For connecting a Bluetooth receiver or MP3 player, use the TAPE INPUT . It is high-level (-10dBV). The AUX input works too, but TAPE is usually more forgiving.

Do not go below 4Ω. The manual states clearly: "Total impedance must not be less than 4 ohms." If you connect two 4Ω speakers in parallel (2Ω), the amp will overheat and die in 10 minutes.

But without the manual, that mess of jacks on the back (6-pin mic inputs? screw terminals?) might as well be alien hieroglyphics.

Scan it and upload it to HifiEngine. Save the next tinkerer from the frustration of guessing those mic pins. Did you just fix a dead 12-150? Tell us about replacing the thermal paste on the output transistors in the comments below.

If you picked up a dusty, heavy metal box at a garage sale or inherited one from a ham radio operator’s estate, chances are you’re holding the Realistic MPA-150 (often catalog # 12-150 ). This little workhorse was Radio Shack’s answer to affordable 150-watt PA amplification in the 1980s and 90s.