The AC/G from Cosmos (Metro Vick) was an early AC indirectly heated valve and was released at the September exhibition in 1927 as the advert declares.
The aim of the design was to enable people to take advantage of the newly installed electricity in their homes to replace the battery valves in their wireless set with these AC valves with the minimum of alteration to the set. To this end the base was a modified B4 where pins 1 to 3 were as the B4 pattern and designed to plug into a B4 valve-holder and pin four was replaced by a pair of studs. The valve was supplied with an adaptor that slid over the longer than normal base pins and made contact with the studs and brought the heater out to a twisted pair of wires for separate connection to the AC heater supply.
The valves released in 1927 were the AC/G and the AC/R output triode.
These AC valves featured a revolutionary dull-emitter cathode thereby reducing the possible hum induced into the mechanism by heating the cathode directly. In production the heater was dipped in liquid porcelain clay and fired to produce a hard, very thin insulation. The tubular cathode was a ground-breaking 1 mm in diameter. It would be five years before this technique was beaten for rapid warm-up time, efficiency and low operating temperature.
The base with the heater connections on the right. Metro Vick produced a special valve-holder as well as the adaptor. In 1928 with the advent of the B5 base Cosmos adopted the new base for their valves.
Standard valve pins for a B4 base were 15 mm long, these are 20 mm long to allow for the adaptor.
The base adaptor allows pins one to three to pass through and provides connections to the heater.
The complete adaptor with cotton coated green twisted pair wires and connectors.
The shortpath construction showing the closed end of the anode.
The open end of the anode. The single thick grid support is at the front and the wire loops of the grid project back into the anode space.
The insulated heater wires with hairpin bend held in a spring pass through the small diameter cathode tube.
The grid can be seen more clearly in this image.
The BVA logo.
The BVA logo photographed with dark field illumination.
The Cosmos identification etched into the glass together with the green paint spot.
Cosmos and the MetroVick logo as seen under dark field illumination.
The balloon envelope is 60 mm in diameter, and excluding the special base pins is 125 mm tall.
References: Advert, Emil Dudek (Vintage Technology website) & 1043 Type AC/G was first introduced in 1927.