25B6G

The 25B6G is an indirectly heated output pentode for receivers with series heater chains. Sylvania specifies output stages of 110 V universal and DC receivers; Tung-Sol emphasizes available power in such supplies. Nominal heater rating is 25 V, 0.3 A. The glass ST-14 envelope and octal base with seven actual pins belong to the documented construction.

Historical documentation includes RMA Release 84 of 5 August 1936, presenting tentative Raytheon data and announcing a sample. The attached Raytheon sheet, dated 6 July 1936, gives 95 V anode and screen, −15 V grid, 45 mA anode current and 2 kΩ load: output is 1.75 W at 10% distortion. This documents an early stage of the valve; later tables give different conditions and performance.

RCA's 1941 and Tung-Sol's 1940 sheets give a class A1 point at 105 V anode and screen, −16 V bias and 1.7 kΩ load. Quiescent currents are 48 mA anode and 2 mA screen; maximum-signal currents 55 and 10 mA. Output is 2.4 W at 12.5% total distortion with 16 V peak drive. At 135 V on both electrodes, −22 V grid and the same load, output rises to 4.3 W at 14%; quiescent currents are 61 and 2.5 mA, maximum-signal currents 69 and 14.5 mA. Transconductance rises from 4.8 to 5 mA/V.

A third RCA/Tung-Sol condition separates anode and screen: 200 V anode, 135 V screen, −23 V grid and 2.5 kΩ load. The table gives 7.1 W at 15% distortion with 23 V peak drive; anode current rises from 62 to 71 mA, screen current from 1.8 to 13 mA. Higher power thus accompanies different conditions and stated distortion. Curves separately show second and third harmonics and load effects.

Source differences are retained. RCA and Tung-Sol rate maximum anode voltage 200 V, screen 135 V, anode dissipation 12.5 W and screen dissipation 2 W. The French Sylvania extract instead gives 180 V anode and 9.5 W anode dissipation, retaining screen 135 V and 2 W. RCA permits grid-circuit resistance 0.1 MΩ with fixed bias and 0.5 MΩ with cathode bias; Sylvania gives 0.1 and 0.25 MΩ for resistance coupling. RCA also recommends keeping heater-cathode potential difference as low as possible. These are not merged into a manufacturer-independent specification.

The octal bottom-view diagram has 2 and 7 heater, 3 anode, 4 screen, 5 control grid, 8 cathode and suppressor; pin 6 is absent. RCA and Tung-Sol mark 1 no connection, whereas Sylvania labels it S; the detail remains source-specific. RCA and Tung-Sol specify maximum diameter 1 13/16 inches, maximum seated height 4 1/16 and maximum overall length 4⅝: 46.0375, 103.1875 and 117.475 mm. The earlier Raytheon drawing gives maximum overall length 4 19/32 inches, 116.68125 mm. These are separate outline revisions.

Documentation consulted: Sylvania, Le tube essayé sur poste, pages 119–120, 25B6G entry; Tung-Sol 25B6G, sheet of 30 December 1940 and curve page; RCA 25B6-G, Power amplifier pentode, 1 May 1941; RMA Tube Standardization Release 84 of 5 August 1936 with Raytheon Engineering Service QE-1203 of 6 July 1936.