Tube of the Month : The 6BH11

Hi!

While rummaging through my tube storage I came across some boxes of Compactron tubes which I never used. So I decided to take a closer look and present them. Starting with the 6BH11.

The 6BH11 makes use of all the 12 available pins of the Compactron base and contains 2 triode systems for general purpose use and a pentode which was designed for use as horizontal deflection oscillator in TV sets.

The base pin out is shown on the left. The two triodes have the same parameters. An amplification factor of 46 and plate resistance of 5.4kOhm. This results in a rather good transconductance of 8500 micromhos. Quite an interesting medium mu triode actually. the pentode section is of the sharp cut off variety. Each of the three sections has a maximum plate dissipation rating of 2.5W. The heater runs off 6.3V as the '6' prefix indicates. It consumes 800mA which is moderate. See the General Electric datasheet for all technical details. Most of such Compactron tubes are largely ignored nowadays although many of them could be used in audio or guitar applications. They can be picked up for mere pennies. As always let's have a look at the plate curves, starting with the curves of the triode sections as shown in the datasheet:

Not exactly as linear as other triodes but for low level applications that would not be critical. Here the curves of an actual tube:

The pentode section:

The pentode section triode wired:

I only have 6BH11 made by Westinghouse:

The base:

A few close ups:

The top:

Some views from the side:

Pentode on the left and the two triodes on the right:

Close up showing the shared heater of the three sections:

Ok let's open one up to see more details of the construction.

The pentode section has a large cutout in the plate and beam forming plate :

Pulling out the heater:

One of the triode sections:

Quite unusual how the grid is shaped in a circular fashion around the cathode rather than a box shape as is more common.

The second triode :

The pentode:

Here we see the control and screen grid. The control grid is also bent in a circular fashion around the cathode.

Grid nr 3 is actually a beam forming plate and it extends on one side as an internal shield.

The cathode.

Unfortunately I completely wrecked the grids when removing them from the rest. So only the separate beam forming plate / internal shield to show:

Quite an intersecting tube and advanced construction details which is typical for such tubes which were manufactured in the 1960ies.

The 6BH11 in operation.

Stay tuned for more coverage of Compactron tubes in upcoming posts.

Best regards

Thomas