Fender Princeton Reverb Half Power Mod

Half power is not all that noticeable. A master volume would give you some more flexibility. Or an attenuator as an option.

 

A master volume cuts drive to the outputs, so you take the output tube mojo out of your sound. Maybe that's OK. If you want 1/2 volume, that would be a 1/10th power attenuator, and keeps the flavor of the output section.

 

The master volume on the 80s Superchamp works well - it's a Princeton Reverb preamp minus trem into a Deluxe Reverb power section, so it should be easy-ish.

Unless you fit a dual-secondary power transformer the only real way is triode mode which alters tone a bit. Plus on beam forming grids on 6V6s, don't know how it'd work.

You could look at the power attenuation in Vox AC4s and other amps using big, switchable cement block resistors.

 

I bought a 5E3 Tweed clone from Carl's Custom Amps about 3 or 4 years ago and had a London Power Scaling and SAG Control mod built into it. I can go from full power down to 1/2 watt or anything in between playing with the power & limiter controls. I love that I can plug in at 2am and overdrive the hell out of the thing & wake up no one. He also does a bypass Master Volume mod. Personally I think the London mod is more useful.
He did BF Princeton Reverb's in the past...not sure if he still does. Others do this mod too & for me it's been the most useful thing I've ever had on an amp.

 

Maybe a set of these, though switching to EL84s 'straight' might not make a huge difference in volume...

Tube Converter - Yellow Jackets®, YJUni, Triode version | Antique Electronic Supply (tubesandmore.com)

but Yellowjackets AND a set of these? Might get you what you need, maybe.

Vacuum Tube - EL844, JJ Electronics | Antique Electronic Supply (tubesandmore.com)

 

Well this is a change from the normal, how to make a PR into a TR type thread.
If you don't go hog wild with speaker sensitivity to start with that helps.
As far as half power one way is to put in a switch to alter the operation of the power tubes, instead of trying to cut one out completely.
But even if you achieve "half power" that doesn't mean "half volume".
An attenuator does seem like the simplest way to go through. Or just use a drive pedal of some sort when you can't turn the amp up, that's even cheaper and easier. I'm not a pedal guy but that's the way I'd go.

 

Do you want pre-amp distortion? Then add a master volume.

Do you want power tube distortion? Then add an attentuator.

Don’t want any distortion at all? Then do whatever is easier (or add an effects loop and stick a volume pedal/box in it)

 

what's the actual goal here?

a PRRI wound up to 10 is loud.... but it ain't THAT loud.

PRRI cleans are pretty dang reasonable sound levels.