atelier 13 audio
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.