost every day I find new brands, models, or projects in the magazines or on the internet which confirm my point of view that nowadays they are the most searched of all tube amplifiers by the wise audiophiles that wants to get the most realistic sound reproduction possible. Kits range from very inexpensive $359 to the fully assembled $292,500 Audio Note Gaku-On. The Gaku-Ons are the most expensive in the world today. From the more simple to the more complex projects with three chassis, by mono unit to six chassis in stereo (Ed Billeci 212) and weighting more than 500 pounds the whole system! I will try to show on the listing below the loudspeaker systems more adequate to be driven by SET amplifiers. Giving advice to take care with many factors that could annoy the marriage. By example; very low impedance, very large impedance range, quantity of ways (four- or five-way systems) and consequently the resistance offered by the required complex crossover, deficiency on cabinet construction in the case of loaded horns that imparts large amount of distortion etc.

SET amplifier 0.5 to 1.9 watts  and loudspeaker with 104 to 108dB/W/m
2 to 5 Watts               100 to 103
SET 6 to 20 Watts        96 to 99
20 to 40 Watts              92 to 97
45 to 60 Watts              90 to 97
65 to 100 Watts            89 to 97
< than 100 Watts          87 to 94

Low power SET amplifiers require high efficiency loudspeakers. Those who wish to get high SPL levels of the last few octaves of deep bass response on acoustically dead rooms or reproduce music with large contents of sounds like symphonies, coral music, rock concerts etc. at high volumes may want to supplement their system accordingly. In this case it seems better to use some adequate horn loaded loudspeaker system (Lowther, Tannoy, Altec, Fostex etc) or more powerful amplifiers with medium to large loudspeakers or to acquire one or two subwoofers with dedicated amplifiers to obtain those required low bass passages and to increase the general dynamics of the whole system. Therefore a two or three way tube crossover (Marchand XM26 for instance), multiple amplifiers, and adjusting the volume of both loudspeakers (the main system and the subwoofer) for the same level.

To choose a suitable SET amplifier in our country (Brazil) I will recommend a model that uses the 300B tube on the output (Sonic Frontiers Assemblage ST300B kit, Audio Note Kit One, Audiopax 300B or Aetherius AMAT300B etc.) and suggest the prospective buyer to get a set of spare tubes like two KR Enterprises 300BXLS, two Western Electric WE300B and two JJ Electronics (Tesla) 300B which implies in the possibility to have a different sounding amplifier for each type of tube used and allowing the owner to adjust the sound palate at his will.

With so many options of adequate loudspeaker systems for use with SET amplifiers of whatever power, most of them exhaustively explained on the Internet or on the DIY specialized magazines. These can be built by the proper audiophile with little cost. It has become fairly easy to choose the right one for your specific room, personal option, or SET amplifier power. As in Brazil, the matter is not well covered. Only a few audiophiles knowing the subject, I think the best options are that supplied by Audiopax of Eduardo de Lima of Rio de Janeiro due to the fact that he sells SET amplifiers in a pack with a pair of adequate loudspeaker system (or the option that includes high efficiency full range units from the British Lowther). Eduardo mainly offers the new types DX2, DX3 and DX4 with neodymium magnets. There is a vast cult in other countries (notably in Japan) for horn loaded loudspeaker systems with the most expensive types from Tannoy (Westminster Royal, Westminster TW and Kingdom) and the models derived from Lowther (Audiovector, Bel Canto, Academy, Fidelio, Side Vivace, Bio Mauhorn) earning great respect. In the end SET amplifiers as used with the right speaker can give glorious music. After all, it is the music that matters most.

I wish to thank Mr. Scott Frankland for his work and also the books "A Taste Of Tubes" and "A Little History" as they were my main reference during the history section of the above article.