TUBE AMPLIFIERS - BY POWER TUBE
Developed by Philips in 1953 for use in the British Mullard 5-10 amplifier, the EL84 pentode tube, or commonly known as 6BQ5 in North America, was intended for use as an inexpensive alternative to the larger audio tubes of the time, such as EL34, 6L6, and KT66 power tubes. The 1959 "Miniwatt" Technical Data book from Philips lists the 6BQ5 as the R.E.T.M.A. (American) name for the EL84 in its "Type Number Cross Reference",and hence an exact substitute. American and Japanese manufacturers might label their versions of the EL84 as "EL84/6BQ5" or "6BQ5/EL84" or simply "6BQ5".
Other manufacturers followed with their versions, such as the N709 from General Electric Co. Ltd. of England that were designed to be "drop-in" substitutes. The CV2975 is the military designation (Common Valve) for EL84. Other equivalent tubes are the 7189 and 7189A, an extended-ratings version of the tube for industrial applications, E84L (7320) long life, professional version with more than 10000 hours expected lifetime, and the directly equivalent 6P14P (Cyrillic: 6П14П) produced in the USSR by the Reflektor plant.
The EL34 pentode tube was introduced in 1949 by Philips the parent company of Mullard and, although no longer made by them, it is manufactured by JJ Electronic, Shuguang, Svetlana and Reflektor (Sovtek, Electro-Harmonix, Tung-Sol), amongst others. Some firms make a related tube called an E34L which is rated to require a higher grid bias voltage, but which may be interchangeable in some equipment.
The American RETMA tube designation number for this tube is 6CA7.
Other closely related with similar specifications and characteristics are tubes from the Beam-Tetrode and Kinkless-Tetrode families. These include most notably the KT77, and more distantly the KT66 and 6L6. In many cases EL34 amplifier circuits will allow for 'rolling" the aforementioned tubes as substitutes for the EL34.
The USSR Beam-Tetrode 6P27S tube (Cyrillic: 6П27C) is also a 'rolling" candidate. It can be substituted for the EL34 but is very rare.
6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936 under license from MOV, owned by EMI/GE. At the time Philips had already developed and patented power pentode designs, which were rapidly replacing power triodes due to their greater efficiency. MOV's innovative beam tetrode design also allowed RCA to circumvent Philips' pentode patent. MOV had licensed the design to because their engineers did not feel the kinkless tetrode could be successfully mass-produced. They later did finally introduce it themselves, as the KT66..This choice of denomination came about because the sobriquet "KT", stood for "kinkless tetrode".
RCA's later versions, included the 6L6G, 6L6GA, 6L6GB, 5881, 5932, 7027, and the final version 6L6GC which was the most powerful - rated at 30W plate dissipation. Earlier variants included the 807 (1937), the 1625, the 6V6 and the 6BG6G (1946), a modified 807. Other close equivalents also include the 7581A and the 7591A. The 6L6 became the most successful and most produced tube family in vacuum tube history.
The tube was also produced in Russia under the designator 6P3S...as was the 6P6S, the Russian version of the lower-power 6V6.
The KT88 was introduced by GEC in 1956 as a larger variant of the KT66. It was manufactured in the U.K. by the MOV (Marconi-Osram Valve) subsidiary of G.E.C, also labelled as IEC/Mullard, and, in the US, where it was known as the Genalex Gold Lion.
The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has similar ratings to the American 6550 which was designed for use as a servo amplifier. It is one of the largest tubes in its class, and can handle significantly higher plate voltages than similar tubes -- up to 800 volts.
Other than the 6550, the KT88 has a number of close-equivalents and near-equivalents , namely the very rare MOV TT21 and TT22, the KT90, KT94, KT99 and KT100. Distant relations include the EL509 and PL519 tubes via relationship to the KT90.
Type PL519 was first introduced in 1970 and was a line output valve, or sweep tube as they were known in America. This tube type is of late manufacture and probably the last to be produced before transistors took over the role. They would be found in large screen colour televisions with an EHT of around 25 kV and thus requiring considerable line drive power. Another application of the EL509 / PL509 is that of the amateur radio output stage. Today we encounter this tube and its variants in audio amplifier power output stages. Its Russian equivalent is the 6P45S.
A near equivalent, and substitute, the newer EL509 variant with octal base (no anode cap) was specially designed for use as a power amp tube.
The 500 family of tubes includes the E-types -- EL505 / EL509 / EL519 -- and the P-types PL505 / PL509 / PL519. The higher the number, the newer the tube, and the higher the anode power dissipation. Ultimately, only the EL519, the PL519 and the 6P45S were produced.
Although today only a handful of amplifiers implement this tube in their power stages, a few renowned designers, like Tim de Paravicini, recognized its talents. And for those lucky enough to have heard this tube in action, it is without a doubt one of the finest sounding tubes available.
The KT120 and its successor the KT150 are current production tubes and are manufactured for Tung-Sol at their New Sensor Russian factory.
Tung-Sol KT120 claims a storied history that stretches back to September of 1950 when the company introduced the 5881, an industrial version of the 6L6GA with a 23.5W plate dissipation, thus opening the doors to the hi-fi movement. This rugged tube also found a home in the '59 Fender Bassman and servo amplifiers used in B-52 bombers. In 1955, Tung-Sol raised the bar of high fidelity with the introduction of the 35W 6550, which could deliver 100 watts with a pair in push-pull configuration. It was used in the iconic Dynaco Mark III and Sunn amplifiers.
