Tube of the Month: The 6A6

Hi!

One of my favorite indirectly heated drivers for small output tubes is the 6N7 which was already presented as tube of the month almost 2 years ago. There is an electrically equivalent tube which is a predecessor of the 6N7. I have used it in the past as drivers in 45 amps. It's time that it get's it's own place in the Tube of the Month series. Here is the 6A6.

With it's gain factor of 35 it is just right for a 2 stage amp with useful input sensitivity. Plate resistance is low enough to drive small triodes like the 45 with ample head room.

It is actually a double triode with a common cathode. It was designed for Class B push pull output stages. With grids and plates wired in parallel it is great as Class A driver. It has a 6.3V heater which consumes 0.8A. Some might be scared away by the fact that it is a double triode. There is a misbelief that paralleling triode halves creates some smear or other loss of resolution. Not really a concern with this tube in my experience. Please see the 6A6 datasheet for all electrical parameters. The pinout of the 6A6 can be seen on the left. It has a medium 7-pin base which is also referred to as UX7. Two of the pins have a slightly larger diameter. These are the heater pins. Since there is no key in the center of the base as with octal tubes, the heater pins can be used to make sure the tube is inserted into the socket with correct orientation. The photo below shows the base with indication of the individual pins.

Although this is a less common base, availability of sockets is good. They are still produced. Below a photo of a chinese made ceramic UX7 socket.

As always we are mainly concerned about the linearity of the tube. Let's see what the datasheet curves tell us:

Let's see how this looks on a curve tracer with a different scale:

Looks like anything a tube audio designer can wish for!

A tube which is quite overlooked. NOS availability is good and prices are low. While the prices for 6N7, especially those in ST glass rise, the 6A6 stays in the tube dealers warehouses widely ignored. As far as I'm aware the 6A6 was only made in ST shape. No straight sided glass or steel bottles. Also no globes as far as I'm aware. It seems that the 6A6 was only manufactured over a rather limited time frame until it got replaced by the 6N7. The size of the bottle is actually quite large for such a tube. It has the same dimensions as a ST shaped 45 triode. Apparently the 6A6 was produced by pretty much every major vacuum tube manufacturer. All of the big ones: RCA, GE, Westinghouse, Sylvania, National Union. There are also some lesser known brands: Cosley, Delco, Arcturus, and the usual rebranders offered it as well. Below just a small selection of 6A6 tube boxes.

Some tubes of various brands:

A beautiful Ken-Rad 6A6 with blackened glass which was often done by this manufacturer:

The top of the Ken-Rad tube:

A Westinghouse:

This is a 6A6 from my collection which came in a generic white box. No brand marking visible on the tube. It has a dark brown base:

The top construction is a bit different from others. It only has a tiny mica strip for alignment of the electrodes and large springs which hold the entire structure:

Now let's have a closer look at the internals by dissecting a tube. We will examine this General Electric 6A6:

First the glass has to go off:

Now we can see the internals much better. Here a close up of the topside which shows the grid cooling fins:

If the tube is used in Class B push pull operation which can push the grids into the positive region there is some grid dissipation. These fins help to cool the grids a bit. Let's take the upper mica disc off so we can actually see the grid and cathode structure inside the plates:

One of the plates removed:

A closer look at the heater as it enters the cathode sleeve:

A close up of the grid structure with a scale in the background. The black vertical lines are 1mm apart:

The cathode and grid:

Heater removed from the cathode:

Maybe not as impressive as the D3a which I showed last month, or the EC8020, but still a marvelous piece of vacum tube engineering.

Finally some photos of a 6A6 lit up:

A close up of the heater:

The top side:

A close up of the cathode, through the hole in the mica, the grid is visible:

I hope you enjoyed the second Tube of the Month presentation of 2013. Stay tuned for more articles on interesting tubes during the coming months.

Best regards

Thomas