Telefunken EL34

Name: Telefunken EL34
Brand: Telefunken

A power pentode that outlived the amplifiers it was built for - and outlived the company that built it.

Overview

The EL34 is a power pentode with a 6.3-volt heater drawing 1.5 amps. It is arguably the most consequential European audio tube of the postwar decades. Telefunken's own datasheet lists it under the German spelling "Endpentode" - power pentode. At a plate voltage of 250 V, a screen voltage of 265 V and a grid-1 bias of −13.5 V, the tube idles at 100 mA of plate current and 14.9 mA of screen current. Transconductance is 11 mA/V and internal plate resistance is 15 kΩ. That made it efficient and predictable, and it became the default output tube for a generation of high-fidelity amplifiers.

The datasheet documents operation in single-ended class A, push-pull class AB, and push-pull class B, often on the same sheet at several supply voltages. In single-ended class A at 265 V, the tube delivers 11 W into a 2 kΩ load at 10% distortion, or 8 W into 3 kΩ with a grid bias of −14.5 V. Wired as a triode, with the screen grid tied to the plate, it makes 6 W into a 3 kΩ load at 375 V with a 370 Ω cathode resistor. Push-pull goes much higher. Two tubes in class B at 800 V on the plates and a 750 Ω common screen resistor produce 100 W into an 11 kΩ plate-to-plate load at 5% distortion. At 500 V, the same pair makes 70 W into 4 kΩ. Push-pull class AB at 375 V yields 35 W into 3.4 kΩ. All of that happens in the same socket.

The maximum ratings are these. Plate voltage is capped at 800 V, screen voltage at 425 V, plate dissipation at 25 W (27.5 W with a signal applied), screen dissipation at 8 W, and cathode current at 150 mA. Grid-1 resistance is limited to 0.7 MΩ in class A or AB and 0.5 MΩ in class B. Exceed it in a fixed-bias amplifier and you invite thermal runaway. The bulb temperature is rated to 245 °C. Capacitances are modest: roughly 15.2 pF grid-1 to everything, 8.4 pF plate to everything, and less than 1.1 pF from grid-1 to plate. That is low enough that the tube rarely causes trouble in high-impedance driver stages. The tube weighs no more than 50 grams.

Period coverage treats the EL34 as a fixture rather than a product. A tube survey in Vacuum-Tube-Valley (2001, p.29) tested it in a restored 1959 Marantz Model 8 stereo amplifier, with an Aesthetix Calisto line stage and a 47 Labs Flatfish CD player up front. That is a deliberately high-resolution chain for a tube shootout. The same survey (p.13) ranked the Telefunken sample against Sovtek, Tungsram, and KT77 examples on measured distortion at 1 watt, with second harmonic only shown. The Telefunken landed in the middle of a tightly clustered field, not at the top.

The record also separates EL34 types by construction, and Telefunken's name shows up in it in a specific way. According to the same survey (Vacuum-Tube-Valley, p.19), the Type II has a large-diameter dark red or dark brown bakelite base, numbered pins, a small hole in the center of the locating pin, two small-diameter getter rings, and a rounded top with no glass seam. It first appeared around 1957-1958 and was seen labeled as Amperex, Mullard, and Telefunken, among others. So the Telefunken-branded EL34 of that era was not necessarily a Telefunken-manufactured tube. The survey (p.9) notes that the Type I was believed to have been manufactured by Dutch Philips, and that the Type II was the original tube seen in 1960s Dynaco amplifiers under the Dynaco brand. A separate passage (p.20) records that in 1958 a major Mullard competitor moved to poach EL34 business, motivated by the fact that pentode construction ran the screen grid hotter than an equivalent aligned-grid beam tetrode. The EL34's commercial history is one of cross-licensing and relabeling, and Telefunken was partly a brand on tubes that came from elsewhere.

For anyone buying today, the survey (p.12) warns plainly that NOS supplies are unreliable and that prices can only increase. Dealers exist, but their stock is unpredictable. Collectors report that well-used Type I examples often show heat-stress cracks in the metal ring yet still test like new, so visual inspection is a poor substitute for a tube tester. The EL34 was never rare. It was made in enormous quantities by multiple manufacturers across two decades. Original-production examples in good condition are still a different proposition from current production, and the price gap reflects that.

Telefunken's datasheet is clear. It gives measuring values, maximum ratings, capacitances, and typical operation for single-ended, push-pull AB, push-pull B, and triode-strapped configurations, all on a few pages. It is the most useful document the archive holds on this tube. It does not say which of the tubes bearing the Telefunken name actually came out of a Telefunken factory. The period survey suggests that at least some of them did not.