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Wednesday, July 8, 2015

Tube of the Month : The 37


When I introduced the 27 in last months post, I mentioned that it was the first in the development of a series of small signal triodes which led up to the release of the 6SN7. This month I'd like to continue with this series of tubes and introduce the first descendant, the 37.

The 37 is very similar to the 27 and basically only differs in the heater specification. Also the construction is very much the same.

It shares the same UX5 base and pinout with the 27 and has the same dimensions. The internal structures are also very similar upon first look. In fact they can be hard to distinguish visually if the printing on the glass has faded. This happened to me in the post about the 27 and I mistakenly pictured a 37 tube. try to guess which one. Of course the difference would be immediately apparent when the tube is tested. The 37 was developed with a 6.3V heater which only consumes 0.3A current. But this is already where the differences end. All other electrical parameters are very close. The amplification factor is slightly boosted to 9.2 from the 27s mu of 9. This the 37 would drop right into a circuit designed for the 27 if the heater voltage is adapted. The plate curves are also very similar.

No surprises when comparing these to actual plate curves taken from a tube sample:

Here there different 37 triodes made by RCA:

These are identical internally and only differ in the printing/engraving of the base. The RCA Cunningham has a beautifully engraved base:

This one only has the Cunningham name printed on the base, but it's made by RCA:

RCA Victor with the Nipper besides the RCA logo:

These two with engraved base seem identical:

But apparently one was produced for the military with a military number engraved on the other side:

A close up to the internals of a RCA 37 shwowing the folded heater wire in the cathode sleeve:

Philco branded 37, probably made by Sylvania:

Contrary to the RCA above the heater wire of this one is wound in helix fashion:

Close up:

A close up of the top end:

Comparison of the top sides of a RCA and a Philco:

An earlier 37 in globe shape, Cunningham branded:

This one also has a ceramic tube as insulator inside the cathode. Similar as seen on globe shape 27s. While the heater wire looped back at the top and went back into the ceramic tube, there is only one run of heater wire in this one and it is wired back to the base with the thick rod.

A close up:

View of the top with Cunningham 'C':

These little globe 37s are simply gorgeous:

Next we have a very special 37 made by Arcturus in blue glass:

This little beauty is simply stunning.

A nice shiny plate is visible through the blue glass

Close up:

Similar heater arrangement as in the globe Cunningham. A single heater wire through the center:

returned back to the base outside of the plate:

Here the heater connection from the top to the base running beside the plate can be nicely seen:

Another angle:

37 printed in orange on the glass:

These came in beautiful boxes with the observatory motif:

The boxes are still sealed and have never been opened.

These boxes are cleverly done such that the tubes can be tested without breaking the seal:

Normally the corners would get broken to insert the tube into a test socket. But these boxes even had the corners intact so they are really untouched. To take photos I had to break the seal of one:

But it was worth it to get these foots taken in the moon light:

tube at the lake front:

Moon light reflecting in the water behind the tube:

Another shot:

The 37 has one drawback however. The heater glow is very dull and barely visible even in the dark, here a ST shape 37 in operation:

Close up:

The heater at the bottom end:

The globe version even glows less:

It takes a long exposure time on the camera and a dark environment to make the glow visible:

Even less light comes through the blue glass of the Arcturus:

So not a tube for glow fetishists. But who could resist the beauty of this cool Arcturus master piece ?

Best regards



  1. Really interesting -- how common was the use of coloured glass in vacuum tubes..?

    1. Hi!
      As far as I know only Arcturus used blue glass in the old days.
      Some recent manufacturers might offer this as well. I think I once saw some blue glass tubes made by Ei

      Best regards


  2. I think it's probably JJ you are thinking of. They briefly did a line of blue tubes.

    1. Yes, you are right, it was JJ... Thanks... Thomas

  3. AVVT did a few tubes in blue glass as well. Additionally there was a manufacturer of 01/01A tubes in the 1920s that did red and blue colors (they also did a clear-glass version, so often times old radio collectors who use 01s/01As will put the red-white-blue combo of tubes in the radio, mostly for visual effect.

  4. Very nice review Thomas !
    Best regards Claudio, IT