All rights of photos and text reserved. Usage of photos or text from my blog on other websites or for any other purpose only with prior permission. If you want to use any material from my blog please contact me by email.

Friday, November 26, 2021

Tube of the Month : The 6BH11


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


Tuesday, November 16, 2021

Silver D3a LCR RIAA Phono Preamplifier


last week I finished an entire silver electronic chain consisting of phono,  line and power amps. Here some photos of the phono.

This is the single ended version of the D3a phono but with all signal transformers in silver.

This particular unit is equipped with 2 inputs with the selector switch on the back for short wiring of the tiny signal from the MC cartridge.

Here a peek to the inside. The main signal transformers are hidden under the black transformer covers:

The underside of the top plate pre wired:

Top part finished:

All chokes remain in copper wire as they do not carry any signal. The chokes are mounted on a sub plate:

The sub plate mounted underneath the top plate:

Almost finished phono stage with 4 more silver signal transformers placed between the chokes:

A total of 8 silver transformers.

Stay tuned for photos of the rest of the all silver system

Best regards