vox ac4 with top boost


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.


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


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!


terminal said...

it looks like you have two sets of inputs, and you jumpered one to the other. does that mean you run the same signal into the EF86 channel and the 12AX7 channel? it seems like you'd have an out-of phase problem since the you run the same signal through two phase inversion stages on the 12ax7 side, and 1 phase inversion stage on the EF86 side.

Anonymous said...

Could you post a schematic? Also, what transformers did you use?