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.

ga-5f1

i call this amp the ga-5f1 because the fender champ 5f1 circuit and the gibson gA-5 circuit are so similar that only 5 components differ! well that is certainly true for the vintage versions. these subtle differences, however, create two different amps. the gibson has an extra 7db of gain and can really start to break up when cranked.

prototype a

this series of four builds started when i did a self-study course on vacuum tube audio frequency amplifiers for my university coursework. i needed a project, so i set out to find an amp to build. i had read some good stuff about the electrar tube 10, one of gibson's offbrand amps. learning more about this amp led me the the kalamazoo model one, another gibson off-brand amp quite similar to the ga-5. i decided that was the amp to build - the model one with a tone control.

this was my first amp build and it was the first amp that i actually built and designed from scratch. sure, i had a schematic, but i had to buy a lot of tools to drill the chassis and stake turrets on the fiberglass board. I used hammond tranformers, orange drop caps and jj tubes to build the amp.



when the amp was first fired up, i noticed a lot of hum - the old 100 ohm resistors trick fixed that one. i had some other problems with parasitic oscillation. i never figured that one out, although i did recently solve a problem that may have also solved this amps problem. the strange thing was that the amp would ring even if a dummy load was connected. it must have been an actually mechanical oscillation!

I really loved the clean sound of this amp, especially after adding a bright cap across the volume pot and with my second attemp of a tone circuit in. The first tone circuit was from the model one, but i later changed it too the tone stack used in the gibson ga-8. It sounded much better. The amp did always have a pretty harsh distortion sound, though. I eventually pieced this amp out and use the parts to build other amps because it just wasn't seeing much use since it was in a naked chassis... not that i didn't have big plans for the amp. i had designed an antique cathedral radio style cabinet for it, but the build of it didn't turn out good.



there were several issues with this amp. first the layout wasn't very good. this likely led to the oscillation problems. these problems were mostly solved by putting a 12dw7 tube in the preamp to reduce the gain a bit. not a bad way to play. the poor layout also didn't leave room for all of the components, some needed to be retrofitted to the amp with terminal strips.

in the end, this was good for my first build. it sounded beautiful when clean - clear as a bell. i certainly learned ways to improve my next build.

prototype 1

one of my aspirations is to teach classes on building tube amps, building ga-5f1 kits and hopefully extending those courses to the internet. i began this endoevour by building the first of three prototypes. the ga5-f1 is based on the classic ga-5 circuit, with a solid state rectifier and el84 output tube- much like the 2003 gibson ga-5 les paul junior reissue.

this protoype was all about functionality and low cost. it was one of my first builds to incorporate the use of radial capacitors instead of axial capacitors. (axial capacitors have leads coming out of both ends, radial capactors have both leads coming out of only one end.) radial capacitors are much less expensive, but all the layouts i have seen are made for axial capacitors. as you can see from my circuit board layout, i didn't exactly accomodate for them in my layout either.



another cost-cutting measure was the use of 70 volt transfomers as output transformers. transformers are simply magnetic cores wound with magnetic wire and have turns ratios and power ratings. power is realted to voltage and current, and current is dependant upon impedance. the 70V transformers is fed a constant voltage and there are several 'power taps' which have different primary impedances to change the power going to the speaker. theses 70 v transformers have a 50Hz-15Khz frequecy response, so they work great for guitar amp transformers.

i used a 10W transformer and with a few calculations using a calculator, signal generator and a volt meter, i found the power tap that would give me the correct impedance for an el84.

i wanted to have plenty of room for this build so i once again built it in a cakepan.

for a power transformer i used a fender standalone reverb unit power transformer. it gave voltages close to those i was looking for, so a dropping resistor was installed before the first filter capacitor. for this build i started with a 100 ohm resistor to see where that got me.

one of the unique features of this amp was that i installed two output tube sockets one for a el84 and one for a 6v6, wired in such a way that each would be biased correctly by simply swapping tubes. i did some recording with each type of power tube to see if there was a great difference in sound. i really couldn't tell much of a difference, so i opted to go with the el84 for the build because they are easier to find and modern production el84 tubes more reliable at high voltages. i got about 5.5 watts from the 6v6 and 5 watts from the el84.

the tests of this amp were a success! it sounded great, had no parsitic oscillation and the cost cutting measures didn't seem to affect the tone in a negative way.

prorotype 2

this prototype was an attempt to build prototype 1 in proper sized chassis. for the chassis i used bud 9x5x2- close to the specs of the orignal champ chassis.

