The State Of Hollow State Audio E BookThe State of Hollow State Audio in the Second Decade of the 21st Century Vacuum tube (or valve, depending upon which side of the pond you live on) technology spawned the Age of Electronics early in the 20th Century. Until the advent of solid state electronics near mid century, hollow state devices were the only choice. But following the invention of the transistor (after their process fell to reasonable levels), within a couple of decades, the death