Sounds quite a bit like The Legion of Space by <PERSON>. As I recall, the secret weapon (AKKA) was mostly mental so the physical portion of the device was really simple.
From Wikipedia:
The Legion is the military and police force of the Solar System. It was created to keep the peace after the overthrow of the "Purples", a dynasty that ruled all humanity for generations. <PERSON>, a young graduate of the Legion academy, shares a surname with the <PERSON> but is an enthusiastic supporter of the Legion.
A weapon called AKKA was used to defeat the Purples. Using a space/time distortion, it erases matter from the Universe—any matter, of any size, even a star or a planet. The secret of AKKA is kept in one family, descended from its creator, and is passed down from mother to daughter. 322
One of the Legion's most important tasks is to guard the current Keeper, a beautiful young woman named <PERSON>.
Through the machinations of his uncle, a powerful politician with a hidden agenda, <PERSON> is assigned to <PERSON>'s guard force at a secret fort on Mars. When she is kidnapped by a huge alien spaceship, <PERSON> and the three other survivors of the guard force follow her kidnappers to a planet of Barnard's Star. They crash-land and must battle their way across a savage continent to the sole remaining citadel of the Medusae.
<PERSON> uncle and his nephew have allied with the Medusae as a means to regain their empire, and have kidnapped <PERSON> to ensure that AKKA is not used against them. The Medusae, however, turn on the Purples, seeking to destroy all humans and move to the Solar System, as their own world, far older than Earth, is spiraling into Barnard's Star.
<PERSON> and his companions rescue <PERSON>, but the invasion of the Solar System has already begun. The Medusae conquer the Moon, set up bases there, and bombard Earth with gas projectiles. <PERSON>, <PERSON>, and their companions land on a ravaged Earth. Fighting off cannibals maddened by the gas, they build AKKA and destroy the Medusae fleets (and Earth's Moon as well). 786