Has anybody ever try the diode trick?

[soundguruman]

comment withheld, due to low rating.

[trobbins]

I think it's much more likely that power supply cap directly after the rectifiers is getting hammered by the power tubes, and the PT simply cannot keep up. The main filter cap then has 2 choices of where to source it's replenishment current - from the filter caps downstream (the pre-amp filters), or from the overloaded transformer. The problem is, is that there is probably 10-50k of resistance between successive filter caps, which means the time constant is quite large - it cannot instantaneously sap current from these filters, hence the pre-amp tubes should be safe, unless the 'problem' persists for several seconds. The poor power tubes on the other hand have to essentially starve until the transformer recovers. That's my theory, anyway.

I don't think the main filter cap would ever 'source' current from preamp filter caps - the main cap is unlikely to droop low enough to go below the other caps especially if you have largish R droppers.

Diode isolation may be of benefit if powering preamp stages from a screen supply which is designed to slide significantly. An example is when its easier to take screen and preamp rails from the middle of an output HT rail, such as from a typical doubler configuration with series output caps that run over 600V.

After power turn-off, the rapid drop on the main filter cap could be influencial in pulling down the preamp rails, and using diodes without some discharge path could keep preamp caps charged if a valve or two were not in place.