Why tube rectification?

@invalid This is an interesting assertion. If you were building a 60 Watt PP amp based on KT88s what rectifier would you use?

Could appropriate HEXFRED diodes be used to replace the existing diodes in each amp without other modifications?

If the equipment is already designed with solid state rectifiers then yes, there are HEXFREDs rated at 1200V with plenty of Amperage.

Because it sounds better.

@sounds_real_audio This is really debatable (I’m doing it right now...).

I’ve found that using properly snubbed semiconductors in the power supply sounds better and measures better than a tube rectifier. They have different design requirements of course (apparently not everyone understands how to snub them for example). If used correctly I find no tradeoffs.

FWIW, one of the best tube amps from the vintage era was the HK Citation 2, which employed a solid state supply. It was a Voltage doubler style which was common with many American tube amps (ex.: Fisher).

there is no reason to have voltage sag at higher power with tube rectified equipment if the proper rectifier tubes are chosen.

@invalid Yes- I like the plural of the word as you chose, as that is what is needed. For example the Dyanco ST70 was designed with one rectifier tube when they really needed dual rectifiers. Because they cheaped out on this issue, the 5AR4 is more likely to fail than any other tube in the amp. You would need a number of tube rectifiers to really prevent sag. It is the Voltage sag which is why blues players in particular really like tube rectifiers in guitar amps.

IOW there really isn't a good reason to use them in a tube amp meant for home hifi use other than the nice glow. But the designer might not have figured out how to keep solid state rectifiers quiet so resorted to a tube rectifier instead. Tip: HEXFRED rectifiers are nice and quiet; so much so they often need no snubbing at all. They can handle high Voltage and currents that no tube rectifier used in the home can hope to match. So in a tube amp they can work quite well.

These two competing solutions both supply the basic power to the audio tubes and output transformers, so only indirectly interact with the sound signal. I have not read anything that explains what tube rectification brings to the party.

Voltage drop across the rectifier, especially when the amp is making power, is one very large reason rectifiers can affect the sound. The other reason is noise.

Solid state rectifiers are often blamed for making noise but its really how the power transformer interacts with the rectifiers. Between the two an electrical resonance can occur which is set into oscillation by the rectifiers turning on and off. The resonance might be at 2MHz but can cause the rectifiers and transformer to make noises much lower in frequency, which can leak into the signal chain and cause IMD. This is known as a 'swept resonance'. Its not something you often run into with a tube rectifier.

But it can be solved with proper application of snubber networks. The best approach is a small capacitance in series with a resistor across the input to the rectifier(s), directly across the output of the transformer.

Once the swept resonance is shut down there's no going back to tube rectifiers as the power supply sags less at higher power levels- which makes for less distortion at those power levels.

An obvious advantage of a solid state rectifier is the capacitances in the power supply can be quite a lot higher. There is often a fairly low limit to capacitor values when tubes are used. There are timing constants involved with these values. What you want is that none of the timing constants in the amplifier circuit be lower than those in the power supply- else the amp can much more easily modulate the power supply, resulting in higher distortion.