Heater can be wired in series (12.6V / 0.45A across pins 4-5) or parallel (6.3V / 0.9A with pins 4 and 5 tied together). For 6.3V operation apply heater voltage between pins 4&5 connected together and pin 8 (heater midpoint).
The 7119/E182CC is pin-compatible but has significantly different electrical characteristics: higher mu (~28 vs 16-18), higher gm, and lower heater current (0.64A vs 0.9A at 6.3V). Not all circuits designed for the 5687 will perform identically with a 7119 — verify operating point compatibility.
The 7044 is electrically very similar to the 5687 but rated for higher total plate dissipation (8W both sections vs 7.5W).
Originally developed by Tung-Sol (registered February 1948) for computer and industrial applications. Also manufactured by GE, RCA, Raytheon, and Sylvania. Widely adopted in high-end audio as a driver tube due to its high current capability and low plate resistance.