7868

The 7868 is a power tube for audio output stages. RCA and Sylvania sheets describe it as a power pentode with a nine-contact Novar base for high-fidelity amplifiers; RCA also mentions radio receiver output stages. Their tables include examples with one tube, two in push-pull and ultralinear connection. The output figures for these configurations are not interchangeable: they depend on bias, voltages, load and how the screen grid is connected.

The coated cathode is indirectly heated, and the heater requires 6.3 V and 0.8 A. The Novar base differs in shape and contact arrangement from an octal socket. In the bottom views of the sheets, pins 4 and 5 serve the heater, pin 9 the plate, and pin 3 the cathode and suppressor grid; the control grid connects to pins 2 and 6, and the screen grid to pins 1 and 7. Pin 8 is treated differently in the RCA editions consulted: one marks it as an internal connection not to be used, while another permits limited use as a tie point at specified potentials. For actual equipment, follow the sheet applicable to the tube and its construction rather than assigning that pin a universal function.

Under the design-maximum rating system, the sheets list 550 V for the plate, 440 V for the screen grid, 19 W plate dissipation and 3.3 W screen-grid dissipation. The latter may reach 6 W during peaks of speech and music signals under the conditions described by the manufacturer: the peak figure does not replace the normal limit. Heater voltage and dissipation limits should be considered together with the equipment's thermal and load conditions.

In class A1 with one 7868, RCA and Sylvania list about 11 W output at 300 V on both the plate and screen grid into 3 kΩ, with stated total harmonic distortion of 13%. With two tubes in class AB1 push-pull, their tables give, among other examples, 24 W at 300 V on plates and screens, 34 W at 400 V on the plates and 350 V on the screens, and 44 W at 450 V on the plates and 400 V on the screens. The plate-to-plate load is 6.6 kΩ at these points, but bias voltages and currents change. Sylvania highlights the 44 W point in its summary and specifies 5% total harmonic distortion: it is the result of that operating point, not a promise of equal output at lower distortion.

For ultralinear connection, the sheets describe screen grids fed from output-transformer primary taps that apply 50% of the changing plate-signal voltage to each screen. The RCA examples for two tubes give about 23 W output with fixed bias and a 400 V plate supply; another point with cathode bias and 425 V gives about 21 W. Both use a 6.6 kΩ plate-to-plate load. This is a specific connection that must be read together with its load and bias values: the 7868 designation alone determines neither the power nor the distortion of an ultralinear stage.