Tube of the Month: The EC8020

Hi!

So far tubes presented in this series were less known in the audio scene, cheaply and easily obtainable. Now it's time to also go to the other extreme and to present some of the rarer kind. I'll start with the tube many of you probably waited for, since I'm known for using it quite a lot. The Telefunken EC8020.

This is my all time favorite of the indirectly heated tubes. This tube has been developed in the 60ies by Telefunken. As far as I'm aware no other manufacturer ever made real EC8020s. There are currently manufactured tubes which are claimed to be close in parameters, namely the russian 6C45. While it might come close in terms of transconductance it is light years away in terms of stability of parameters, quality of manufacture and most important sound. The only tube which comes close is the WE437. But for me, the Telefunken EC8020 is the queen of the small signal high transconductance tubes.

The EC8020 comes with a noval 9 pin base. In contrast to other typical noval tubes like 12AX7, ECC88, 6DJ8 it has a much fatter glass bottle to contain the rather large plate structure. In fact it is a small power triode with a plate dissipation of 8W. Detailed technical data can be found here. The tube has a whopping transconductance of 65.000 micromhos (65 mS). The amplification factor is 55. This results in a plate resistance well below 1kOhms making the tube suitable for phono preamplification using transformer coupling. This is the main application for which I'm using this tube, transformercoupled LCR RIAA phonostages. The low plate resistance allows to drive a 600 Ohm LCR RIAA network with a transformer with a slight step down, preserving the majority of the gain. The EC8020 really shines in this application, enabling very low noise phono preamplifiers.

With the high plate dissipation the EC8020 can even be used as a power tube. A single stage, 1 tube per channel SE amp can be built with it. It is able to deliver up to 1.5W in such a circuit with a suitable 5k output transformer. As with all high transconductance tubes, careful layout and some precautions are necessary to keep it from self oscillations. I usually apply ferrite beads to the grid leads. With these measures it will work even with tiny input signals as those from a phono cartridge with very low noise and low microphonics. The tube has exceptional linearity as can be seen from the photos below which show the plate curves, taken from an actual tube with a curve tracer. The two shots are taken with different settings for the grid voltage steps.

Unfortunately the EC8020 has become very rare and is difficult to find. A good friend of mine once said, EC8020s are carved out of solid block of unobtainium. Even at current prices which are being asked for EC8020s, I think they are absolutely worth it. I have never seen original price lists from Telefunken, but a reputable source once told me that Telefunken sold these in the 1960ies for 80 Deutsch Marks to the industry in quantities. They probably have never been available to amateurs in single quantities back then.

Given the fact that they have been manufactured to very tight specs this seems to be realistic. The grids have very fine spacing. This required extreme care in the manufacturing process. The photos below show some details of the internals of the tube:

A truly magnificent piece of engineering. Given the tiny tolarances and grid spacing this can be considered as micro electronics.

I'm about to start construction of a custom LCR RIAA phonostage with this tube. I will document the assembly process in upcoming posts. The photo on the left shows a close up of a phono preamplifier based on this tube. This is my main phonostage which I use every day. I've been using EC8020s for a very long time and never had any reliability issues with it. It was classified as a long life tube by Telefunken. They guaranteed an average life of 10.000hrs.

Best regards

Thomas