Motorola B12WA recap and mods

[wa2ise]

My brother found this radio someone tossed. It needed the usual electrolytic caps, and more unusually, needed a few new tubes (tubes are usually not the main issue in sets this old, this one dates back to 1964). Needed a new 12BE6 and some 12BA6s. But while I was in there, couldn't resist doing a few simple mods. Changed the power supply from a single silicon rectifier diode to a bridge (making it a full wave rectifier circuit), and reworked the heater string so the 12AV6 has the minimum AC waveform on it, but has half the B+ voltage on it (elevated DC on heaters helps reduce hum into the tube). Sure, the radio's internal ground is always hot no matter which way you plug the power plug in, but as the radio is packaged inside a wooden and plastic cabinet, it's not an issue. I've seen this done in 1980's era RCA color TV sets, where the entire circuit is hot.

To get a little more gain, changed the AM and FM IF stage from a 12BA6 to a higher gain 12BZ6 (which requires a shield, but this set already has a shield on this tube). And the screen and cathode pins are swapped, but here both go directly to ground, so not an issue.

And built a separate AM AVC detector circuit, to allow the detected audio to have less backfed from the AVC filter cap. And thus less audio clipping. Used a geranium diode (as the 12AV6 tube with its unused diodes was too far away).