My Hot Tubes/ Craig Anderton Tube Sound Fuzz variants from the '90s. Nowadays I would get the cheapest Aliexpress booster pedal (Kokko FBS-2), and tack the last 2 stages of the "Basic" arrangement onto the output. That would be about $US 20, and an hour's work.
This is the "House Pedal" - an overdriver/distortion/fuzz/octave up pedal. It's not a just a tweaked copy of something else, although it functions as a simplified Electro-Harmonix "Hot Tubes". No opamps, just a standard 1-FET booster to drive the CMOS inverters. Overdriven CMOS inverters actually sound a bit like a tube guitar amp, moreso than most opamp diode clipper circuits. I think. It's all the mind anyway.
A lot of distortion and volume from this thing, and I've reduced the gain from R.G. Keen's "Hot Tubes" schematic. I don't think the EH Hot Tubes was this hot (it used a CD4049BE, this may be the reason). The EH Hot Tubes is a guitar amp simulator. You have 2 inverting asymmetrical clipping stages, a tone control and a symmetricial clipping stage. This is a crude model of a push-pull guitar amp.
It does seem that the manufacturer of the CD4049UBE chip used makes a big difference. It's worth trying a couple of manufacturers. Harris (CD4049UBE H9801) and ST Microelectronics (HCF4049UBE W990A9828 MALAYSIA) work for me. I plan to try the CD4069 next (Ed Rembold's suggestion).
You can replace the input transistor with your favourite volume booster, a tube screamer, or an opamp as in the Guitar Player Anderton Tube Sound Fuzz. It does actually sound a bit like an overdriven tube amp.
The Basic Hot Harmonics has about 15 components, which is almost as minimalist as it gets. There is no FET preamp stage, so the CMOS gain has been increased. Like a Fuzz Face, the pickup loads the input, reducing high frequencies (which is good). Turning the volume control down to about 7 takes nearly all the distortion away, giving a bright rhythn guitar sound.
It doesn't sound like a Fuzz Face of course, more of a "amp at 11" sound.
See also the Slow Finger on Aron Nelson's page.