[35Z5]

The "center bass channel" was the in thing just prior to MPX stereo FM.  Most(if not all) used a PP output transformer for punch that combined R&L channels into one for center. On combo AM/FM was also common to see selector position that one channel was fed AM, the other FM. A few stations transmitted R & L channels separately on the different bands. Short lived as it was useless after FCC approved FM MPX in late '61.

I had a small 1960-ish(I believe J-whatever model) Philco amp with center channel I got from a old friend. Used a pair of 6AQ5, feeding R&L output transformers in series with center channel. Kind of a cheezy setup, I removed the separate transformers as R&L had all the punch of the common AA5. After removing the high freq rolloff cap. It was fair as a single channel PP amp, used it briefly as a driver for a plate modulated transmitter.