The El Rey Dorado is your go-to box for that plexi tone. Yes, there is a wide range of 1960s Plexi Amp Tone in terms of power, tubes installed, speakers, and/or cabinet used, but this hits the core-tone bulls-eye! One of the many reference points for the tone was my own JTM 45/100® full stack. (It’s a dead-on reissue Marshall® did with crazy cool custom made vintage recreation speakers and dual transformers to transfer the load of the 4 EL-84s!)
The El Rey covers a lot of Classic British Amp tones by featuring two gain modes with the Power Toggle. With the Power Toggle switch you have control over the compression and harmonics. Low Power mode is louder, has more girth, and has fewer harmonics for a more old school tone. The High Power mode has more compression, more harmonics and a tighter, more defined tone. Old school crunch in Low Power mode, super lead tones in the High Power mode.
https://www.riffcityguitaroutlet.com/products/keeley-electronics-el-rey-dorado-overdrive-pedal
https://www.andertons.co.uk/keeley-el-ray-dorado-overdrive
RKFX.com
Also used in this video:
Fender American Vintage '57 Stratocaster
Vintage Fender Champ through Vintage Fender 2x12
Boss RC03- Phrase Looper