Summary
The EL34 (American RETMA tube designation 6CA7) was a power pentode
type with an octal base and was predominantly found in the final output
stages of amplification circuits.
History
One of the most popular output tubes of all time, the EL34 was originally
intended as a cheap means of getting 40 watts with high sensitivity.
The purpose was to allow amplifier designers to make a driver/phase
splitter with a minimum components count. Low distortion was incidental,
as this tube was intended to be used in a push-pull pair with negative
feedback. Although beam-power tetrodes had taken over the market in
the 1940s (see 6L6), this new tube was a true pentode, an archaic design
by the standards of the day. This may have been done in order to circumvent
RCA's patents on beam tubes, however Mullard was taking a big chance
when they introduced the EL34 in late 1953.
In spite of the low-cost intent, the early EL34 was an excellent tube. The Mullard early version (recognizable by the metal ring around its base) could easily outperform any 6L6, and was almost a match for the 6550. It gave easy drive and considerable peak power from a tall and thin glass envelope that saved space on chassis. Mullard's book CIRCUITS FOR AUDIO AMPLIFIERS, first published in 1959 and recently reprinted by Audio Amateur Publications, featured the EL34 in the largest power-output design. All the other amps in that book used the smaller EL84 or ECL82.
Among the first amps to use the EL34 were the Marantz 2 (1955), Dynaco Mark II, and the Pye Mozart, a single-ended 9-watt British unit from 1956. The easy drive requirements allowed a 30-40 watt amp to be built using only 3 tubes: the two EL34s, and a triode-pentode as the voltage amp and phase splitter. This made the EL34 a perfect choice for Dynaco to use in their Mark II and its low-cost stereo version, the Stereo-70. At least half a million Stereo-70s were sold from 1958 to 1977. The EL34 is also found in the Marantz 5, 8A, 8B and 9; the H. H. Scott 240, 250 and 290; the Eico HF60 and HF87; the Heath W-7A; the Acrosound UL-120; the Fisher X-1000; and many, many others. These are some of the best tube hi-fi amplifiers ever made, all based on "a cheap way of getting 40 watts".
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