3Q5

The 3Q5 is a battery audio output valve of the beam-tetrode family, with an octal base and directly heated filament. Documents also identify it as 3Q5GT or 3Q5-GT/G; retained photographs show Philco, Sylvania, Mullard and Brimar specimens. Its documented role is the output stage of portable receivers, including sets intended for several power-supply systems. A centre-tapped filament permits parallel connection at 1.4 V and 100 mA total, or series connection at 2.8 V and 50 mA. Current must therefore always be tied to the arrangement: 2.8 V at 100 mA does not describe normal series operation of this type.

Radio Valve Data, 1958, lists the 3Q5 with parallel filament, 110 V anode and screen, −6.6 V grid bias, 10 mA anode current, 1.4 mA screen current and 2.2 mA/V transconductance. Into 8000 Ω it gives 0.4 W at 6% distortion. The RCA 3Q5-GT/G datasheet of 1 May 1942 confirms this operating point and separates series operation: at the same electrode voltages, but with a different potential distribution along the filament, it gives 8.5 mA anode current and 0.33 W at 8.5% distortion. RCA sets design-center anode and screen voltage limits at 110 V: maximum ratings and typical operating points serve different purposes.

The half-watt output deserves an interesting qualification. At the 110 V point with parallel filament, RCA obtains it by increasing peak grid signal to equal the bias voltage, with distortion rising to 10%. With series sections, the same criterion gives 0.4 W at 10%. Thus these are not contradictory power figures for one valve, but results tied to different filament arrangements and drive levels. The relationship to the 1Q5 is also specific: RCA states that 1Q5-GT/G curves apply to the 3Q5-GT/G when the filament sections are in parallel.

The base brings the filament to pins 2 and 7, with its centre tap on 8; anode, screen and control grid are on 3, 4 and 5 respectively. Manufacturer notes draw attention to shunt resistors needed in series-filament circuits: the particular manufacturer document and equipment wiring must be followed. Photographs reveal a bright box anode, mica spacers and filament-tension springs. One Mullard specimen described by our sources has a raised spring associated with a broken filament section; this is a feature of that specimen. Illustrated specimens measure about 29–30 mm in diameter and 64–73 mm in body height excluding pins, also showing physical differences between constructions.