25A7G

The 25A7G combines a half-wave rectifier and an audio power pentode in one bulb. Sylvania specifies universal receivers where compactness matters; its type-specific sheet identifies ST-12 bulb and octal base. The heater requires 25 V, 0.3 A AC or DC. RCA and Tung-Sol cover the 25A7GT/G family jointly: declared common electrical data must be distinguished from cylindrical-bulb drawings, which do not automatically describe the shaped G envelope.

The common documented audio point uses anode and screen 100 V, control grid −15 V. Quiescent currents are anode 20.5 mA and screen 4 mA; anode resistance 50 kΩ, transconductance 1.8 mA/V, load 4.5 kΩ. Output is 0.77 W; RCA and Tung-Sol specify 9% total harmonic distortion. This is class A1 amplifier performance, not power available from the rectifier to the entire receiver. Anode curves use screen 100 V and various biases.

Pentode limits in RCA and Tung-Sol are 117 V anode and screen, anode dissipation 2.25 W, screen 0.8 W. Maximum control-grid-circuit resistance is 0.1 MΩ fixed bias and 0.5 MΩ cathode bias. RCA recommends transformer or impedance coupling without excessive grid-circuit resistance. Maximum 2.25 W dissipation must not be confused with the typical 0.77 W audio output.

The half-wave rectifier has a separate cathode. RCA's March 20, 1943 sheet gives peak inverse voltage 350 V, peak anode current 450 mA, maximum DC output 75 mA and maximum DC heater-cathode potential 175 V. The capacitor-input example uses 117 V RMS AC, 16 µF capacitor and minimum effective source impedance 15 Ω. DC at filter input is about 130 V at half load 37.5 mA and 110 V at full load 75 mA: drop depends on load and filter.

A source discrepancy must remain visible. Sylvania's French 25A7G table gives maximum DC 60 mA and maximum AC anode voltage 125 V RMS, but its application text on the same page says limit the rectifier to 75 mA. This is not an unambiguous confirmation of the higher figure: the page is internally inconsistent. The 75 mA and 117 V example above therefore belong to RCA and Tung-Sol's explicitly designated 25A7GT/G family, without silently replacing Sylvania's table.

Octal basing 8F clearly separates the cathodes. Bottom view: 1 rectifier cathode, 2 heater, 3 pentode anode, 4 screen, 5 control grid, 6 rectifier anode, 7 heater, 8 pentode cathode with suppressor. Any mounting position is permitted. Separate cathodes identify the two circuits sharing heater and envelope. T-9 drawing dimensions in combined sheets are not taken as ST-12 dimensions for Sylvania's G type.

Documentation consulted: Sylvania, Type 25A7G, Pentode redresseur combiné, page 118 of French manual Le tube essayé sur poste; Tung-Sol, 25A7GT/G Rectifier–Pentode Power Amplifier, November 17, 1941, two pages; RCA, 25A7-GT/G Rectifier-Pentode, data dated March 20, 1943 and curve page June 28, 1938.