3Q5GT

The 3Q5GT is a beam output tube for portable receivers' audio stages. Tung-Sol describes it for “three-way” sets with a low-current filament; RCA calls it a beam power amplifier. Its centre-tapped filament permits two supplies: 2.8 V DC at 50 mA with halves in series, or 1.4 V at 100 mA in parallel. The T-9 bulb uses an intermediate-shell octal base described as seven-pin and allows any mounting position.

RCA's table shows that filament connection also matters to the audio data. With halves in parallel, 90 V plate and screen and −4.5 V grid, it gives 9.5 mA plate current, 2.2 mA/V transconductance and 270 mW into 8 kΩ at 6% total harmonic distortion. With halves in series at the same plate, screen and grid voltages, it instead gives 8 mA, 2 mA/V and 230 mW into 8 kΩ at 8.5%. A single output rating cannot be transferred to both arrangements without its conditions.

At 110 V plate and screen and −6.6 V bias, RCA lists 400 mW into 8 kΩ at 6% distortion in parallel, with 5.4 V peak audio grid signal; in series it gives 330 mW into 8 kΩ at 8.5%, with 5.1 V peak. With peak signal increased to the magnitude of the bias, RCA's note gives 500 mW in parallel and 400 mW in series, but at 10% distortion. Higher output therefore also involves different drive and distortion levels.

RCA sets 110 V maximum plate and screen. The zero-signal cathode-current limit is 6 mA for each 1.4 V half in series operation, or 12 mA in parallel; grid-circuit resistance must not exceed 1 MΩ with fixed or cathode bias. Series filament is between pins 2 and 7; parallel supply is between 8 and joined pins 2 and 7. Grid voltage is referenced to 7 or 8 respectively. Plate, screen and grid are on pins 3, 4 and 5. RCA's note requires a series-operation shunt between 7 and 8 to bypass excess half-filament current, and possibly another between 2 and 7 when other series-chain tubes contribute current.

RCA gives a useful correspondence: the 1Q5-GT/G curves also apply to the 3Q5-GT/G with filaments in parallel. This refers to electrical characteristics and conditions, not identical pinouts. Brimar's manual instead refers to the 3V4 for 3Q5GT characteristics. When comparing different designations, these references must retain the configuration stated by the document.