🇺🇸【 Catalinbread 】 Formula 51 Tweed Champ-style 
All the way back in the ‘40s, Fender introduced the Champ amplifier, then known as the Champion 800. After a lengthy series of part swaps and circuit revisions, Fender arrived at the 5F1, otherwise known as the “Tweed Champ,” named so because of its unmistakable wrapping. It featured just one knob—volume—that went to 12 (in your face, Nigel), and if you’ve ever played one, that’s about all it needs. Just that one knob can coax a myriad of tones from the humble circuit, from gently gritty cleans to honking, mid-heavy blown out distortion. Unlike most amp-in-a-box devices, the Formula 51 features a preamp, a discrete power amp simulator and an output transformer to deliver the full touch-sensitive Tweed experience, with a grip of extremely interactive controls. The Sag control loads down the transformer to your taste, giving you some real-deal blown-out leads with the twist of a knob. The Tone control is straight out of the vintage amplifier playbook, incorporating the mid scoop control of the Framus Cobra amp. This time-tested mod essentially turns the mid-leaning Tweed profile into the mid-scooped “blackface” sound of later Champs. Run it at 18V for more headroom or stack it with other pedals to really dig into that transformer!
Controls
SAG: Start with the knob all the way down until you find a tone you like, then inch it up until it does things you like. If it starts sounding farty or weird, it's just too much degradation for the signal level to handle. *More on this knob below. 
VOLUME: The Formula 51 sounds great when this knob is set at any position other than zero. Try it!
TONE: Some time ago, back when amp geeks had just started to trade information over the Internet, a “mids” control mod began popping up that was pulled from an amp called the Framus Cobra. This control, wired in reverse, essentially scooped out the mids and turned a burly, mids-focused Tweed amp into a more scooped “blackface” model. And so, we have given you this control as a Tone knob. When it’s all the way down, you’re in Tweed town. When it’s maxed, you’ve got a “blackface,” Jack.
GAIN: This knob functions as the “volume” control on the original Tweed Champ; it controls the preamp gain, or how much juice hits the power amp and transformer. When it’s all the way down, you’re treated to the slightly gritty cleans of the original, as you roll it up you get a cranked Tweed and finally some heavy distortion all the way at the end.