The EL34 was intended for audio output. It is a pentode, with a maximum anode dissipation of 25W. It was introduced by Philips/Mullard in 1953. It was intended for hi-fi and public address systems, but was also made popular for guitar amplifiers by Marshall. As a true pentode, it is easy to drive. However some US-produced versions, which may be labelled 6CA7, are beam tetrodes and so will tend to have higher grid current.
There is a fascinating account of the story of the EL34 on a German website. See also some information from the Mullard description of the EL34. It is still in production in Russia, China and eastern Europe.
A now-popular version of the EL34 was produced in East Germany by RFT. At the time this was looked down on by people who preferred branded Western goods. What they did not realise was that some of their Western brands were simply labels attached to Eastern products. This effect can be seen today: a Brimar-branded RFT EL34 will sell on ebay for more than an identical valve labelled, say, Zaerix. For a while even some 'Mullard' EL34 were actually RFT, and they still sell for silly prices to those not in the know. Now that some modern EL34 are not so good, the RFT is appreciated and regarded as a good NOS buy.
There are some features of the RFT EL34 which make it fairly easy to recognise, especially if you have seen one in the flesh. They are:
I have described here the most commonly seen RFT EL34. There are variants, but they are quite rare (in the UK at least). The Saratov plant in Russia produced a very similar EL34, using RFT-sourced tooling, but I understand that this does not have the dimple in the glass and may have a patchy anode coating.
As mentioned above, the RFT version was branded by many different companies. It is sometimes colloquially known as the 'Siemens' EL34, as many appeared with this branding. I have shown eight examples here, as these show a brand name. I have unbranded valves too. From left to right, top row then bottom row, these are:
The same valves appear below with their boxes. Another pair of RFT EL34 are shown in their Marshall box. At first sight these ones appeared to be unmarked, but on closer examination I found that they have a partially removed Trigon label - an example of double rebranding?
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updated 18 July 2008: more on Russian lookalike