The double triode 5608 was designed for AC anode operation in control applications. The 5608A for Barber-Colman - Wheelco Instruments - was selected type with tightly controlled characteristics. It was used in the Capacitrol range of temperature control instruments.
The capacitrol looks to have been an optical sensor responding to smoke density in a furnace flue. If too much particulate was present the controller would open the dampers.
The 5608 was a development of the Type 53 Class B double triode. There is a suggestion that the warning label was more to keep customers replacing valves with the company supplied ones rather than using generic types.
The two triode sections. Note that the heaters are wired in series and consist of an insulated single wire open helix. The bridge rod can be seen at the top of the image.
The interesting feature is the paper label. A milky getter indicates a soft vacuum and the new valves have a hard vacuum and thus gas must be evolved from the glass or electrodes in use.
The Instrument company names are on the Bakelite base.
A closer view of the triodes. The grids are wound on copper supports and in the centre is an electrostatic screen connected to the cathodes.
The classic envelope is 44 mm in diameter and, excluding the UX7 base pins, is 102 mm tall.
Reference: Data-sheet. Type 5608A was first introduced in 1948. See also1948 adverts.
