6V6Ss are power amp valves commonly used in American styled amps. They're an octal power valve of approximately 14 Watts of output. The 6V6S is a beam tetrode output valve, meaning they have additional plates to focus the electron stream into beams to increase the output of the valve. They function in a different way to a pentode (like EL84s for instance), giving them an different tonal structure.
6V6s are commonly considered to be scooped, smooth and clean. They contribute to the thick bass end and crystal clear top end found in many early 6V6 equipped amplifiers of the 50s. They are often found in cathode bias class A circuits for a spongy, low wattage power section.