The 5687 has a low mu and high transconductance. If you put it on a chart with the 12AX7 and the 12AU7, the 12AU7 would be in the middle. The 5687 has even high

The 5687 has a low mu and high transconductance. If you put it on a chart with the 12AX7 and the 12AU7, the 12AU7 would be in the middle. The 5687 has even higher transconductance and a little lower gain. If it ran hot, part of that was probably the fact that the 6.3 V heaters ran at 900 mA, instead of the 300 mA. This is a full 3x as much power, just for the heater. You could use it in a guitar amp, but I would probably just use a 12AU7 instead. The 5687 has a different pinout. > It wouldn't be a good fit for the final output stage before the output transformer because of it's relatively low heft compared to a beam power tube or something, [...] There are guitar amps out there with less powerful dual triodes used as output. Yes, they're low-power. There are amps out there using a 12AU7 for 500 mW of output power. It seems like such a small amount of power, but it can be loud--loud enough to damage your ears, loud enough to get evicted, not loud enough to keep up with a drummer.