This before and after schematic illustrates the wiring changes needed to beef up a Vox AC30C2X, including the addition
of a footswitch controllable DPDT switch.
Dear Amp Man,
Great articles. At the moment, I'm using some of your ideas rebuilding a Pignose G40V.
I have a Vox AC30C2X. Sounds great. It's my amp for non-in-ear gigs. It's bright and I play a Fender through it, so I've done some minor modifications to take some of the brightness out of it—nothing major. I run the amp with the two channels patched together, and one channel cleaner than the other. I never run it flat-out.
I was looking at the schematic the other day and wondering if it would be practical to run the normal channel's output from V1 through a switched jumper of some kind to the top boost's input on the other triode of V1 to cascade the channels. Would a setup like that work and get me into Mesa/Boogie, 800-pound violin territory, or would I end up with distorted mush? Foot-switchable would also be way cool. Or should I save my lunch money and get a Lone Star Special?
Any other thoughts or ideas on the Vox? Appreciate it!
Regards,
Mitch Morgan
Hi Mitch,
Thanks for reading, and glad you're getting some useful information from the column.
There have been so many iterations and re-issues of the Vox AC30 over the years, so thanks for being very specific as to which model you have and even supplying the schematic! There was one small issue I ran into regarding the schematic, however. The definition was not quite good enough for print, and even though I also searched the web, I could not find a better version. That being the case, I've re-drawn the necessary sections here and present them in both a “before" and “after" version, which should make it a bit easier to see exactly what's going on.
WARNING:
All tube amplifiers contain lethal voltages. The most dangerous voltages are stored in electrolytic capacitors, even after the amp has been unplugged from the wall. Before you touch anything inside the amp chassis, it's imperative that these capacitors are discharged. If you are unsure of this procedure, consult your local amp tech.The modification is not that complicated, and although this is a circuit-board amp, the procedure I propose should not require cutting circuit traces. I've also made it switchable using a DPDT on-on switch, so that one position is stock and one is modified (cascaded channels). In the “modified" position, the normal volume control will act as a pre-amp and the top boost control will act as a volume, with the master, of course, remaining a master. So let's get to it. By the way, the steps and descriptions below are also nicely mapped out on a drawing of the rear of the DPDT switch appearing on the schematic.
Fig. 1 — Start by lifting the side of the R37 resistor not connected to the amp's integrated circuit and attaching a new wire to it. This wire will connect to one of the common terminals of the DPDT switch.
Viva la resistors! First, locate resistor R37 (Fig 1), which connects the output of the normal channel to the op-amp that drives the effects loop, and find which side of this resistor connects to a pin on U1. Since a direct trace cannot be seen on the top of the circuit board, this can be found by using a multimeter with a continuity function. Once you find which side of the resistor is connected to the IC, lift the opposite side from the circuit board. You will then need to connect a wire to the lifted side of the resistor. This wire will run to one of the common terminals (#5) of the DPDT switch.
Next, locate R7 and R8 (Fig 2), which are the input resistors from the top boost jacks. Find which end of these resistors connects to the grid of V1 and lift this end of both resistors from the board. Now connect a wire to one of these pads and connect the other end to the other common terminal (#2) of the switch. Return to R7 and R8, connect the open leads to each other, and attach a wire to this junction. Connect the other end of this wire to a switch terminal on the same side as the previous wire (#1). Now connect a wire to this same position on the opposite side of the switch (#4) and connect the other end to a ground connection on the circuit board. A good spot may be the ground side of R16, which can be verified with your meter.
Fig. 2 — The R7 and R8 input resistors from the top boost jack must be connected to another of the switch's common terminals and brought to ground through an additional easy-but-important step.
Now, the last part of the procedure is to connect a jumper between the two remaining open terminals on the switch (#3 and #6). Also, connect a wire to one of these terminals. The opposite end of this wire needs to be connected to the wiper terminal of the normal volume control. This can be done by connecting it directly to the center terminal of V1 or to the pad where you removed a lead of R37, which may be easier to connect to.
Tune-up tips. That's it! You're all done. I'm really not sure if this will get you into 800-pound violin territory, because only a few amps are truly capable of that, but here are a few tips for this mod. First, it may be best to use coaxial cable to make these connections to reduce noise and the chance of oscillation. If you do, remember to connect only one end of the shield to ground. This will prevent any ground loops. Next, if the result here is just too aggressive, overdriven, muddy, or just plain hairy, you will need to better control the signal from the normal channel. This can be done by replacing the jumper on the switch with a resistor and capacitor in parallel. You can arbitrarily start with a 220k resistor and a 220 pf capacitor. Decreasing the resistor value will raise the signal level, and vice-versa. The capacitor controls the amount of high-frequency content in the signal. A smaller value will be brighter and a larger value will be thicker.
