25L6G

The 25L6G is a glass-envelope beam audio output tube intended for universal AC/DC and DC receivers on 110 V mains. Sylvania stresses its ability to deliver power at relatively low anode and screen voltages. A precise historical source survives: RMA release 110 of March 30, 1937 presents preliminary ST-12-envelope 25L6G data supplied by Arcturus Radio Tube Company. This is more than a reconstructed date: the document directly identifies the type, data supplier and registration stage.

Heating is indirect, 25 V and 0.3 A AC or DC; our sources indicate series-heater-string use. The base is octal, 7AC: heater pins 2 and 7, anode 3, screen 4, control grid 5 and cathode 8. Arcturus declares pin 1 unconnected. Sylvania instead shows its glass 25L6G as ST-14 alongside the metal 25L6, so a specimen’s envelope should be compared with the relevant documentation. Any mounting position is allowed.

In Sylvania’s 25L6G class A1 column, anode and screen are 110 V, grid −7.5 V. Idle anode current is 49 mA and screen current 4 mA; at maximum signal they are respectively 50 and 11 mA. Internal resistance is 10000 Ω and transconductance 8200 µmho, or 8.2 mA/V. A 2000 Ω load gives stated output 2.2 W at 10% total distortion. Second and third harmonics are respectively 3.5% and 8.5%. The 2000 Ω is the example’s anode load, not loudspeaker impedance.

Arcturus 1937 gives two examples for the same 25L6G: 2.1 W into 1500 Ω at 11% total distortion, or 2.2 W into 2000 Ω at 10%. Both use 110 V anode and screen and −7.5 V bias. They illustrate load, power and distortion relationships, not a universal distinction between metal and glass envelopes. Sylvania also explains that a high-density electron beam uses space charge to counter secondary emission, referring to the same construction principle as the 6L6. Sharing a principle does not mean identical characteristics or interchangeability with 6L6.

For input Sylvania recommends transformer or impedance coupling. Grid-resistance instructions differ: Arcturus limits fixed bias to 0.1 MΩ and self-bias to 0.5 MΩ; Sylvania’s French text gives 1 MΩ fixed and 0.5 MΩ self-bias with the stated heater-voltage condition, no more than 10% above the specified value. The fixed-bias discrepancy remains explicit, a reason to consult the relevant manufacturer’s sheet rather than merge different historical instructions into one limit.