Recently this pedal came to my door. I've always been a big fan of the EHX Hot Tubes pedal and CMOS overdrive pedals, but this little unit just sounds even better.
The EHX Hot Tubes (not to be mistaken for the tube version (the EHX Hot Tubes tube pedal) and the EHX Hot Tubes Nano (the small version of it) is a pedal made from 1978 up until 1984 and is an overdrive / fuzz pedal based upon the CD4049UBE CMOS chip. This version was made in 1980.
CMOS overdrive
CMOS (Complementary Metal Oxide Semiconductor) is a transistor that was designed to be used as a switch. The advantage of the design behind the CMOS design (1967) was that the unit didn't use a lot of current and could easily be implemented into a various of designs.
Using it in the audio world became common by a design made by pedalDIY-guru Craig Anderton in the feb issue 1977 of guitar player magazine.
He called it the 'Tube Sound Fuzz' and was a double footswitched pedal with a switchable gain option and no EQ knob. This design, minus the extra gain function, was copied for the Way Huge Red Llama.
EHX also used this info by building the original Hot Tubes, but they added something else. EHX added a dedicated input section (buffering), a tone knob (the mid dip tone knob which could be found in a Big Muff) and the option of a tone knob bypass.
The unit featured buffered bypass, an AC cord and a relative big box. A direct out output was added.
A lot of musicians used the Hot Tubes. Most known users are Kim Gordon (Sonic Youth) and Stevie Ray Vaughan (although be mentions it to be used only one occasion). As a big fan of Kim, I got into this pedal by listening to Sonic Youth and trying to get the same basssound.
The circuit
Two versions of the Hot Tubes (vintage) were made (the EH3075 and the EH3075B). Only small components vary. The CMOS IC (which has six CMOS stages) is used for four stages. The other unused stages are kept at a voltage to keep them from interfering. Next to this CMOS IC the unit uses a standard RC4558 IC for the preamp.
As you can spot the circuit uses 9V and can be run fine on a a dedicated 9V PSU. a very critical component is the 27p cap over the first CMOS stage. In the official schematics these caps are let out, but without them, the unit would sound drasticly different.
The Sound
The sound is different then other overdrive or fuzz boxes. It really follows the rules of a fuzz box: it responds well to volume knob changes and gives a nice 'in your face' grit that reminds me of a good fuzz. Also, when the gain is raised, the unit becomes more and more a 'real' fuzz box and can bring you nice fuzzy tones.
The Tone knob adds low end or spit into the high end range. The unit handles low really well and keeps it straight, bold and apparant in the mix. The EQ of the tone knob has a slight mid dip that works well with P90 or humbucker guitars. The Tone Bypass switch adds a nice lot of mids in the sound.
To me, this unit on bass or guitar really makes you stand out, but also gives you a very characteristic overdrive sound that reminds me of old Tweed amps or the late Tom Petty's tone (he, as well as the other guitar player in his band, are using CMOS OD pedals). It can also easily go into fuzz territory with a lot of volume on the tap. In the FX loop the unit really comes alive and makes the sound really fit! I just love it!!
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