Push / Pull starts with a push-pull output transformer, a design that hardware fans often compare with the P1 terminal. Its primary winding is center-tapped, which means a wire connects to the electrical midpoint of the coil.
At the top middle of the screen, the P1 Terminal’s camera is housed in a small cutout.
The GRiD Compass has a magnesium clamshell body.
Shaun Wellens P1 Terminal Circuit Basics
In a push-pull stage, two tubes, VT1 and VT2, alternately conduct and amplify the positive and negative halves of the input signal. In the standard connection, the screen grids of both tubes are tied together and fed a lower, stabilized voltage. They do not connect to the output transformer. The cathodes are usually tied together and connected to ground through a shared cathode resistor, often bypassed with a capacitor.
Swapping vacuum tubes for solid-state parts changed everything, as the 1954 pocket radio showed. The Regency TR-1 and its role as the first transistor radio is a useful companion read for anyone curious how that shift began.
The voltages at the B+ point and on the tube plates are lethal, often in the 400V to 500V range or higher. Anyone servicing a vintage terminal or similar unit should treat these points with extreme caution.
Jeremy Hindle is a production designer on Severance.
The push-pull concept depends on the two power tube inputs being 180 degrees out of phase. The preceding phase inverter circuit provides that phase relationship. A quality transformer keeps the winding from P1 to B+ balanced with the winding from P2 to B+. That symmetry lets the stage cancel even-order harmonic distortion and prevent DC core saturation.
The GRiD Compass inspired the Shuttle and was designed by Bill Moggridge. Xerox’s Alto, a pioneering GUI computer beloved in retro computing circles, dates to 1973. France distributed free Minitel terminals about a decade later.
Terminals and early personal computers were built for hands-on tinkering, and that spirit lives on in modern retro hardware. InkBoy e-paper display revives Game Boy gaming shows how a classic device can be reimagined with new display technology.
A Community Memory terminal was installed at Leopold’s Records in Berkeley in August 1973.
The GRiD Compass flew aboard Columbia in 1983.
Sources: The Heart of Push-Pull Power: Understanding Output Transformer Wiring, yankodesign.com
Frequently asked questions
How do the two tubes in a push-pull stage work?
In a push-pull stage, two tubes, VT1 and VT2, alternately conduct and amplify the positive and negative halves of the input signal. Their screen grids are tied together and fed a lower, stabilized voltage, and their cathodes are usually tied together.
What voltage levels are dangerous in a vintage terminal?
The voltages at the B+ point and on the tube plates are lethal, often in the 400V to 500V range or higher. Anyone servicing a vintage terminal or similar unit should treat these points with extreme caution.
What does a center-tapped transformer winding mean?
A center-tapped winding has a wire connected to the electrical midpoint of the coil. The article uses this to describe the primary winding of a push-pull output transformer.
Why must push-pull power tube inputs be 180 degrees out of phase?
The push-pull concept depends on the two power tube inputs being 180 degrees out of phase. The preceding phase inverter circuit provides that phase relationship.
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