6CD6 and other interesting valves/tubes

[Refugee]

When going for first light during restoration of a lovely workshop instrument a beam tetrode that glows like the rectifier next to it made me very curious indeed.

On closer examination it appears to be a double beam tetrode with the two sections internally connected in parallel but with a common anode.
It obviously has a lot to see above the pinch with all the linking straps.
The cathodes look like the strap is above the mica and can only be seen with a light shone through the envelope at an odd angle.
The control grids are linked with a strap made from sheet metal crimped around the supports.
The screen grids are linked with a part that looks like the frame of a dolls house wheel barrow. This may be to do with heat expansion having been a problem on early examples of this very curious valve.
It looks very much like there is only one pair of beam forming plates shared by both sections.
The top cap connecter is very stiff and I will not be risking damage to the valve or worse so it had to stay in the chassis for the photos.

[Alan Douglas]

6CD6s here have single cathodes. That Brimar must have been more difficult to manufacture, but presumably it was cheaper to use existing element tooling. The anode may have been a stock part from some other valve.

[Refugee]

The radio museum in Germany has a photo of the American version and indeed it is taller and cylindrical with a tall anode and less connections below the structure.
This would be consistent with it having a single cathode.
The one I have is in a Marconi sig gen and is branded Brimar so would be British.
The Valve Museum has one just like this one in my Marconi sig gen.
http://www.r-type.org/exhib/aaa0321.htm

[Merseyswimmer]

I remember coming across the double cathode Brimar 6CD6 doing service as a line output valve in certain KB TV sets. It was a long time ago so I can't remember which model(s) it/ they were. I also seem to remember that the 6CD6 was also popular for homebrew amateur AM transmitters. All the specimens of 6CD6 I had/saw all had that double cathode construction.

Was that bit of kit a regulated power supply? Another application for 6CD6.

[Brianc]

The 6CD6 is also used as the line output stage in their MkIII Image Orthicon Camera of Grandstand fame. It was obviously a favourite of Marconis.

[Refugee]

It is indeed a series regulator in a lovely Marconi signal generator.
It has a nice distortion free output unlike modern reconstituted sine wave function generators.

[Murphyv310]

The 6CD6 valve is an excellent valve as is it's series fed heater cousin the 50CD6.
Oddly the valve wasn't hugely used, the EL38 was used in more applications and was inferior. I've a few NOS ones and two pulls that are just about 100%.

[Refugee]

I have seen a photo of one from the US that had a single cathode.