An Ancient IBM Vacuum Tube Mainframe Computer

In early 1957, I saw my first computer during a high-school field trip to an IBM office in Cleveland, Ohio. It was a vacuum-tube mainframe in the IBM 700 series. Though I can find no record of that particular IBM office, its mainframe was most probably an IBM 705 Data Processing System, which had been designed to process business data.

The office was a single, large, street-level room that combined a reception area and a data-processing workspace. An IBM employee, probably an office manager, gave us his ‘grand tour,’ which was lecturing to us while we stood in the center of the room. The equipment included an operator’s station, a large card punch and reader, and a few cabinets for tape drives and power supplies. Through a glass wall, we could see multiple racks of glowing vacuum tubes in the back room.

Each IBM 700 series mainframe included a vacuum-tube processor, a magnetic-core memory and data-storage devices for punched cards and magnetic tape.

Vacuum Tube Processor
A vacuum-tube processor in an IBM 700 series mainframe was roughly similar to the CPU in a modern PC. This processor executed program instructions, performed calculations, and communicated with magnetic-core memory and data-storage devices.

I had recently seen a classmate’s transistor radio. Therefore, at the end of our host’s ‘sermon,’ I asked him whether IBM would ever use transistors in its computers. He said, “No,” and explained that transistors could never work as quickly as vacuum tubes.

Magnetic Core Memory
A magnetic-core memory in an IBM 700 series mainframe was roughly similar to RAM in a modern PC. This magnetic-core memory was the working memory through which the processor made calculations. A magnetic-core memory consisted of ferrite rings in a wired array, with three wires threaded through each ring. Current pulsed simultaneously through two wires would magnetize a ring in either of two directions (states), on or off (1 or 0). The third wire would sense a change in state.

Data Storage Devices
Each IBM 705 mainframe would use data-storage devices to store data to, and retrieve it from, punched cards or magnetic tape. A business customer would use cards or tape to bring its payroll, or other business-operation data, to the IBM data center. After processing these data through its mainframe, IBM would use cards or tape to send the results to the customer.


* – Wikipedia, “IBM 700/7000 series”,

* – IBM Archives, “IBM 705 Data Processing System”, IBM

* – National High Magnetic Field Laboratory, “Magnetic Core Memory”, Florida State University