6F6G

The 6F6G is an indirectly heated power pentode intended for the audio output stage of radio receivers. Sylvania describes its use both as a single tube and as a push-pull pair; Brimar specifies it for AC-powered equipment and car radios. The G version has an ST-shaped glass envelope and an octal base. Its heater requires 6.3 V and 0.7 A. Sylvania's documentation notes similarities between the electrical characteristics of the 6F6 family and type 42, but this does not remove the socket and circuit differences that must be checked before substitution.

In one class A example with a single tube, at 250 V on both plate and screen grid and −16.5 V on the control grid, the sheets give about 34 mA of quiescent plate current, 6.5 mA of screen current and a 7 kΩ load. The stated useful output is about 3.2 W, with total harmonic distortion around 8% under those test conditions. At 285 V with different bias, an operating point of about 4.5–4.8 W is given, depending on the sheet. Brimar also provides a push-pull example using two tubes, 315 V on the plates, 10 kΩ plate-to-plate impedance and about 10 W output: that is the power of the pair, not of one 6F6G.

The 6F6G can also operate as a triode by connecting the screen grid to the plate. Sylvania proposes a tube connected in this way as the driver for a pair of 6F6 or 6F6G tubes in class AB, and gives about 0.85 W output for its single-triode example. This flexibility does not make the operating points interchangeable: load, bias and the role of the screen grid change. Brimar's ratings for the 6F6G include 375 V on the plate, 285 V on the screen, 11 W plate dissipation and 3.75 W screen dissipation. In repair work, these limits must be kept separate from the voltages and powers in the application examples.