Tube of the Month: The D3a

Hi!

After the series about transmitting tubes from the 800s during the last 4 months, I will present a very different and more modern tube today: The D3a pentode.

I'm constantly asked for suitable alternatives to the hard to find EC8020 for use in transformer coupled LCR phonostages. Various pentodes work very well in this phono stage topology when triode connected. I already presented the E55L as an alternative which I used in two different LCR phono stage builds. The frist one can be seen here and another one here.

The D3a is a quite modern tube developed for wide band amplifiers for telecommunication applications. It has a nine pin miniature base. The pinout can be seen on the left. Triode connected it has a very high transconductance almost approaching that of the EC8020. Amplification factor is 77 at a plate resistance of 1900 Ohms. See the Telefunken datasheet for all electrical parameters also for the pentode connection. The low plate resistance makes this tube very suitable for transformer coupling. With a step down transformer, for example Lundahl LL1692A in 3.5:1 configuration or LL1660 in 4.5:1 the tube is perfect to drive a 600 Ohm LCR RIAA network. In such a configuration there would even be some gain left after the EQ network. With a step up transformer after the RIAA, for example Lundahl LL7903 in 1:4 confguration this could be boosted up for further amplification in a second D3a stage using another step down transformer for low output impedance. With a Lundahl LL2745 wired 2.8:1 at the output such a 2 stage phono preamplifier would have a MM gain of about 46 dB with an output impedance of less than 300 Ohms. This gain can be brought up to suitable levels for moving coil cartridges with a MC step up transformer.

Although signal levels are very small in a phonostage we want the amplification device to be as linear as possible. These are the plate curves for the triode connection from the datasheet:

This looks promising! Let's see how this keeps up with reality. Here are the plate curves taken from an actual tube with a curve tracer. The scale is 5mA and 50V per step, 0.5V grid voltage spacing:

Looks perfect for audio applications.

The D3a was made by various european manufacturers: Telefunken, Valvo, Siemens, Philips. Although it has an american designation, the industrial type 7721, I'm not aware of any US based company who made this tube.

A Siemens D3a:

This tube has been manufactured by Siemens in the former western part of Germany. It has the characteristic pinched glass around the electrode structure.

Siemens purchased tubes from the former eastern Germany in the 1970ies. These D3a are probably relabelled, but I'm not 100% certain:

A comparison between pinched and straight tubes:

The boxes of the older Siemens tubes had a seal to prove that the tube is unused:

A closer look at the tubes:

Zooming into the internal structures:

This photo gives already a hint at the delicate grid structure inside. Another view.

In order to get a better view of the internals, we have to take a tube apart. First the glass has to be taken off:

Now the details are better visible:

A view of the top showing the ring which held the getter material before it got flashed during the manufacturing process:

This ring is often wrongly referred to as the getter, while it only held the getter material before it got flashed and condensed as the silvery mirror on the inside of the glass. After the flashing of the getter this ring has no function any more. It especially has no influence on the sound of the tube as some audiophiles seem to believe.

The ring and upper mica disc removed:

This gives a nice view of the heater wire inside the cathode sleeve:

The heater in the cathode, as seen from the other side:

The base removed:

This shows the details of the internals nicely. The plate actually consist of two pieces at each side of the grids/cathode assembly. Grid number three actually consist of two beam forming plates, the silvery frames between grids and plates.

A close up of the grids:

This gives already a hint of the control grid which is visible below the screen. To get an even better view we must take the tube further appart.

Removal of one of the mica discs which aligns the electrodes:

Removing the anode to give a good view to the beam forming plate:

With the beam forming plates also taken away we see the grids and cathode. of course the alignment is screwed up now without the mica holding everything in place:

Removing the screen grid:

The ruler gives a feel of the scale. Now with the actual control grid is exposed and we can have a close look at it:

Removing the cathode is impossible without destroying those extremely thin grid wires:

Another zoom in showing the tiny control grid wires placed on the ruler:

The black lines on the ruler are 1 millimeter apart!

Now some pictures of a D3A when it's lit up:

Closer up:

I hope you enjoyed this months tube presentation. A LCR phonostage with the D3A is already in the works. Stay tuned!

Best regards

Thomas