Guitar combo amps with effects jacks

Effects loop (send/return)

— in this case, we mean a set of connectors designed to connect an effects unit between the preamplifier and the combo's power amplifier. One of these connectors (preamp output) is usually labeled “Send”, the second (power amplifier input) is labeled “Return”. With such an interface, various “gadgets” can be connected to the combo amplifier, supplementing it with effects that were not originally provided for in the design.