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Last Update: 29-Mai-2016

Portrait of a tube

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We refer to Telefunken EF94

Author: Jac van de Walle

Tubes in this family are: 6028 - 5654 - 6AK5 - 6AU6 - EF94 - 403A - 408A - 713A - 717A - VT296

  Filament Ua Ug2 Ug1 Ia Gm
EF94 6.3V 130 130 -2V 7.5mA 4mA/V

408A

20V 130 130 -2V 7.5mA 5mA/V
6AU6 6.3V 130 130 -2V 6mA 4mA/V
717A 6.3V 130 130 -2V 7.5mA 5mA/V

Above operating points are chosen the same to compare the tubes
though they have different maximum ratings

These are sharp cut off pentodes, indicating they have other applications that just HiFi. They were intended for coaxial applications. However, they can be used for Audio very nicely.

EF94 - 5654 - 6AK5 is a very nice small pentode with 7pin miniature base. Highly underestimated, and a potential HiFi tube. Used by Yamamoto Soundcraft in their phono amplifiers. 6AU6 was used by the Hammond organ company in the small signal paths. When biased at around -2V grid voltage, the tube is very linear both as pentode or triode.

6AU6 looks is constructed with larger, round plates instead of square as with EF94, but still it seems to be a similar tube.

403A, 403B. Most are made by Western Electric, but other brands exist as well. It has 20% higher transconductance as EF94. So it is not just the same tubes in 20V, and also some other ratings may be slightly different. However, when you take care, one can easily make a design which can run on both types. When I try it on a tube tester, the bias point is the same. Also the plate shape of EF94 and 408A is identical. Close to EF94.

408A. 20Volt version of 403A.

While I write this (year 2008) EF94 Telefunken can still be found NOS, though not cheap, it can be found. Some with TFK branding, some with AEG branding, but with <> bottoms. 408A is easy to buy, since it has 20V filaments, and the tube crowd doesn't jump in that so greedy, and probably never will. The good thing is, ever WE408 is affordable.

717A is close, but has an octal base. These are sometimes called "door knob tubes", though I think that's a pretty stupid name for such beautiful items. Designed for radar applications, they put the inside sytsem on the side, to get shorter connections from the tube system to the base. So they achieved faster switching times like that. Same as the 408A. 717A was not intended for Audio use, but actually has remarkably nice curves if you choose a grid voltage around -2V or so. 717A has a metal shielded base, the 713A has a phenol base. For the rest these are the same. Very nice if the 717A is of course, you can actually USE the shield, while stil enjoy the nice glow picture. This is a 403A, adapted for very high frequency operation, and we find them often in Aviation Radar. Yet it appears an excellent Audio tube, and microphonics are very low.

NOS Supply situation:

717A is a tube with very little demand and very little supply. So prices can be varying very much. It seems all of those were made by Raytheon only, though they did private branding for Western Electric, and Tung-Sol. Some believe the WE 717A is higher quality, but at least WE costs a lot more more. All of these can sometimes give a very small scratch noise, but that's really rare. If it happens, just replace that tube, and sell the old one one on that famous auction website where everybody sells his second choice stuff for high prices.

VT296. This is the military designation for 717A

384A. Flying lead version. Not quite the same tube as 717A.

713A. Same as 717A, but 717A has a metal base, and 713A has a bakelite base.

datasheets available from our website (Check database):

6AU6 Brimair, General Electric, RCA
EF94 Lorenz
408A Sylvania, RCA, Western Electric
717A Tung-Sol