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Friday, March 17, 2017

Tube of the Month : The UX112A


After the presentation of the rather modern tube last month, I will take you back all the way to the 1920ies today with the presentation of a very early triode, the UX 112-A.

This tube was introduced as detector and amplifier triode. It was meant as an improved version of the UX201A.

The UX112A shares the same UX4 socket and pinout with the UX201A as shown on the left. They have almost the same amplification factor but the 112A has about twice the transconductance which results in half the plate resistance. So definitely a major improvement in specifications. This was achieved by changing the thoriated tungsten filament to an oxide coated one. Filament voltage and current stayed the same (5V, 0.25A). In most applications the UX112A could simply drop in to a socket meant for the UX201A. See the Cunningham data sheet for all technical parameters of this tube. The UX201A gained some popularity recently and is used in line preamplifiers. I also experimented with it and consider it as a nice sound processing device. I have not really done much with the UX112A yet, but as mentioned a circuit could easily be designed such that it accepts both tubes which would enable to compare the specific differences between the thoriated tungsten and oxide coated filaments. The 112A definitely has nice plate curves:

While the UX201A was made in large numbers and can still be found rather easily, Less UX112A tubes are around nowadays, but still not impossible to find. Here some 112As from my stock.

Here we have a beautiful Cunningham CX-112-A.

Nice shiny nickel plate.

Unlike the UX201A the glass bulb is not almost entirely covered with getter so the internals are usually visible from one side while the other is completely mirrored:

Some more views:

RCA Radiotron UX-112-A:

Some detail close ups:

Another Radiotron with different construction:

Later Versions by RCA / Cunningham:

These dropped the 'UX' prefix.

Nicely sealed box which allowed testing of the tube without breaking of the seal.

These already have the more modern ST-shape.

Later versions dropped the 3 digit naming and were simply dubbed 12A. Here an example from Sylvania:

National Union 12A:

Some close ups of the plate structure:

A JAN (Joint Army Navy) NU 12A/112A made for the military:

An older National Union in globe shape:

Beautiful coloured box and gorgeous globe shape tube.

Nicely engraved base.

The top:

Side views:

The oxide coated filament of the UX112A only shows a rather dim glow:

With the room lights off:

That's all for this beautiful triode. I hope you enjoyed the journey back to the 1920ies.

Best regards



  1. It is nice to see this wonderful tube get its due. People forget how tough they can be. This mechanical engineer loves seeing how they kept changing the plate supports on the tubes, constantly trying new ideas. I had a Bosch 112A shipped to me off Ebay in an envelope with no padding. It somehow made it across the USA no worse for wear.

  2. What is everyone's favorite 112A tube in terms of sonics?