Tube of the Month : The 53

Hi!

This month I am presenting a tube which I had in mind for this series since a long time. Whenever I came across my small stash of these I told myself to write about them. Now the time has come, here is the 53.

The 53 is electrically and mechanically equivalent to the 6A6 except it has a 2.5V heater instead of 6.3V.

It comes with the same 7 pin (UX7) base as the 6A6 and has the same pinout which is shown on the left. It was designed for the same purpose, a twin triode for Class B output stages or as Class A driver stage with both triodes wired in parallel. As mentioned it has a 2.5V heater which consumes 2A of current. Since the 6A6 is a UX7 equivalent of the 6N7, the 53 can also be used in applications of the 6N7. Readers of my blog know that I am quite fond of the 6N7 and 6A6 for use as drivers for small triodes. Refer to the tube of the month posts of these two types for further technical details or to the RCA data sheet. Like the 6A6 the 53 was only made in the small shoulder type (coke
bottle) while the 6N7 was also made in straight sided glass and as metal type. I have never actually used the 53 but there is no reason why it shouldn't perform as well as the 6A6 or 6N7. Probably due to it's ancient heater voltage spec of 2.5V it is largely ignored by audio amplifier designers who prefer to stick to the standard 6.3V types. I am guilty myself for not using it for the very same reason. So it is still available at modest prices and easy to find.  Since I never used it in any of my designs I only acquired a small stock of them. As usual, let's have a look at the plate curves in the data sheet and at actual curves taken by a curve tracer. Most data sheets I have seen don't even bother to show a set of plate curves for the 53 but only have a common diagram for all the three types:

 No surprises there, not distinguishable from either the 6N7 or 6A6.

 Let's have a look at some samples, here we have the 53 made by Sylvania.

 Nice green boxes which allow testing of the tube without breaking the seal.

The UX7 base:

The old leaf logo on the base:

The top view shows the cooling fins attached to the grid rods.

These allow extra cooling of the grids which are driven positive in Class B circuits.

Some details:

A close up showing the heater wires going into the cathodes:

Philco branded 53:

RCA:

 Here we have an interesting one which I have never seen before:

 The box is marked 'sub standard' and also has a ceiling price printed on the box.

 A little notice inside the box explains this. Apparently these are tubes which would not pass the regular quality tests but would still be good in many applications. Instead of discarding them RCA sold them at a lower price.

There is also a 'sub standard' marking on the tube base:

I tested the tube and it came out fine with maybe the transconductance a bit low but still within acceptable limits.

Next we have a Ken-Rad 53:

Like many Ken-Rad tubes this also has the characteristic black coating on the glass.

Raytheon:

And lastly National Union:

These have the getter applied in the top.

Like the 6A6 the 53 does not give off much light when powered up:

I hope you enjoyed this first tube of the month post of 2018!

Best regards

Thomas