of perceived loudness for a given amount of amplifier power. Even though tube amplifiers produce more heat than solid state amplifiers, few manufacturers of tube amplifiers include cooling fans in the amplifiers' chassis. Usually adequate cooling is provided by passive convection. Adequate airflow is needed to prevent excessive heat from shortening the tubes' lifespan or producing tonal inconsistencies. Tube amplifiers require more maintenance than solid state transistor amplifiers, such as replacing vacuum tubes and electrolytic capacitors. Tube amplifiers are usually heavier than an equivalently powered transistor amplifier. As tubes are made of glass, tube amplifiers are more fragile than a solid state amp. Solid state amplification By the 1960s and 1970s, semiconductor or transistor-based amplifiers (also called "solid state") began to become popular. This was in large part because for a given wattage level and feature level, solid state amplifiers are less expensive, lighter weight, and require less maintenance than tube amplifiers. As well, transistor amplifiers are more reliable and less fragile than tube amps. The output transistors of solid state amplifiers can be passively cooled by using metal fins called heat sinks to radiate away the heat. For high-wattage amplifiers, a fan is often used to move air across internal heatsinks. Hybrid Hybrid bass amplifier heads typically pair a tube preamplifier with a solid-state power amplifier. This provides the player with the best elements of both amplifier technology. The tube preamp gives the player the ability to obtain tube amplifier tone, which tube enthusiasts state is "warmer" than a solid state (transistor) preamp. As well, tube users state that tube preamps have a more pleasing-sounding, natural tone when the preamp's volume is pushed up so high that the bass signal becomes overdriven; in contrast, a solid state preamp that is pushed to the point of signal "clipping" can be harsh-sounding. Some hybrid amp heads have a bypass switch, so that the tube preamp can be bypassed, if the tube breaks or develops a technical problem. The tube preamplified signal in a hybrid amplifier head is then sent to a solid state power amplifier. Compared with tube power amps, solid state power amplifiers are more reliable, require less maintenance, less fragile and lighter in weight. A hybrid tube preamp/solid state power amp thus provides a bass player with the benefits of both technologies' strengths: tube preamp tone and solid state reliability for the power amp. Power in watts and volume