In the last post about the European Triode Festival, I showed a photo of the two very old directly heated triodes which I bought at the auction in Denmark. After returning from the ETF I tried to gather some information about them and it turns out that these are very interesting tubes and very little is known about them. So let me share the little information I have about the Spheria.
What a cool name, no? The name seems totally out of time for a tube which was manufactured about 90 years ago. Could be a name for a product of our time.
The Spheria is a small directly heated triode. It has a European 4 Pin base. The base connections are shown at the left. This base was the european counterpart of the american UX4. All information I found indicated that it is an equivalent or very similar to the Telefunken RE074 or Philips Miniwatt A409. These have a filament voltage of 4 at a very low current of only 60mA. If the tube is similar to those it should have a mu of about 10 with a plate resistance of 10k which would mean a transconductance of 1mS (1000 micromohs). So I measured those values to see if they are really the same. I started with a low filament voltage of 3V and the power supply set to current limiting. Then increased the voltage until I reached 50mA. The plate curves would saturate quite early with filament voltages below 3.5V. At 3.8V they start to look good. and the filament current settled at 60mA. The 4V/60mA filament spec seems to be correct. The the other values however are quite different from the RE074 and A409. I measured an amplification factor very close to 7 coupled with a plate resistance of 15kOhms and about 0.5mS transconductance. These values turn out to be very close to those of the Philips A406. Below the set of plate curves which I generated:
Nicely linear curves as expected from a directly heated triode! Now let's have a look at some photo shots of this beauty:
Isn't that gorgeous?
All information I could find about the origins of these tubes was that they were made in Belgium in the 1920ies. I talked to many old tube heads and most never even heard of the Spheria.
No markings of manufacturer or information about the country of origin can be found on the tubes.
And they came without any packaging which probably disintegrated decades ago.
After some digging around and talking to people I assume that the tube was probably made in a small batch and was most likely a custom development for a specific application.
The manufacturer was most likely the company M.B.L.E. in Belgium which started as a light bulb company and later produced tubes under the Mazda and Adzam brands.
Let's try to get a glimpse of the internal structure.
As in many other early triodes the internals are mounted horizontally rather than vertically as in more modern tubes.
The plate is cylindrical and the grid is a nicely wound spiral inside
The filament consists of a single very thin wire which is connected and supported at both sides. Theoretically the perfect arrangement. Let's see how the tube looks with the filament lit up.
As can be seen it has a tungsten filament which is most likely thoriated.
View from the side: