Tube of the Month : The 6J5

Hi!

Today we will have a close look at an indirectly heated medium mu triode, the metal type 6J5 and it's glass equivalents 6J5G and 6J5GT.

The electrical characteristics of the 6J5 are identical to the ubiquitous 6SN7. It only contains a single triode instead of two as in the 6SN7. The 6SN7 is very popular among amplifier builders and also found it's way into many commercial designs. There are good reasons for it's popularity. It is very linear and has a decently low plate resistance making it suitable as driver in power amps. Pretty much everybody who builds amplifiers at some point used or considered the 6SN7. Hence it got quite costly. Therefor it is surprising that the electrically identical 6J5 seems not to be very popular and is still available at low prices.

The 6J5 uses an octal base. The pinout is pictured on the left. It has an amplification factor of 20 which classifies it as medium mu triode. The plate resistance is about 7kOhm which makes this tube usable with transformer coupling. Of course it is equally well usable LC or RC coupled or as a cathode follower. It is very often used in the famous and once popular SRPP and mu-follower circuits. The complete datasheet can be found here. Although it is a very nice tube, I rarely find it to match my needs. As a driver tube in a power amp it has just too little gain for a 2 stage design. Using two of them  cascaded (or two halves of a 6SN7) provides way too much gain. That's why I prefer tubes like 6N7 as drivers for small output tubes. For large output tubes like 300B or even 211 where the 6N7 has too high of a drive impedance, the 6J5 is not much better either. While still ok for a 300B maybe, I would not consider it for the large transmitting tubes.

Let's have a look at the plate curves. This is what the datasheet claims:

This looks very goo,d let's see how reality holds up to this. Here the curves of an actual 6J5GT tube taken with a curve tracer:

A fine tube, which deserves more use! That's why I will be using it in a stand alone phono preamplifier, which is based on the phono section of my Octal preamplifier. It will replace the 6N7 in that circuit for getting a lower output impedance to make that phono section usable stand alone.

There is another good use for the 6J5. In power amps which use both halves of the 6N7 paralleled, a 6J5 can be used without modifications. Compare the two pin outs:

The heaters are brought out on pins 2 and 7 on both of them. Both have the cathodes on pin 8. And since both are available as metal types, pin 1 is either connected to the metal shell or unused. In a socket wired for the 6N7, pins 4 and 5 would be connected together for the input signal. The 6J5 has the grid on pin 5 and pin 4 is unused. The plates of the 6N7 are on pins 3 and 6 which again would be tied together. The 6J5 has the plate on pin 3 and pin 6 is unused. Perfect! That means the 6J5 can drop into a socket wired for paralleled 6N7 without any changes. It would also bias up correctly in a circuit for the 6N7. It would only have less gain. This can be helpful in systems with too much gain (a common problem). I have recommended this substitute to users of amps from me which use the 6N7 as driver when they had a preamp with too much gain.

Now let's have a closer look at the different versions of 6J5s. Let's start with the metal shell variety:

And some close ups. This Sylvania has a nice shiny black:

The General Electric:

Two varaints of RCA:

The 6J5GT is a glass version, the GT stands for Glass Tube. Here a nice one from Dumont:

The plate structure:

Apparently they varied the construction over time. I have some with the getter applied at the top and some with the getter at the bottom of the tube:

Here a quite peculiar JAN 6J5WGT, made by Sylvania:

It has a nice brown micanol base:

But what's peculiar about this tube are the two plate structures inside as would be expected from a double triode: 

If you look closely, cathode and grid are only assembled inside one of the plates. It is better visible when the tube is lit up:

Obviously the same tooling and internal structure as for the 6SN7 was used, they kept the second plate inside for mechanical stability, but left the cathode and grid out.

Next a 6J5G in the 'coke bottle' or shoulder type (ST) shape:

Another view:

The plate structure:

The small fin on the to is attached to the rods which support the grid. It provides extra cooling for the grid.

Now, let's examine the internals of a 6J5, as has become a tradition of the Tube of the Month serious. Caution: If you are offended by photos showing the destruction of a vacuum tube, don't read any further.

Let's take one of the metal tubes, a General Electric:

The base can be easily taken off, when the metal is bent away in those spacings:

Slowly prying it open:

The base comes off:

The metal shell opens like a tuna can:

Part of the sealing is gone now:

Breaking the air tight shell. A first peek inside:

The electrodes are already visible:

Removing the triode from the metal shell:

The ring which held the getter:

The triode system:

Grid and cathode removed from the plate:

A close up:

Showing the scale of the grid spacing:

Another close up, the distance between the black lines is 1mm:

cathode removed from the grid:

The cathode and heater:

A close up:

That was all I have about the 6J5. I hope you enjoyed it. Stay tuned for the presentation of the phono stage circuit using this tube.

As a last one, a 6J5 with a panorama view:

Best regards

Thomas