Tube of the Month : The 31

Hi!

After the presentation of the two 'twin preamplifiers' in recent posts, the choice for the tube of this month is probably no surprise : The 31.

This is a small directly heated power amplifier triode developed for the use in battery operated receivers. Such tubes are also often referred to as battery tubes.

The 31 shares the same pinout as most directly heated triodes which use the UX4 base. But it comes in a much smaller bottle. The coke bottle shaped version has the ST-12 size. This is the same size as the 27 for example. Being designed for battery operation means rather lowish plate voltage and more important very low filament power. This required the development of very efficient filaments. In the 31 the filament operates at 2V and only consumes a tiny 130mA. This translates to just 260mW filament power. Compare this to other types which need more than an Ampere and run on higher voltage. The typical plate voltage for this triode is 135V and the maximum is at 180V. In typical operation it only draws 8 to 10mA. Of course not much power output can be expected when only so little is put in. Still it can produce a whopping 375 milliwatts. Now that would be something to power hyper efficient horn speakers.
Anybody out there who is man enough to join the bragging contest of his audio buddies with that kind of power rating? I am prepared to take the challenge to build a suitable power amp around this tube. The rather highish plate resistance would require an output transformer in the 15k impedance range for single ended operation. Obviously it does not need a huge voltage swing to drive it to it's maximum power output, maybe the 30, another directly heated battery tube would be just right for the job. But that will be another story maybe. What I have used it for was in a line preamplifier only and there it worked extremely well as can be read in the previous post. I could also see this tube to work well in a headphone amplifier or as driver for other small power triodes, like the 71A or even the 45. Many possible uses for this little beauty! As was expected from it's excellent performance in the preamp it has exceptionally linear plate curves.  Very typical of a directly heated triode. See the set of curves from the data sheet below:

And as always an actual measurement:

As can be seen in the first photo, the 31 was made in 3 different glass shapes. The early tubes, named UX231 came in the usual globe shape but also in a tubular bulb. Later the tubes moved to the more modern coke bottle.

Here a beautiful sample, made by RCA for Cunningham and named CX-331:

Globe shaped tube made by National Union:

Beautiful old box!

Engraved base:

A later version by National Union:

Triad:

They named it T-231

Nice warranty sticker on the tube still intact:

Although it says T-231 on the box, there is a '31' printed on the glass:

Let's have a closer look at some details of the globe tubes:

Two different globe tubes in comparison:

The one of the right has a little glass stem inside with rods molten into the glass which in turn keep the right distance between grid and plate and one which holds the filament at the top:

Close ups:

The other one has a small mica disc at the top which does this job:

A globe shape 231 from Sylvania:

Some close ups showing the construction details:

The other earlier shape is a tubular glass, here in comparison with the globes:

Two different tubular 231 tubes:

Some close ups:

Here we also have the different variants of support structures on the top.

Next some more modern ST-shape 31 tubes:

Made by RCA

Some different versions

Sylvania:

The top:

Another version made by Sylvania:

This Philco branded 31 is still in a sealed box, so I am not opening it:

Now let's have a look at the glow of the 31:

As expected from the low filament power, the glow is quite dull.

They are also called dull emitter tubes for this reason.

I hope you enjoyed the first tube of the month in 2016. Stay tuned for many more interesting tubes to be presented.

Best regards

Thomas