Siemens 0.9 volt screen grid bias cell

[Gridiron]

The picture is of a grid bias cell from a 1930-31 catalogue, some Wireless World set designs of the period used this cell permanently screwed down to the set baseboard. I was wondering if anyone would know what type of battery it is that would be used to give a 0.9 volt output stable over a long period at virtually zero current?
Regards, Mike.

[AC/HL]

Two different battery technologies wired back to back ?

[Gridiron]

Just recently found an answer to this thread of mine from long ago, Scott-Taggart in the Manual of Modern Radio (1933) on page 149 says that this 0.9 volt battery uses a special Cadmium cell.
Mike (Gridiron).

[Silicon]

Presumably it is to bias the control grid of a 'screen grid' valve.

[Alf]

The Ambassador model 6778 was fitted with a gwz mono cell no. 4

[emeritus]

Not a battery I have ever heard of. I'll have to dig out my dad's copy of Scott-Taggart and see if it's mentioned there. Judging from a table of electrochemical potentials, and given that one electrode was Cadmium ( -0.403V) , the cell's other electrode was probably Copper (+0.521V). Immersed in a suitable electrolyte, these would produce a potential difference of 0.934V, close enough to a nominal 0.9V. I guess the current drain would have been minimal.

[Nicklyons2]

To use an almost unique battery/cell; strange and expensive way to provide grid bias. Given the impendance/source resistance of the circuitry in this era why on earth not use a standard battery and a potential divider? A few micro-amps from a 'U2' for example would hardly shorten its life from shelf-life.

[Hartley118]

Seems an eccentric way to avoid a cathode resistor and decoupling capacitor providing auto bias.

Maybe the battery salesman was generously inclined to take the circuit designers to the pub at lunchtime.

Or am I missing some hidden advantage?

Martin

[Nicklyons2]

Originally Posted by Hartley118

Seems an eccentric way to avoid a cathode resistor and decoupling capacitor providing auto bias.

I wholly agree, although in the early years maybe no-one had thought of it yet?

Maybe the battery salesman was generously inclined to take the circuit designers to the pub at lunchtime.

Not beyond the bounds of possibility.

Or am I missing some hidden advantage?

It would guarantee a service call every couple of years-ish; nice little earner....

[AC/HL]

You have to be careful judging the past by the present. To be in a catalogue there must have been a demand of sorts from set makers or hobbyists (not as much distinction back then as there is now). Obviously they didn't survive the test of time, but neither did other forms of biasing. Quirks like this are what makes the evolution of radio interesting to me.

[SteveCG]

Were there any directly heated valves for which this bias battery would be needed ?

[Mike. Watterson]

They had battery packs anyway, so often used a tapped 6V, 9V or 15V pack.

Some mains sets used direct filament output triodes for a while, but I think those were replaced by indirect Pentodes.

Many output valves, battery, direct or indirect mains needed about -4V to -6V, or -8V to -12V for class B. So would have needed a stack of button cells. The 0.9V ones were replaced by Mallory 1.35V mercury cells, which are very stable.

The Mallory battery, or similar (the company now called Duracell), was the idea of a long life sealed cell as it could be sold cheaper than a capacitor + resistor to bias mains valves.
It wasn't very popular, then WWII came and the long shelf life and good performance at temperature and humidity made Mallory mercury cells popular for electronics of proximity fuses in shells and bombs.
This made Mallory very successful. After the war the cells used in cameras (light meters) and eventually in transistor hearing aids.
Mallory saw the value of marketing Eveready's Alkaline battery (not Ever Ready but Union Carbide / National Carbon Company, whose predecessor formed British Ever Ready Electrical Company). NCC/UC and UK Ever Ready regarded the battery type as too expensive, but Mallory marketed it as Duracell, the general purpose complement to their mercury button cells. Later the button cells too used the chemistry. The selling point was the x5 life.
Actually not quite honest as that only applied at high current drain. At low drain the capacity is only 2x to 3x. However shelf life can be x10 longer!
Eventually they changed their name to Duracell.
US Eveready then later launched it as Energiser, using similar marketing and also later changed their name.
The UK Ever Ready produced them too late as "Gold" and basically lost a lot of market to Duracell, Japanese and Chinese batteries. Hanson Trust's Vulture Asset strippers bought UK Ever Ready about 1981 or 1982, then a conglomerate that had taken over US Eveready/UC/NCC/UCARS bought UK Ever Ready about 1996, I think.

[Gridiron]

In reply to the above posts, Emeritus's post seems a to provide a likely answer to the composition of the cell, in the catalogue a 1.5 volt version is also listed at a cheaper price presumably because the cadmium for the 0.9 volt cell would make it more expensive to make.
The 0.9 volt cell was used in at least one professional application, the Marconi 352A ship's receiver used it to bias the anode bend detector valve (LP2 triode), a friend of mine had one of these sets with the cell still present, screwed to the set's baseboard (no voltage present, of course!). This receiver was commonly used on ships from the early 1930's to the war years so, presumably, supplies of the cell would have been available during this period.

Mike (Gridiron).