The 3Q5GT is a battery pentode with a centre tapped filament. When the two halves are in parallel for a 1.4 Volt supply the valve is identical to the 1Q5. But the filaments in series for a 2.8 Volt supply require only 50 mA.
As an audio output valve the maximum output is 500 mW for 10% total harmonic distortion.
The Mullard name on this American designed Pentode.
The electrodes. Note the raised filament tension spring indicating that one half of the filament has failed.
The wide glass tube envelope is 30 mm in diameter, and excluding the IO base pins is 73 mm tall.