this build also incorporated a switched volume pot. turning the pot down past zero flips a 'piggyback' switch that turns off the amp. this is pretty nice because now i don't have to drill a 1/2" hole in the chassis for an additional power switch.

i also added some cost-cutting jacks to this amp. in the end, they ended up being a great pain becuase they are quite difficult to plug into or unplug from. it's defintately worth the extra 25 cents to get some good jacks like neutrik jacks (still less than switchcraft).




i also tried a different wire with this amp. it had a thinner insulation and looked a little easier to work with. i only bought one color, light blue. the wire seemed to work fine, but i soon realized that it is impossible to trouble shoot an amp without multiple wire colors. being that the wire was not commonly produced in many colors, i decided against using it again.

you will also notice that i had found a way in this layout to incorporate the radial capacitors in a way that was reminicent of axial capactitors. this avoids the silly look of radial capacitors with thier butts in the air on a circuit board and makes them easier to work with and integrate with other components.



the circuit board material is used on this build is grade xx garolite from mcmaster carr. the board is 3 x 6 x 1/8" cut from a 4 foot length. its is much less expensive than fr4/g-10 garolite and more rigid than the fender type vulanized fiber boards (all of which can be found at mcmaster.)

the eyelets i used are some that i found in my boss's electrical kit. he gladly let me have them. the are smaller than the classic fender sized turrets, and they fit into the same holes as do the hoffman turrets. you will notice a mixture of eyelets and turrets on this board based one the needs of the particular electrical node.

the dropping resistor used in this build is 82 ohms, the closest to the value that gave the closest desired voltage from the duncan amps power supply designer software. using the power and output transformer from protype 1, i got a little more juice out of the el84 this time around.

another thing you will notice on the turret board is an axial fuse. the reason this fuse is there is simple- if this fuse blows something bad has happened and the amp needs to be looked at. this also frees up some room on the front panel in case any extra features required an additional pot, switch, etc.

you may notice on this particular layout that i did not use the entire 3" of space. this is cimply because many modern componets won't span the full 3".

ultimately the layout was an improvement, except for the fact that i actually had the layout flipped vertically. this also made troubleshooting difficult because nearly every wire crossed the board on it way to where it needed to go!

intital tests of this amp showed severe problems with parasystic oscillation. shilelded wiring eventually solved the problem for a while and the amp sounded great. by this time i had also found a cheap speaker to use - the jensen mod 8-20. it didn't sound as good as the weber signature 8, but it did have a bit of a smoother distorted tone than the weber.

this build was a partial success. i had attributed the parsitic oscillation to the inverted layout that cause all of the wires to cross the circuit. i would later find out that was not the case, but that story is yet to come...

prototype 3

this prototype improved upon the first two in a few ways. first, the layout problem was fixed and the layout was tidied up a bit. the bridge rectifer doesn't use as much space and the mounting holes were place in areas that were much easier accessed. other than that, this layout is pretty close to the classic 5f1 layout. i wanted a tried and true layout in order to build this amp without parastic oscialltions.

for this amp i used a new power transformer. i used a hammond 261g6 this was a bit more expensive than the one i used previously, but it had better specs and was available through my usual supplier.

you will notice that i am back to nuetrik jacks on this build also. much better!

i am also using a new wire, which you can get as a complete kit of 8 colors from radiodaze. its a great deal and very sturdy wire!

i also bought a proper cabinet for this amp- a 5f1 champ cab from mojotone. it looks great. the tweed covering isn't my favorite, but it would have cost more to go with a nice cream tolex. you will notice that the 2" chassis leaves a little gap between the top of the chassis and the control panel cutout. that problem was easily solved with some large gold piping. i later found that there exists a 9.5 x 5 x 2.5 chassis that would have avoided this problem!


zero signal electrical tests of the amp showed the amp dissipating 6.5 watts, but no redplating was occuring so i didn't bother. i did use a different 70.7 v transformer for this amp and its higher dc resistance likely played a part in this.

i was pretty dissapointed when i fired this amp up and it had the same oscillation problem as prototype 2! i spents hours trying to find the problem, even poking around with the oscilloscope. something was causing a signal to be induced into the inital gain stage, but how? these questions led to a revision of prototype two to find the answers.

prototype 2a

this time around, i wondered if the 5f1 layout was that great of a layout afterall. i wanted to modifiy it so that all of the wire paths exited the board on the same side they were destined to go. once again the layout was modified. this time i had run out of xx garolite. i had order a vulcanized fiber 5f1 board from mojotone to see what the classic fender stuff looked like, so i hacked up the "insulator" fiberboard and applied my new, improved layout.



i worked really hard to clean up all the wiring and get everything just right.

