25A6GT

The 25A6GT is an indirectly heated cathode audio output pentode intended for universal AC/DC radio receivers and sets powered from DC mains. The Hytron Bantam sheet stresses the output obtainable at the relatively low voltages available in these sets. Tung-Sol relates the 25A6 family’s electrical characteristics to type 43; Hytron states that its own 25A6GT is interchangeable with its own 25A6G. These references belong to their respective manufacturers’ contexts.

Its heater is rated 25 V at 300 mA AC or DC. The envelope is tubular glass with an octal base; Hytron specifies T-9D and also depicts a T9C version, while RCA groups GT/G under T-9. Main pins are heater 2 and 7, anode 3, screen 4, control grid 5 and cathode 8. RCA and Hytron specify a 90 V DC limit between the negative heater end and cathode: this uses a particular measurement convention and should not arbitrarily become a symmetrical limit.

An operating point common to the examined sheets uses anode and screen at 135 V and grid at −20 V. Anode current is 37 mA and screen current 8 mA; transconductance is 2450 µmho, or 2.45 mA/V, and internal resistance about 35000 Ω. A 4000 Ω anode load gives stated output 2 W at 9% total harmonic distortion. At 95 V anode and screen, −15 V grid and 4500 Ω load, output is instead 0.9 W at 11%. Anode load means the impedance seen by the tube through the output stage, not directly the loudspeaker impedance.

The sources differ at the third operating point. Hytron lists 180 V maximum anode, 135 V screen, −20 V grid, 38 mA anode current, 5000 Ω load and 2.75 W at 10%. RCA and Tung-Sol instead list a 160 V anode maximum and, at that voltage, 120 V screen, −18 V grid, 33 mA and 2.2 W into the same load at 10%. These latter two sheets specify dissipation limits of 5.3 W anode and 1.9 W screen. This documentary difference does not authorize applying Hytron’s 180 V to every family member.

Hytron recommends transformer- or impedance-coupled drive. With resistance coupling, it limits DC grid-return resistance to 0.25 MΩ for self-bias and 0.1 MΩ for fixed bias, provided heater voltage never exceeds its rating by more than 10% under any operating condition. The sheet references RMA release No.134 of April 11, 1938 and refers to 25A6G for curves. This is a useful trace of its technical history, without establishing an unproven first-production date.