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Friday, May 25, 2018

News from the ELROG Tube Factory 11


Today I am sharing some insights into vacuum tube production.

You will see some of the steps of the making of the 845 triode.
We start from the bottom with the glass stem with the wires which make the connections from the inside of the tube to the outside. We make these in house from the same glass as the bulbs.

For this we have special graphite moulds with holes to keep the through wires in place.

These get filled with an exact amount of glass.

The lid also has holes to keep the wires tightly in place.

Then they go into the oven over night at a predetermined temperature.

The glass melts and takes the desired shape.

The finished stem:

Then the wires are cut to the exact length and the mounting of the internals of the tube can begin.

Above the main support rods are attached to the stem. The different mica discs prepared for assembly:

Mica discs and support springs added.

Now the electrodes can be mounted. First the filament wire needs to be cut to length and shaped into an M:

The filament mounted to the assembly:

Next the grid wires, here we see the making of a grid:

A freshly wound grid:

The exess wire is cut off before the grid is added to the tube assembly:

The tube assembly with filament and grid in place:

Now the anode:

Almost a complete tube already, but the top mica discs and support springs for the filaments still need to be added.

Now the only things missing are the getter rings...

... and serial number:

Next the complete the assembly is washed with purified water and special detergents. Since the water purity required for this is not readily available we have or own water purifying station for this.

Finished and washed tube assemblies waiting to be put into glass bulbs:

The pre shaped glass bulb with evacuation stem and our signature black coating:

Now fire comes into play:

The sealing process requires close attention to the temperature and a lot of experience with glass.

Too hot and the glass drips off.

Too cold and the seal will not be air tight.

Here we see the process in motion:

From a different angle:

The freshly sealed tubes are stored in pre heated holders so they can cool down in a controlled fashion.

After cooling the tubes get evacuated.

Each tube is heated with an RF coil to drive out gasses:

Here we see the rings holding the getter material which is not activated yet:

After evacuation is complete the top stem gets sealed and the getter gets fired:

As such the tube has no emission yet and needs to go through a forming process during which a one atom thick layer of thorium is formed on the filament surface.

This is a procedure with various steps of applying different voltages to filament and plate.

During this process the tube comes to life. Once this is done the base is added:

Then the tubes go onto the burn in rack for 24 hrs:

After that they go through final test and get the printing on the glass.

I hope you enjoyed this little journey through the process of tube making!

Best regards



  1. Hallo Thomas,...toll mal zu sehen wieviel Arbeitsschritte nötig sind eine Röhre entstehen zu lassen. Die Vorwärmöfen kenne ich aus der Zahntechnik.
    Wir verwenden zum Brennen der zahnkeramischen Massen spezielle Vaccuum Öfen die auf 1 Grad genau geeicht sind. LG

  2. Thank you very much for this detailed and interesting report, and above all, for the incredible work you do with it. I guessed it, but did not know it. Best regards. Frank from Berlin

  3. Amazing! great detail and high quality tubes

  4. Wow, Never thought about how tubes Are manufactured before. Very impressive and detailed Report.
    Good, that You keep The craft alive.
    I Hope to Be able to listen to them, soon.
    Chris from Fulda

  5. Impressive! Can you explain the burn in circuit and why a period of 24 hours was chosen versus a shorter or longer time?

    1. Hi! During burn in the tubes get pushed very hard to drive out any residual gas (which then is captured by the getter) and fully activate the filament. 24hrs proved to be the right amount of time to get the job done. The circuit is very simple it applies 1000V to the plate and sets bias for about 90ma plate current