when i fired up the amp, i got the same oscillation! what? what is going on here? again, poking around with the oscilloscope showed a voltage being induced in the first gain stage from somewhere.

eventually i figured it out... it was 'blocking distortion.' the grid of the el84 was being pushed to the point that it would start to conduct, this in turn was creating a strange signal to be induced on the first gain stage that would make the problem worse. after reading some technical info, i installed a 1.5k resistor on the grids of the el84s and instantly the probems went away on protypes 3 and 2a. what a relief! now they both sound great!

in the end, these builds helped me learn a lot about tube amps. one of the most important things i learned however was take your time and do things right the first time around. rushing things only leads to poor design choices. luckily most of the time they won't matter much, but every once in a while they do..

so now that i've built 4 ga-5f1 amps, what is next? well, i just happen to have enough sapre parts layoung around to build a gibson les paul junior / fender 5c1 champ ( both have a 6sj7 pentode preamp and 6v6 power amp.) this amp will also incorporate another crazy idea i had- using a power transformer as an output transformer! stay tuned!

vox ac4 with top boost

concept

this build started around the vox ac-4. i had heard a lot of the ef86 hype, yet never heard an ef86 myself. when vox released thier 'heritage series' of amps with ef86 and top boost channels, i wanted to build one. the ac-4 seemed like the perfect amp to get to know the ef86 without spending an arm and a leg.

i had to ask myself, what if i don't like the amp? what then? so, i decided to throw in a top boost channel to make the amp more versatile.



power supply

i had picked up a old 'voice of music' tube pa amp at a thrift store for $3. it didn't sound particularly well, but a review of the schematic showed it to be very similar to a fender champ or gibson ga-5. i salvaged the transformers and tubes for future builds. the high voltages from the power transformer could only realistically be used with the 6x4 rectifier. even then, the voltages were too high for the vox ac-4. i found a schematic for the badcat minicat which was close to the right voltages, so i built the power section around that amplifier. i still needed to put a 2k power resistor in the power supply to drop the voltage to the right levels, however.

you'll notice that i have placed an axial fuse on the circuit board. this is there simply because if the fuse blows, something is most likely wrong with the amp and it needs to be serviced properly. this is a feature i am starting to make a habit in my diy builds. it also saves a little space on the control panel and on this build you'll notice that i needed all the space i could get.



power amp

the ac-4 power amp utilizes an el84 cathode-biased output stage. the ac-4 output stage is a little different that the one i used which was based off of the badcat mini-cat. There is about a 80 volt difference in the plate voltages between these too amps! I wanted to keep the voltages high so that the top boost preamp would sound right.



the preamp: ef86 channel

the ef86 channel seemed to have a low number of parts. this is one of the reasons i decided to add the top boost channel. i had heard many speak of the excess amount of bass in the ac-4 and some of the fellers at the plexi palace forum suggested that the .047uf coupling cap be replaced with a .01uf cap. after playing the amp myself i had to agree that the .047 made the amp sound too dark, so the .01uf cap went in instead.

as for the gain available with the ef86, i was a little let down. i was expecting much more with all the hype i had heard. maybe it was the electro-harmonix ef86 tube. maybe i hadn't turned the amp up enough to drive the power stage in clipping. either way i much preferred the top boost channel for gainy sounds, good thing i added it!



the preamp: top boost channel

one of the goals of the amp, as always, was to keep costs down. a traditional top boost channel would require two 12ax7 tubes and a wasted triode. vox got around the wasted triode in thier heritage series by running the triodes of the first stage in parallel. these then fed into the second stage then to the cathode follower and tone stack.

to keep the amp simple, the cathode follower stage was removed for the top boost channel- a cost-cutting measure used in vox's ac15 custom classic. the cathode follower provides a current buffer and a lower impedance to the tonestack. the tonal difference is likely negligible.



the preamp: mixing the channels

the channels were mixed using a passive mixer consisting of a 220k summing resistor for each channel. the summed signal was then sent to the el84 grid resistor via a .01 uF coupling cap. with the .047uF coupling cap installed on the ef86 channel, i installed a 4700pf cap to let more treble through. this had a negative effect on the volume and tone of the top boost channel, however. at full volume is was barely enough to hear a bedroom levels. after swapping the .01uf coupling cap in and removing the mixer bypass cap, the amp was sounding good again.



dedication

after the initial ac-4 concept was explored, i was made aware that my brother would be coming home after being away for a few years. he is a big fan of the vox ac30 sound and i wanted to be build him an amplifier as a congratulations for his great accomplishments during his two years away from home. this was probably the greatest driver to add a top boost channel to the amp.

