Little has been said about this tube in the internet. We've Google and didn't find much articles on ED8000 in tube amplifiers. This tube often appears in eBay at very decent price still, considering it is a Telefunken tube (perhaps audiophiles do not like it since it is supposed to be a series/pass regulator tube). Most are German made (Valvo, Telefunken and Siemens) and price are quite decent too, around U$10 - 50/pc, depend on whether it is NOS or used. We suspect all of them are from Telefunken factory (anyone can confirm?).
Datasheet:
http://tdsl.duncanamps.com/pdf/ed8000.pdf
Why do we like this ED8000 tube? There are a few reasons!
1. We love Telefunken tubes. Most sound good, and very reliable.
2. It is a long life tube, guaranteed for 10,000 hours operation.
3. Tight tolerances grade (Telefunken tubes are known to have tight tolerances even for the general type).
4. Special grade vibration and shock proof tubes.
5. High transconductance and low Rp (easy to make the output transformers).
6. It is a TRIODE!
If the circuit is made right, we can actually make an ED8000 tube amplifier that is compatible with 6AS7 / 6080 tube! 6AS7 / 6080 SET, to some, are poor man's 300B amplifier. Their Rp, operating point, output transformers and other electrical parameters are close enough to be made compatible. Do no try this at home! Only special made amplifiers can do this.
Looking at the ED8000 tube Ia/Ua curve, with Pa =17W, the tube can be biased at 140V plate, 100mA Iplate, and -26V Vgrid. That gives Pd = 82%, which is near the 75% ball park, which is the usual values that one would bias their tubes for class A operation. Since this is a special quality tube, I do not mind biasing it hotter to get more power out of this.
With such bias, ED8000 needs only 52Vpp to bring it to full power. A simple 2 stage driver will be sufficient, such as 6SN7-6SN7, or 5687-5687. If one employs a step-up interstage transformer of 1:2 (recommended), single driver tube will be enough. Or, go for a higher gain driver tubes, for example: 5842 / 417A, EC8010, E55L / 8233, 6SL7, 12AX7 and similar tubes. Low Rp tube are recommended though to have better bandwidth.
Output power of ED8000 single ended amplifier will be around 3W-4W, depending on what distortion figure you are looking at. It is a poor man's 2A3 SET. ED8000 is priced like a Chinese 2A3 replicas, and you get old school Telefunken quality (sound!)! This is definitely worth considering.
With high quality OPT (from us of course, LOL!) to go with (not difficult to make since primary impedance is only ~1K Ohm, no complex winding scheme is required), it should be awesome. Did I mention that it can be a good headphone driver too?
Important note: with high transconductance tubes such as ED8000, cathode bias is recommended to prevent runaway. I would use a CCS at the cathode to keep it locked in place.
Now, go build it and let us know how it goes! Let us know if you need the transformers to go with this wonderful ED8000 tube. We can supply a whole kit to you.
J&K Audio Design
10/3/2018
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