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Thursday, June 25, 2015

Tube of the Month : The 27


This months tube is the indirectly heated cousin of the 26 which we saw in May. This is one of the very first indirectly heated triodes which saw a wider commercial use and is the ancestor of a long row of low to medium mu triodes which followed after it. Let me introduce the 27.

Like the 26, the 27 was introduced in the golden twenties of the last century. Initially named UY-227 which was later changed to the two digit designation.

The 27 has a UX5 base. Compared to the directly heated 26 it has an additional pin for the cathode. The tube was developed for use as detector and amplifier in radio receivers. The introduction of the indirectly heated cathode made it easier to heat the tubes with AC and to obtain hum free operation. The heater operates at 2.5V and consumes 1.75A. The amplification factor of 9 classifies it as low mu triode. The plate resistance is 9 kOhms which translates to a transconductance of 1000 micromohs. The 27 is popular among amplifier builders for use in line level preamps or as drivers for small output triodes. It is perfectly usable to drive a 45 output tube. As mentioned above the 27 is the first tube in a long row of small signal triodes which ultimately led to the 6SN7. The availability of the tube is still good even in globe shape.
Many years ago I used it with an interstage transformer to drive a 45 and it worked well in this application. I have also built a line stage with it. These trials have been more than 10 years ago and it is probably time to use this tube again in a project. It's characteristics are not too far off those of the 26. That's why I called it it's indirectly heated cousin. If you are thinking about building a line stage but want to avoid the effort to build an elaborate filament supply which is needed to get a 26 quiet, the 27 might be the right choice. Although it needs a rather hefty heater current, this is still manageable. It can be heated from AC without any hum if some care is taken in the layout and wiring of the heaters. As driver in a power amp it could be used either with transformer, LC or RC coupling. But I would restrict it's use to drive small triodes like the 45 or maybe a 2A3. In a line stage I would prefer to use it with a step down line output transformer in order to achieve a low output impedance. For complete technical details please refer to the RCA data sheet. As always let's have a look at the plate curves:

Not quite as linear as the curves of directly heated triodes but still very good which makes the tube well usable in audio applications. The curves of a tube on the tracer confirms what we see in the data sheet:

Now let's have a look at some tube samples. Here a beautiful ST-shape 27 from RCA Cunningham:

The internals are secured with mica spacers inside the glass which have a distinctive cross shape on the top:

Lafayette branded 27s:

National Union:


The internals:

A close Up:

27 made by Arcturus:

These tubes have holes punched into the plates:

Such punched plates have a reputation for improved sound. I have not analysed this myself but since I have such tubes in stock it might be worthwhile to check this some day. But there is a plate structure which has an even better reputation to contribute to good sound, the mesh plates as seen in the globe type UY-227 tubes. here two samples from RCA with different construction style.

Close ups to the mesh structure:

A Philco branded mesh plate 27 :



There was a sticker with the Sylvania logo on the top which unfortunately is almost peeled off:

When I received this Tung-Sol TS227 box, I had high hopes for a pristine NOS-NIB tube:

But inside was a RCA. It is in good condition though:

Cunningham C327:

Another one with a sticker:

Some close ups to the mesh plate:

In the photo above the ceramic insert in the cathode is visible. The heater is threaded through this ceramic tube which provides insulation to the cathode. A closer look:

Due to this ceramic insulator these old mesh plate 27s have an extremely slow warm up.

Here an unbranded 227:

Although the electrode structure is slanted sideways, the tube operates perfectly fine. This is often seen in old mesh plate 227 tubes.

Here is an interesting find, a batch of Sonatron branded tubes which apparently was used for life time tests:

The sticker on the boxes indicates this use:

Also a L.T. etched into the bases:

Not sure if these tubes have actually been subjected to life time tests. They all still measure good.

Isn't his a beauty?

They also have the mesh plates:

Here a batch of tubes with a brand name I have not seen before:

Unfortunately two of these tubes have distorted plate curves. So let's open one up to see what might be the reason.

Now we can have a much better look at the woven mesh:

Taking the electrodes apart:

And here we see the reason for the problem. The grid has been damaged:

It seems that the grid saw some excessive current maybe due to a fault in the amp or receiver it was used in, or maybe it got misaligned and touched the plate.

Here we see nicely how the heater wire is threaded through the ceramic tube:

Next my most beautiful samples of 27 type tubes. DeForest 427:

Contrary to the other globe tubes, these have punched plates similar to the Arcturus.

These came with an extensive sheet with specs and technical information:

Even a complete circuit example:

Let's compare how the different plate types show off the glow of the tubes in operation. First a ST shape tube with solid plate:

As expected the glow of the cathode can be seen on top and on the bottom. This tube already has the more modern helix wound and coated heater wire as can be seen in this close up:

The ST shape tube with punched plate gives off some more of the glow through the holes in the plate:

A close up:

This tube has the ceramic tube with the heater wire threaded through it. Another view:

The bright spot on the top is the bare heater wire which comes out of the ceramic tube and loops back in as seen on the photo of the heater assembly.

The DeForest globe tube with punched plate:

And lastly the mesh plate globe tube:

Close Up:

Another view:

A globe with slightly different construction style:

The top:

As mentioned above the 27 is the first in a row of small signal tubes which culminated in the 6SN7. Both ends of this series of tubes have been covered now. More descendants of the 27 will be presented in upcoming tube of the month posts. Stay tuned!

Best regards



  1. Nice, I have a set of the blue glass Arcturus punched plates for my preamp, though I've been saving them while waiting for a new amp. Do you have plans for a tube comparison?

    1. Hi!
      Maybe later this year I will build a preamp with the 27

  2. Is the mesh on some of these tubes the plate or an electrostatic shield?

  3. In your opinion/experience, what would happen if you grounded the grid and made the cathode negative enough to accelerate the electron past the grid to the plate? If the plate was connected to a multimeter, do you think a relatively high voltage could be achieved this way?

    1. Not sure what exactly you would want to achieve. But it sounds simple enough to just try it out

  4. I have a theory that it could be used similar to a van de graaff generator. Instead of ionized gasses depositing charge on a dome, we'd have emitted electrons depositing on the outer surface of a concentric anode. I am planning to purchase some low power tube and test it out; but, before I did that I figured you'd have a good opinion on the matter.

    1. I see... But tubes have a limited voltage rating from plate to the other electrodes. If you want to try it, may be use a tube which has the plate brought to a top cap.

    2. Oh man, thank you for that. Do you have any suggestions?

    3. Maybe a Thyratron would be right for such experiments. 354A for example. Or use a TV sweep tube like the 6CB5A (see here: http://vinylsavor.blogspot.de/2013/01/tube-of-month-6cb5a-revisited.html )

  5. Thank you very much. Ill let you know how it goes if you're interested.