matchless hotbox tube preamp pedal

update: want to build one of these pedals? i've revised this hotbox build to fit in a smaller chassis and now offer an information kit to build one yourself. Check it out here.

the matchless hotbox is a tube preamp pedal. unlike most tube effects pedals, the hotbox operates the plates of the tubes at high voltage (250+ volts!) other pedals run the tubes in 'starved plate' configuration at voltages in the tens of volts range.

the hotbox is essentially a vox ac30 preamp with an additional triode stage as a 'clean' channel.

my interest in the hotbox started somewhat when one of my friends became dissatisfied with his guitar amplifier. he is a fan of the blues and many of his favorite blues artists play matchless amplifiers. after reading about the matchless hotbox, i discovered that it functions as not only a effect pedal, but as a preamp pedal. since my friend's amp was a hybrid amp with a tube power amp and a solid state preamp, i wanted to build the hotbox both to satisfy my own curiosity and to prove to him that there was hope for his amp, afterall.

i should mention that i spotted a hotbox 2 in a guitar shop one day and talked my friend into taking his amp to the store to try it out. the hotbox 2 just wasn't doing it for us - not enough volume and the idea of blending the clean and distortion channels rather than switching them seemed a littel silly to us. looks like i'll just have to build the classic hotbox.

one the most difficult parts of this design was to find a low-power tube amp power transformer that would fit into a 2" high chassis and give the right voltages. i looked at the matchless hotbox, matchless dc-30, and vox ac-30 schematics and decided that i would modify the hotbox power supply to run at full ac30 voltage - over 300 volts!

i used duncan amp's power supply design software to figure out the right dropping resistors and chose a low-power tube amp transformer from allied electronics. they sell their own brand of tube amp transformers, but most of them don't have enough juice to run a single power tube. it will work perfectly for this project, though- and at a cost of about $15.



the most difficult part of this design by far was the component layout. the orginal hotbox was point to point wired using terminal strips. i found that any layouts using any other method just simply wouldn't fit. i eventually gave in and used two 8 lug terminal strips to mount my components.

the preamp tubes themselves are mounted horizontally on l-brackets like the original hotbox. this was certianly much easier than creating a bracket with two tube socket knockouts to mount them in. there is also a rubber grommet at each mounting point to give the tubes some mechanical isolation.

in designing the layout i had one principle in mind - keep the components on the lower pins of the tube socket running to the lower lugs on the terminal strip and keep components on the higher pins to the higher lugs of the terminal strip. after that, the layout finally came together.

i'm including the layout here, but i can't guarantee this was my final version. maybe you can make sense of it with the pictures. keep in mind that the two rows of terminal lugs here are actually one terminal strip with the top row and bottom row connected. non-used lugs are circled in case i needed to use them.



i decided to mount the power supply on a leftover piece of xx garolite. it happened to be just the right size to fit the depth of the chassis. you will notice a bridge recitifer, a dropping resistor and then two other power resistors to get the voltages to the right values. notice that i mounted the bridge rectifier with its leads coming through the turrets to make it easier to connect the transformer secondaries to the rectifier pins.


i started this built in a cakepan like many of my builds. i have two chassis sizes in mind and marked those chassis sizes with a sharpee to see what it would fit. it was soon evindent that all the parts wouldn't fit in a 7x7x2 chassis so i aimed for the 7x9x2 chassis which turned out to be just enough space.

after working out the physical location of all the parts in the amp, i mounted them all and began wiring them together. i used shielded wiring for any long signal lines such as the ones to and from the foot switch.

my first test of the amp was with some solid state power transistors ( lnd150 - the kind vox uses for their trem and reverb in the custom classic series.) the amp had some serious oscillation and i was getting very frustrated because i couldn't get it to go away. i final got tired of it and threw the tubes in and the amp started sounding really good. i guess those solid state transistors would make for an excellent troubleshoot tool, maybe. the aren't any oscillation issues at all with the tubes!

after realizing i wired the pots completely backwards, i was ready to finally put the hotbox to work. i grabbed my grommes 20w hi-fi amp, ran it into my marshall 2x12 cabinet and plugged in the hotbox. it sounded really good for both clean and distorted settings.

i called up my freind to have him bring his amp over to try it with. we plugged it into the 'poweramp in' jack of his amp and it was sounded spectacular! he wanted one, too! i knew it would sound good, but i didn't know it would end up sounding that good.

now that i had the hotbox up and running it was time to give it a proper home. i blocked off a 7x9 section of graph paper, traced the outline of the components and found a layout that seemed to make sense - keeping the high current and ac lines away from the signal lines. i also added a green and yellow led to indicate which channel the hotbox was set to.





when the project was finally tongether, i did one more experiment. i have an emu 1212m sound card that came with some built in dsp effects. one of these is a guitar cab sim. i added a little compression to emulate power-tube compression and ran the hotbox into my soundcard with the cab sim on- it sounded fanstatic. it looks like i'll actually be able to do some silent practicing/recording when needed.

i've included a clip of the hotbox with the hi-fi amp though a marshall 2x12 cab on the left channel and the hotbox through the computer with cab sim on the right channel (keep in mind my goal at the time for the cab sim was to find something that gave a real low-fi, raw, dirty tone.)

Listen

this was a great build and a very useful one. i've played the hotbox through a hi-fi amp, tube guitar power amp, 5w champ, marshall 50w and even some small solid state practive amps, and it sounds better than would be expected out of all of them - except a vox top boost amp - it sounds terrible because you've just run one top boost preamp into another with the hotbox plugged in!)

if you're brave enough to try to build one of these - i've seen a few builds on the net - i would highly recommend it.