one of the downfalls of building the amp for my brother was that i gave me a two to three week time frame to build and test the amp. because of this, some of the design checks i would normally go through were skipped. as a result, there were several interesting mistakes make in the build process. the control panel layout was completely backwards of how i had planned it. it seems that i had built the circuit board backwards (staked the turrets on the wrong side.)

also, i wouldn't have time to give the amp a proper cabinet. my father bought a speaker at a thrift store so my brother could use it temporarily until he got a better guitar speaker. i built the amp 'tweed combo style' and drilled mounting holes so it could be mounted in a standard tweed deluxe cabinet. to protect my brother from the high voltages present in the amplifier, i bought a cover for the chassis. the chassis, you will notice is powder-coated in black. I wanted the amp to look nice, even outside of a cabinet. i think the black chassis and the cream knobs look quite nice.

one of the other mistakes you may notice is that i had not checked to see if there was enough room for the input jacks when i placed the turret board. luckily the board was easily modified to account for this.

layout mistakes were also found when putting the amp together. because of this i was careful to use shielded wiring to prevent parasitic oscillations from occurring.

again, one of the downfalls of having such a short build time was that i spent a good deal of time building, testing and tweaking the amp rather than playing it. i didn't have time to make any recordings of the amp either. all i can really say about the amp was that the top boost channel sounded right (good and chimey) and the ef86 channel had a good clean sound and an average gainy sound. long live the vox top boost!

one of my newer experiences in this build was to actually write a users manual for the amp since it was to be delivered to a 'customer.' i was sure to include plenty of statements that could be argued as being true or not to hype up the product. the manual also contained some info about the historical significance of the circuits.

the cabinet

several months after i delivered the amp to my brother, i saw a vox solid state combo up for sale on craigslist. the $85 price tag seemed pretty good, but the best thing about this amp was that it seemed just the right size for an ac4 and had all the right vox cosmetics. i had just blown all the money in my amp fund so i couldn't buy it at the time (we just started saving up for a house, so money is a little tight.) a few weeks later, i had saved up enough to buy it and the seller had lowered the price to $70- even better.

i did a little checking before buying the amp. i asked the guys at the plexi palace vox forum to take some measurments of thier amps. they were quite helpful and i soon realized that with a little modification, the ac4 i build would barely fit into to vox pathfinder combo cabinet. one of the plexi palace folks even offered to buy the solid state amp chassis, after i had fitted the ac4 to the cabinet!

i bought the amp with the understanding that if the ac4 didn't end up fitting the amp that i would build myself a similar amp into the cabinet.

after a few weeks i was able to get the ac4 chassis from my brother and i did a dry fit. it looked like the chassis would barely fit the cabinet. i would have to remove about 1/4" of material from the braces that held the back panel in place and the 2" tall hammond chassis i used wasn't quite wide enough to fill the entire control panel cutout.





after removing the material from the braces, i did another dry fit of the chassis. the amp seriously barely fit. the mounting holes in the pathfinder and the mounting holes i drilled for mounting in a deluxe style cabinet didn't match up (about 1/4" off), so i decided to redrill new mounting holes so as not to modify the pathfinder cabinet excessively. i also used some heftier chassis mounting bolts to hold the chassis in place. With the chassis mounted in the amp, there was still about 1/4 - 3/8" of a gap between the top of the chassis and the edge of the control panel cutout. i put to work a little trick that i used on my most recent ga5f1 build and put some large gold piping in to fill the space. it didn't quite look right, so i placed a little gaff tape (black cloth tape) on the piping mounting flange. it matched the amps vinyl convering pretty well and when i put the amp together again i was amazed by how good it looked.



the back panel would also need some slight modification in order to look its best. the pathfinder had a cutout in the back panel for external connections that exposed some of the components of the amp. that's a potentially bad safety concern. the cutout had a serial plate immediately to its left, so i removed it and reinstalled it so that the left mounting holes on the serial plate now lined up with the right mounting holes in the cabinet. I put some screws in the right serial plate mounting holes and the now exposed left mounting holes on the cabinet and the serial plate now covered the chassis cutout.



unfortunately, the screws that attached the back panel to the pathfinder chassis we in the wrong locations to perform the same function with the ac4. they looked a little silly when they were simply removed. i purchased some nuts to fit the screws, drilled some holes the size of the nuts into the back end of the back panel, installed the screws so that the countersunk nuts held the screws in place and cutout the excess screw length with a hack saw so the screw and nut assemblies were relatively flush with the back panel.



now the amp as a unit was finally finished! and it looked much better than i would have thought it would. while i had the amp for a while, i played with it a little bit and it indeed sounds wonderful. the 8" speaker gives the amp some quirkiness, but cutting the treble seems to negate any unwanted effects from the speaker. with the 8" speaker, the amp is just loud enough to sound good while not hurting the ears!

the ef86 channel on its own isn't my favorite sound, but it sounds pretty good. mixing the ef86 with the top boost channel gives quite the array of tonal possibilities. since the ef86 is out of phase with the top boost channel it makes even more of a difference.

it tested the amp with various distortion pedals and it takes them very well, especially the ef86 channel.