Today, Tung-Sol hand-crafts the KT120, which has a plate dissipation of 60 watts, which made it the most powerful tube in the 6550/KT88/KT90 family, until the introduction of their KT150.
The KT120 can in many cases replace a KT88...and the KT150 can in turn replace a KT120 in amplifiers employing these power tubes. With a massive plate dissipation of 70 watts, the Tung-Sol KT150 was the most powerful octal beam tetrode ever produced until the introduction of the company of the KT170.
A pair of KT150s can allow an amplifier with a power output approaching 300 watts to be built. The Tung-Sol KT150 glass envelope is a special egg shaped balloon that was developed to improve thermal dissipation and maintain a superior vacuum for the best sound. The fact that the glass envelope has no flat sides means the Tung-Sol KT150 has no problems with microphonics.
The Tung-Sol KT170 has a plate dissipation of 85 watts! ... making it the most powerful tube in the Tung-Sol family. A pair of these tubes in push-pull configuration can deliver power levels of 190 or more watts. When used at the parameters found in existing 6550/KT88/KT90 circuits, the Tung-Sol KT170 is impervious to overload, delivering peak power with extreme reliability and long tube life. However, taking advantage of the higher current handling capacity of these tubes, a very unique and super powerful and stable amp can be designed using the Tung-Sol KT-170.
The main 3 characteristics of the 6C33C are very high transconductance, very high current capability and very low internal resistance. This tube is also mechanically very robust as it was to be used in Russian military fighter aircraft.
What distinguishes the 6C33C is that it does not sound like a typical SET tube. What strikes the listener first is that the tube expresses an absolutely luscious rendering of tonal color. More importantly, the 6C33C does this whilst avoiding the injection of an ultimately tiresome syrupy romanticism, a tonal tendency common to many SET tubes. The 6C33C also lacks a typical SET tonal characteristic – an overly warm and ripe midband that directs the musical focus to certain instruments. As for Bass, notoriously the Achilles’ Heel of SET tube design, the 6C33C delivers astonishingly good bottom-end extension, control, dynamics, and articulation. Whilst this would not be mistaken for a solid-state dreadnought design, one would not expect this level of bass control and dynamic drive was being delivered by a pair of 6C33c's in 16W SE mode.
This tube is so natural, organic, and “musically right” - and so greatly minimizes an SET’s traditional shortcomings - that one is led to question the modern paradigm of high powered solid-state amplification. In one word, it is “PRaT-astic” !
To contextualize the 2A3 tube and its DHT triode brethren, let us look at the historical perspective of its heritage. In 1926, the 71A direct-heated power triode was introduced, followed by the 50 in 1928, the 45 and PX4 in 1929, the PX25 in 1930, the 2A3 in 1932, the 300A in 1936, and the 300B in 1938. The 6B4G is an octal version of the 6A3, itself the 6.3V version of the 2.5V 2A3. The Russian equivalent of the 6B4G is the 6C4C [6S4S]
The "power" race started in earnest with the 6L6 and KT66 pentode in 1936, followed by the ubiquitous EL34 in 1951, the EL84 in 1953, the 6550 and KT88 in 1954, and last in the series, the 7591 in 1959 and the 8417 in 1963. (The 6L6 has been in continuous production since 1936, a record unmatched by any other electronic device - we all owe the electric-guitar players a big thank you for keeping the tube factories open.)
Seventy years later, vacuum tubes, and especially triodes, continue to be the lowest distortion amplifying elements ever made. No germanium or silicon transistor, JFET, or MOSFET has ever approached the distortion performance of the direct-heated triodes, with indirect-heated triodes following closely behind. In addition to low distortion in the absolute sense, the distortion spectra of triodes is favorable, with a rapid fall-off of the upper harmonics. (This is less true for beam tetrodes, pentodes, or solid-state devices, which are intrinsically less linear and have higher-order distortion curves.)
The 45 and the PX25 are less commonly implemented in tube amplifiers today, but are actually better sounding than their better known cousins, the 2A3 and the 300B.
NOS versions were also very rare, until Sophia, KR and EML decided to produce modern day copies of them. The 45 followed the 71A and the 50 in 1929. The PX25 was introduced a year later in 1930.
The 45 has the same 2.5V heater voltage specification as the 2A3, but delivers only 1.6W to 2W as opposed to 3.5W for the 2A3.
The PX25 power output, at 4.5W to 5W falls between that of the 2A3 and the 300B. Its heater voltage specification is 4V, as was the case with the British Marconi PX4. For many DHT tube lovers these are the 2 MUST HAVE tubes.
The maker of the original 300B is an old company, Gray & Barton, founded (before radio came on the scene) to make telephone equipment. Gray & Barton became Western Electric Manufacturing Company in 1872. It was reorganized in 1881 as the Western Electric Company. One year later the Western Electric Company became the manufacturing division of Bell Telephone. Western Electric Export Corporation was formed in 1928 to be Western Electric’s foreign distributor and its marketing arm for audio equipment and parts. It became Westrex Corporation in 1942. Litton Industries acquired part of Westrex in 1958.
The 300B is a directly-heated power triode vacuum tube with a 4-pin base, and a 5V heater voltage specification introduced in 1938 by Western Electric to amplify telephone signals. It measures 6.4 inches high and 2.4 inches wide, and the anode can dissipate 40 watts. In the 1980s it started to be used increasingly by audiophiles in home audio equipment.
As of 2012, the 300B and other tubes of similar characteristics are manufactured by EH Electro Harmonix, Emission Labs - EML, JJ Electronic, KR Audio, Shuguang, Sovtek, Svetlana, TJ FullMusic, Takatsuki, and Western Electric.