There you have it. Your very own Vox AC30 cascade. Enjoy!
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Guitarists are used to coveting cranky, old gear, but if you’re not a rack user, the H3000 might not be for you! If you ever see one of these in a studio, be sure to take some time to check it out, and maybe give the plugin a try.When it debuted in 1987, the H3000 was marketed as an “intelligent pitch-changer” that could generate stereo harmonies in a user-specified key. This was heady stuff in the ’80s! But while diatonic harmonizing grabbed the headlines, subtler uses of this pitch-shifter cemented its legacy. Patch 231 MICROPITCHSHIFT, for example, is a big reason the H3000 persists in racks everywhere. It’s essentially a pair of very short, single-repeat delays: The left side is pitched slightly up while the right side is pitched slightly down (default is ±9 cents). The resulting tripling/thickening effect has long been a mix-engineer staple for pop vocals, and it’s also my first call when I want a stereo chorus for guitar.
The second-gen H3000S, introduced the following year, cemented the device’s guitar bona fides. Early-adopter Steve Vai was such a proponent of the first edition that Eventide asked him to contribute 48 signature sounds for the new model (patches 700-747). Still-later revisions like the H3000B and H3000D/SE added even more functionality, but these days it’s not too important which model you have. Comprehensive EPROM chips containing every patch from all generations of H3000 (plus the later H3500) are readily available for a modest cost, and are a fairly straightforward install.
In addition to pitch-shifting, there are excellent modulation effects and reverbs (like patch 211 CANYON), plus presets inspired by other classic Eventide boxes, like the patch 513 INSTANT PHASER. A comprehensive accounting of the H3000’s capabilities would be tedious, but suffice to say that even the stock presets get deliciously far afield. There are pitch-shifting reverbs that sound like fever-dream ancestors of Strymon’s “shimmer” effect. There are backwards-guitar simulators, multiple extraterrestrial voices, peculiar foreshadows of the EarthQuaker Devices Arpanoid and Rainbow Machine (check out patch 208 BIZARRMONIZER), and even button-triggered Foley effects that require no input signal (including a siren, helicopter, tank, submarine, ocean waves, thunder, and wind). If you’re ever without your deck of Oblique Strategies cards, the H3000’s singular knob makes a pretty good substitute. (Spin the big wheel and find out what you’ve won!)
“If you’re ever without your deck of Oblique Strategies cards, the H3000’s singular knob makes a pretty good substitute.”
But there’s another, more pedestrian reason I tend to reach for the H3000 and its rackmount relatives in the studio: I like to do certain types of processing after the mic. It’s easy to overlook, but guitar speakers are signal processors in their own right. They roll off high and low end, they distort when pushed, and the cabinets in which they’re mounted introduce resonances. While this type of de facto processing often flatters the guitar itself, it isn’t always advantageous for effects.
Effects loops allow time-based effects to be placed after preamp distortion, but I like to go one further. By miking the amp first and then sending signal to effects in parallel, I can get full bandwidth from the airy reverbs and radical pitched-up effects the H3000 can offer—and I can get it in stereo, printed to its own track, allowing the wet/dry balance to be revisited later, if needed. If a sound needs to be reproduced live, that’s a problem for later. (Something evocative enough can usually be extracted from a pedal-form descendant like the Eventide H90.)
Like most vintage gear, the H3000 has some endearing quirks. Even as it knowingly preserves glitches from earlier Eventide harmonizers (patch 217 DUAL H910s), it betrays its age with a few idiosyncrasies of its own. Extreme pitch-shifting exhibits a lot of aliasing (think: bit-crusher sounds), and the analog Murata filter modules impart a hint of warmth that many plug-in versions don’t quite capture. (They also have a habit of leaking black goo all over the motherboard!) It’s all part of the charm of the unit, beloved by its adherents. (Well, maybe not the leaking goo!)
In 2025, many guitarists won’t be eager to care for what is essentially an expensive, cranky, decades-old computer. Even the excitement of occasional tantalum capacitor explosions is unlikely to win them over! Fortunately, some great software emulations exist—Eventide’s own plugin even models the behavior of the Murata filters. But hardware offers the full hands-on experience, so next time you spot an old H3000 in a rack somewhere—and you’ve got the time—fire it up, wait for the distinctive “click” of its relays, spin the knob, and start digging.
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