Hi Teleharmonium; your tech was/is ahead of the commerical curve. From Ted Weber's site:
Weber VST Weber is marketing high-wattage, power-dumping zener diodes: application photos from Weber's site:
These are 'break-down' diodes; one end connected to a DC supply leg; the other to chassis ground and when the voltage detected exceeds the diode's rating, it shunts or re-directs the DC to ground until the detected voltage falls below the diode's breakdown voltage effectively regulating the DC voltage as the the specified breakdown voltage.
Most vintage gear was built when AC line voltages were 110V; today they are closer to 118-120V. As a result, tube rectifiers see higher secondary AC voltage and everything is operating at higher B+ voltage. Although these might not help a tube rectifier, zeners would damp B+ voltages downstream.
Weber's page also includes a built-in calculator to ID the diode matching the buyer's volts-to-be-dropped and amplifier wattage. I don't see why these couldn't be used in the primary B+, as a screen regulator, or regulation of preamp supply voltage. Would require knowing the amp's actual power supply out DC voltages v. design and picking a target not-to-exceed voltage for each tap.
Considerably easier than scratch- or kit-building a regulator, requires punching/drilling holes in the chassis, some consideration for location viz heat generated, an ever-so-simple and elegant solution to regulated supplies .... not to mention extending the life of tubes! Cheers, cj