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Friday, July 20, 2018

Tube of the Month : The 8608 / EL5070


This months tube you probably never heard of, the 8608 or the European equivalent EL5070.

The 8608 / EL5070 is a small power pentode. It is electrically equivalent to the E55L.

It shares the same magnoval base with the E55L but has the plate brought out to a small top cap. The pinout is shown on the left. Otherwise it is pretty much identical. For technical details of the E55L refer to the first E55L tube of the month post. Since it is electrically equivalent it can replace the E55L in any application designed for it. For example in LCR RIAA phono stages or headphone amplifiers. It could also be used for a 'spud' (single tube) power amplifier. If you don't mind the top cap, the 8608 is a lower cost alternative to the sought after E55L. The 8608 was originally designed as wide band video amplifier tube as indictated by the Amperex data sheet. But that does not mean it is not perfectly usable for audio as well. It will surely
perform just as good as the E55L, either as pentode or in triode mode. Like the E55L the 8608 has a very high transconductance. In triode connection this is a staggering 50.000 micromohs which comes with an amplification factor of 30. This translates to a very low plate resistance of 600 Ohms which opens a wide array of potential usages. Pretty much any desired coupling method is possible with this tube. The 8608 might be a bit hard to find but if you find some they are usually not overly expensive. I am surprised the 8608 is not mentioned anywhere by audio amplifier builders. Although I have not yet used it myself I am sure it performs well. Lets start by looking at the triode connected plate curves:

As expected, nice and not much better as can be expected from an indirectly heated tube. Here are some Amperex made 8608:

They come in the typical yellow and green boxes.

A close look at the tube:

The base:

Some more details:

Here we already get a glimpse at the delicate grids.

Nice construction! This tube rightfully carries the 'Premium Quality' logo.

Some more details:

Some of the boxes had stickers from Tektronix. Apparently they were replacement parts for some of their scopes.

This one has orange printing on the glass.

The construction is identical.

Next we have a bunch of Mullard 8608:

Let's have a closer look:

EL5070 on one side and 8608 on the other:

Construction seems identical to the Amperex tubes above.

So probably from the same factory.

Here a 8608 side by side to a E55L:

Of course we want to have a closer look to the internal construction. Let's open this one:

Glass removed:

Now we get a better look at the grids:

Now wee see the very delicate grids of this marvellous tube.

The plate consists of two separate pieces.

Connected by a bar on the top which also makes the connection to the cap.

More detailed views:

The heater.

Notice the heavy rods of the grids:

One half of the plate.

More views of the grids:

The cathode:

The grids without the cathode:

Some marks left on the grid wire from removal of the cathode.

Screen grid and control grid separated:

Close up:

The control grid:

The structure with the two separate plate halves gives a nice view to the glowing cathode when in operation:

What a beautiful tube!

Best regards