The Siemens Aa Weitverkehrsröhre – DHT Tubes for the Connoisseur

I would like to bring together some posts that rarely come together normally. So I already started the summer brake some weeks ago with a series of portraits of my major female singers, I will certainly continue this series in the upcoming time. Today I would like begin to address the posts about tubes. Before I introduce design concepts of amplifiers in the coming months, I want to begin to present individual tubes in functional groups. I want to start with my beloved directly heated triodes, briefly called DHT. I will start with those types to be useful in preamp circuits.

For the past 10 years, accelerated by the communication in the internet, some forerunners like Kevin Kennedy, Jim de Kort and also in this country Thomas Mayer widened existing limitations in preamp circuits with this sort of preamp designs. In many cases early U.S. types as the UX-type 26, 45 or 801 are used and these concepts have now made school and have been copied countless times by diy builders. Formerly used in radio receivers mostly in the 1930ties to 1940ties, these tubes have been once widely common. Today, specially in Europe these tubes are hard to find in good qualities.

German Post tubes are originally called "Weitverkehrsröhren" or "Behördenröhren".
From left the original Siemens Aa with tip, in the middle the Valvo Aa and at right the Neuhaus Aa with metal socket.

I would like to present some less common implemented German tube types. The so called "Weitverkehrsröhren" or "Behördenröhren" are normally known as post tubes. The are very early first generation tube designs made by Siemens in the 1920ties called A, B, C, etc. The second generation of this tubes added a second a to the name, so they were called Aa, Ba, Ca, etc.. These tubes have been produced from the late 1920ties over all the wartime into the early 1950ties for official use in telephone applications in both German countries. In the 1950ties the German Post developed the C3-type tubes as indirect heated multi grid successors, more about these phenomenal tubes in some weeks time. The post tubes have been always produced to match extremely high specifications of the German post administration. Best materials were used by companies like Siemens, Valvo, Neuhaus and others to fulfill a general service lifetime of 10000 hours professional use, a very high standard in DHT times, compared with any other brands. The tubes have a specially designed five pin post socket. They were equipped with a thoriated tungsten filaments of different voltages (3,5 to 3,8 V) acting as cathode. This old design of tubes is known for its sonic advantages, in particular their better linearity and their more natural soundstage. The most people do know directly heated triodes mainly as power tubes in modern retro single ended power amplifiers. Here the WE300b and its copies are a very well known DHTs, widely accepted in the hifi community for its unique sound capabilities.

Today I want to concentrate on the Aa tube as one of the best sounding line amplifier devices, when well implanted. For this reason I would suggest a amplifier design with anode chokes instead of a matching output transformer (the first choice with lots of other DHT's). With a nominal impedance of 30 kilo ohms, it is almost impossible to find a output transformer matching the tube for low impedance output of around 600 ohms. I did use the Aa with Lundahl LL1667 anode chokes, configured with a air gap for 5mA anode draw (the tube draws 3mA). In this configuration it will have an inductance of 600 henries, exactly the amount which makes a flawless low frequency response down till 30 hz possible. Together with very good coupling caps like copper or silver foil paper types, this constellation is truly amazing in every respect and outperforms everything I have heard. 

For the anode supply I use a Parmeko mains transformer with tube rectified output, followed by a double choke/oil capacitor filter rail and finished with a three tube regulated output stage. For the heater supply I use Rod Colemans modules. Thomas Mayer here sets the pace with completely passive filtered heater supplies. I am sure this might give another further improvement, if you have the space and money.
This Aa preamplifier gives the most open soundstage I have ever heard from a line preamp. The gap to classic amplifier designs (even to well named and established makes like AN, AR or similar well designed preamps) with indirectly heated tubes is tremendous. The Aa-preamp is so much more refined, with a lot better and wider, almost three-dimensional soundstage, meticulously spread harmonies and finest dynamic details, that I personally will not find back to later IDH-Tubes. It is a perfect solution for mainly line sourced amplifier chains, like digital streaming or digital discs. It brings a unbelievable naturalness back to digital sources, that they can par with very good analogue players. For professional marketed products these designs are not manageable, since the outlay is to expensive to make real profit and good tubes in countable amounts are not in the market for the same reason. So this delicacy is reserved for a solely market of handmade luxury or for the crafted diy aficionado. A parable which covers sooner or later any quality mass product in our globalized world.

On top the Neuhaus Aa with ceramic spacer, underneath the Valvo with ordinary mica spacer.

The Aa has been made by Siemens, Valvo and later on after wartime as well by East German Neuhaus tube factory for their telephone system. All these tubes sound quite different in some aspects, but all will show very similar characteristics. All Aa's are quite microphonic, the internal construction will show a big influence on its sensitivity to mechanical movement. The original Siemens tubes are very complex designed and are very rigid made, so they are favored by some people like DHTRob. They have a glass arm as inside support which carries the heater, the plate and the grid. If you look inside these tubes you will notice that they are made like art pieces. Today they are quite rare and therefor expensive. The more common variant is the weaker Valvo type, I like this tubes a lot as my standard type, but it needs mechanical decoupling from the chassis of the amp. As well very rare is the Neuhaus type, with ceramic supports and metal socket. Where the Valvo shows already the typical mica spacers, which became standard with the most later made tubes. Both tubes are very similar in fidelity.

The masterpiece of all Aa's, the tip-type Siemens Aa with glass support.

Read on soon about line amps using these tubes and their exquisite successors as later IDH-Tubes for hifi, Volker