21st Century Reg

Tom has said
On the output of the regulator, I'd go as low capacitance as possible. Let the regulator do its job.
But then goes on to say
I usually use a 10 uF electrolytic can where the power goes into the amplifier board. That's the supply bypassing cap and it helps with stability. I'd say 1~10 uF is probably enough supply bypassing.
So in what situations would an output/decoupling capacitor (there is already C2 on the pcb for stability) be recommended and when shouldn't they be used
Any advice or "rules"
ps I have R2 versions that I want to use in a Frank Blohbaum 300B MTA amp
The challenge with capacitive load on the regulator is that the capacitor presents a short circuit on startup. So the regulator dissipates significant power as the load capacitor charges. The current is limited only by the ESR of the capacitor (and the resistance of the wiring), which can be extremely low if a physically large film capacitor is used. That's why I recommend a maximum of 47 uF.

Most amplifiers will need some supply bypass capacitance for stability, so follow the recommendations by the amp designer on that. If the design requires more than 47 uF of supply bypass capacitance I'd start asking some pointed questions.

Note that some amps use RCs to filter the supply for the driver and input tubes. Those can be large as they have significant series resistance that limits the inrush current on startup.

Hope this helps. Like many things in engineering this isn't a hardline black/white issue.

Thanks Tom
I had always assumed the 2.2u on the board was for stability
I guess the ESR of 'lytics is why you use those
If I was to use a 10u 630v solen film cap (cause I have them) after your reg are there pros/cons to adding a small R in series to limit the current?
An RC combo would seem to be a "better" snubber for stability but I don't know enough for that idea.
The 2.2 uF on the regulator board is needed for the stability of the regulator.

A bypass capacitor might be required for stability of the amplifier. That's not something I have control over. If the amplifier requires a bypass capacitor it will already be in the schematic for the amp.