Replacing the Columaire’s Interstage Transformer
With a Cathode‑Follower Phase Splitter

When Westinghouse introduced the Columaire at the dawn of the 1930s, it represented the height of Canadian radio craftsmanship. Its tall Art Deco cabinet and powerful push-pull audio stage gave it a presence that was as much architectural as electrical. Beneath its elegant exterior, however, lay a small but essential component that made this impressive sound possible: the interstage coupling transformer.

In early radio design, this transformer served a critical role. The audio signal produced by the detector and first audio amplifier had to be divided into two equal but opposite signals before it could properly drive the pair of output tubes operating in push-pull configuration. The interstage transformer, shown at the right accomplished this elegantly. It coupled the audio from one stage to the next while simultaneously creating two mirror-image signals — one for each output tube.

When functioning correctly, the result was powerful, balanced audio with lower distortion and greater efficiency than a single output tube could provide.

Unfortunately, time has not been kind to these delicate transformers. Their extremely fine wire windings are prone to corrosion, internal shorts, and open circuits after nearly a century of aging. Many surviving Columaires, like this one are discovered with failed interstage transformers, leaving the output stage unable to operate correctly. Without those equal and opposite drive signals, the radio loses much of its audio output — or falls silent altogether.

Finding an original replacement transformer in 1976 was nearly impossible so I decided to design a circuit using a vacuum tube 2A7 to do this job. The circuit I designed is known as a "cathodyne phase inverter", sometimes called a "cathodyne phase splitter".

Although the terminology may sound complex, the principle itself is beautifully simple.

Rather than relying on magnetic windings to split the signal, the cathodyne uses the natural electrical behavior of a single triode tube. The incoming audio signal from the preceding stage is coupled through a capacitor into the grid of the triode. Equal-value resistors are then connected in both the plate and cathode circuits.

As the tube amplifies the incoming audio, two output signals appear simultaneously: one at the plate; and one at the cathode.

Because of the tube’s inherent operation, these two signals emerge equal in strength but opposite in phase. When the plate voltage swings positive, the cathode swings negative by an equivalent amount, and vice versa.

In this remarkably efficient manner, the cathodyne recreates precisely what the original transformer once provided:  — two balanced audio signals, equal in amplitude and opposite in phase, ready to drive the Columaire’s push-pull output stage.

Each signal is then coupled through its own capacitor to the grids of the output tubes, restoring the balanced operation of the push-pull amplifier.

The advantages of this approach extend beyond simply replacing a failed component. Unlike aging transformers, the cathodyne phase inverter introduces virtually no transformer hum, avoids winding deterioration, and provides excellent phase balance using only a handful of reliable components. Properly implemented, it offers performance that is often superior to the original transformer while remaining electrically sympathetic to the spirit of the original design.

In many ways, this solution reflects the natural evolution of radio engineering itself. By the mid-1930s, transformer coupling in audio stages was increasingly replaced by resistance-coupled phase inverter circuits because of their improved reliability, lower cost, and superior performance.

Thus, my replacing the Columaire’s fragile interstage transformer with a cathodyne phase inverter does more than repair a failed radio. It respectfully combines the craftsmanship of early 1930s engineering with the quieter wisdom of later electronic design.

The result is a restoration that preserves both the sound and the spirit of the Columaire — allowing this literal 'Barn-Find'  to continue speaking with warmth, balance, and authority nearly a century after it was first built.