sardobang

A minor correction, fellow Marantz 14/15 fans. In the early days of solid state silicon amps, engineers decided that a positive temperature coefficient (PTC) resistor in the signal path for each output transistor (OPT) would keep the transistor from operating outside of its safe operating area. An incandescent filament is a PTC resistor. Picking one with the right voltage and wattage rating was the secret. In addition to the Marantz 14/15 amps, the idea was also used by Knight Kit in its KG-870 and probably some others. Simply, the normal operating current of the amp is very low and there is not enough current to heat the light bulb. As the current goes up, the light bulb begins to glow gently, and when operated very loud it will glow brightly. The light bulb resistance goes up with current as the temperature increases, so there is a slight lag time, enough to prevent too quick of a reaction that might negatively impact normal dynamics of music. Burned out bulbs are a sign of shorted OPTs, because the lamps should last forever unless blown by a bad OPT. Without the bulbs the amp won't work.