Apco Twin Tube
From the collection of
Bill's Vintage Tube Site

Bill Condon's discription of the Twin tube is: "The Apco Twin has a single triode section but uses an "M" style filament that is center-tapped. Metal tabs on the base allow selection of either half of the filament, or both halves at once. Using one half resulted in '01A performance. If both halves were used at once, the tube could be used as an output tube."

Then in the "Saga of the Vacuum Tube" book the discription says: "The Apco Twin Tube was made by the Apco Manufacturing Company of Providence, Rhode Island, in the period 1925-1926. It contained two complete sets of 201A-type elements. These were connected in parallel ex-cept for one end of each filament. When the first filament failed, the second one could be brought into service by slightly unscrewing one of the filament prongs so that a switch arm could be moved from one contact to another. This brought the second filament into the circuit. Each filament took 0.25 ampere at 5 volts. The base was of the UX type and the only identification on the tube was an oval black paper label on the glass bulb with the words "Apco Twin Tube." At the left of the trade name "5 volt" and to the right ".25-amp" were printed all in dark red on the black label."