Listen to a clip

part 1: "a hard day's night" left channel- top boost, right channel = ef86
part 2: ef86 channel
part 3: top boost channel dirt
part 4: top boost + ef86 dirt


conclusion

this amp build was a great experience. my brother hasn't played the amp much, yet, as he is trying to get back into normal life and apply for graduate school and find some way to support himself along the way (and he doesn't have much for an electric guitar.) i have definitely learned the value of planning, and planning time to build an amp right. luckily, most of the 'mistakes' in this amp build weren't detrimental to the performance of the amp. I hope that i have pointed out those mistakes enough here that anyone desiring to build a similar amp can avoid them. who knows, maybe i'll build myself one, too!

the pathfinder cabinet makes a great ac4 cabinet. a pathfinder can be found new for $120 or so which is still a great deal for a vox-style cabinet. knowing the dimensions available ahead of time would be very helpful before designing the chassis. i looked into other chassis sizes and it seems that the 13.5" x 5" x 2" chassis that i used is the best stock fit, though a custom-made chassis about 12 7/8" x 5" x 2.5" or so would require no modification to the cabinet! especially if the mounting flanges were wide enough to accept the backpanel mounting screws.

i would certainly recommend this cabinet to anyone wanting to build a small vox combo.

in retrospect



vox recently released its ac4tv modern classic model. after building this amp, i would certainly rather have it over the ac4tv! the ac4tv has neither an ef86 nor top boost channel. a schematic has not been leaked yet, so who knows what is in there. it also does not use a tube rectifier. it has the ability to be turned down from 4 watts to 2 watts to 1/2 a watt. still, the ac4 i built was not loud enough to hurt the ears at full volume. if i were to buy the ac4tv, i would certainly want to modify it to match a classic vox circuit. in the end, it would probably be cheaper to just build a classic vox circuit in a pathfinder cabinet. i like the looks of the pathfinder cabinet better, as well.

update 6/27/2014:

at long last vox has released a small-power combo version of the top boost channel, the ac4c1 series. after building two 8 inch-based combo amps, the 8 inch speaker is really the weak point in the circuit. i was looking into putting a 10" alnico speaker in the pathfinder chassis, but found that it wouldn't fit.

after weighing my options, i purchased my first production amp in over 10 years!

the ac4c1 is the true classic top boost circuit with a few variations:

1) single-ended el84 output stage
2) buffered master volume (with bright cap)
3) increased filtering in the power supply
4) single input jack

the bright cap (c20) on the master volume control needs to be removed to retain the true top boost sound. i think that vox installed this to make the amp sound louder at low volumes, but it makes it sound thin.

of course, i won't be able to leave this alone. i will be installing a weber alnico 10" blue pup, some brown diamond grill cloth and..... converting it to a handwired circuit. the design is almost done and i'll post my results as soon as my conversion is complete.

if your looking for a quick way to get the topboost sound in an small powered combo, i would recommend the ac4c1. at 300 usd, it may even be less expensive that my pathfinder to ac4 conversions.

update 12/24/2009:

for the past year or so, i've been thinking about how to improve this amp, wanting to build one for myself. i've been watching the local ads for pathfinder amps for sale to steal the cabinet from.

i had a breakthrough tonight. i've have figured out how to implement my concept for a single ended, 5w vox amp with ef86 and top boost channel (complete with cathode follower), a phase inverter simulator (to give the ef86 that extra boost) with the channels mixing together in-phase with minimal additional components. one more preamp tube must be added, but the signal should be much more close to a vox ac15 than my previous attempt... now to find one of those pathfinders!

update 11/19/2011:

the in-phase channel mixing concept proved to be a failure. the concept did work, but it allowed too much interaction between channels. i could make the EF86 channel sound good or the top boost channel sound good, but never both of them.

i will work on a putting up a post about a top-boost only amplifier.

also, a note about the pathfinder speaker. i order an 8" weber blue pup for the amp and while it did sound slightly better, the vox pathfinder speaker was remarkable similar. if you are looking to cut cost on your amp build, stick with the stock pathfinder speaker!