Mercury Magnetics

This chapter begins some actual circuit modifications, since the release of the EVJ in summer of 2006, many folks have picked up on modifying this amplifier. It has quite a following, well built on a stable platform, a simple Class A single-ended circuit design and packaged in a pretty durable and attractive enclosure -- makes this amplifier a great choice for modifications.

Bruce Hutcheon is one of the folks that took a serious step towards modifying the EVJ, forming his company "BitMO," he has several mod "kits" now available, Bruce's BitMO kits can be found at these locations:

Bruce, like many of us, is a guitar player and gear head who has put a lot of time and experimentation into his mods, through trial and error -- finding some of the best kits to be had for the EVJ. There are many other advertised mods, especially on eBay -- be aware that many of the $5 to $15 variety are just a set of instructions for mods that may or may not work very well.

BitMO kits come complete with parts selected to work well within amplifiers and a set of fairly well written instructions. In this chapter, I will describe the modifications, some of the major steps to install and an overall assessment of the BitMO "TRIO" modification kit.

3.1 -- BitMO TRIO Stage 1

The BitMO TRIO mod is actually two kits combined, one formerly know as the "DUO" and a second mod known as the Voicing/Gain mod. In stage one, we will be installing the "DUO" portion of the kit. This mod involves installing a control pot on the front panel of the EVJ, the control pot is a push/pull type, when pulled -- you boost the current tone setting.

The Tone control side of the pot (pushed in) is fairly standard, used on many early tweed style amps from Fender, Ampeg and Gibson, and it is used extensively in many boutique style amps still today.

When boosted with the knob pulled out, BitMO refers to the tone as, bottom heavy chunk to sweet overdrive to full on mayhem as the control is rotated.

So lets get started!

3.2 -- Preparations

You will need your tools for this mod, and I will assume that you have the amplifier unplugged and the back panel already off. You will also need a drill for this mod, fitted with a 5/16" drill bit, for mounting the control pot on the front panel of the EVJ.

First step will be to remove the chassis from the enclosure, you will find 4 plastic cover/inserts on the top of the EVJ, using a thin bladed knife -- work the edges up (taking care not to mar the covering material), until you can pull the inserts out. In the holes will be 4 large Phillips head cap screws. Remove the 4 screws and then extract the chassis from the EVJ enclosure.

The yellow highlighted area in Figure 3-1 illustrates where the Stage 1 modification will be installed.

Figure 3-1. Chassis Removed

3.3 -- Stage 1 Installation

Open the baggie you received in the kit, there should be 3 capacitors, 1 electrolytic (polarized) capacitor, 1 resistor, a control pot with "chicken head" knob and some jumper wire, see Figure 3-2.

Figure 3-2. Stage 1 Parts

The first steps will be to follow the included instruction and wire the control pot up. See the illustration in Figure 3-3 for a completely wired control pot. Notice that I use a pair of vice-grip pliers to lightly hold the pot while I work on it -- makes it easier to solder connections.

Figure 3-3. Wired Control Pot

Now that you are confident that you have the control pot wired connected, it is time to mount the control pot to the front panel of the chassis.

BitMO supplies a drill template to locate the placement of the control -- I recommend you use it, placement is critical, as different areas within the chassis are more prone to interference than others. Bruce found that mounting the knob where indicated, reduces the need to provide shielding for the control. Figure 3-4 shows proper placement of the knob hole where the "X" is.

Figure 3 4. Locating the Control Pot

After drilling and installing the control pot, your EVJ front panel will be sporting a new knob and look as illustrated in Figure 3-5.

Figure 3-5. A New Tone Knobs

Continue to follow the instructions and wire the control pot to the PCB at 4 points where indicated in the documentation.

Note: I chose to drill and mount the pot before wiring it in as stated in the instructions, I find it a little easier to keep gangly wire lengths down to a minimum if the pot is already mounted to the chassis.

Figure 3-6 illustrates the PCB and is highlighted in the areas we will be working in.

Figure 3-6. PCB

Figure 3-7 illustrates the stock schematic for the PCB and is highlighted where the circuit changes occur with this stage of the TRIO mod.

Figure 3-7. Stock Schematic

Double check your work, compared to the illustrations in the instructions, if it looks good, let's continue to the next section, which is testing this stage of modification!. See Figure 3-8 for how I installed the control pot to the front panel and wired it to the PCB.

Figure 3 8. Control Installed and Wired

3.4 -- Testing the Stage 1 Mod

I'll say it again -- even though this is a very simple modification -- double check your connections -- BEFORE applying power!

Figure 3-9. Stage 2 Parts

The first steps will be to follow the included instruction and wire the toggle switch up.

Note: At this point, if you have a Version 1 or Version 2 PCB, it will be wired differently than a Version 3 PCB. This is explained in the BitMO instructions clearly, for this project -- I'll be following the Version 3 wiring diagram.

See the illustration in Figure 3-10 for a wired Version 1 and Version 2 toggle switch.

Figure 3-10. Wired Toggle Switch (Version 1 and Version 2 only)

See the illustration in Figure 3-11 for a wired Version 3 toggle switch.

Figure 3-11. Wired Toggle Switch (Version 3 only)

Now that you are confident that you have the toggle switch wired correctly, it is time to mount the switch to the front panel of the chassis and wire it to the PCB.

BitMO recommends placement of the toggle switch between the volume control and input jack to keep the wiring away from the big caps on the PCB, placement is critical, as different areas within the chassis are more prone to interference than others. Figure 3-12 shows how I placed the toggle switch and wired it into the PCB circuit. For Version 1 and Version 2 owners, read the Version 3 insert that comes with the documentation -- the connections made to the circuit board are much easier to make as shown in the insert, and work for all versions of the PCB.

Figure 3-12. Toggle Switch Wired In

Figure 3-13 illustrates the stock schematic for the PCB and is highlighted where the circuit changes occur with this stage of the TRIO mod.

Figure 3-13. Stock Schematic

After drilling and installing the toggle switch, your EVJ front panel will be sporting another new feature and look as illustrated in Figure 3-14.

Figure 3-14. A New Switch!

3.8 -- Mod-2 Summary and Samples

First off, just let me tell you folks -- this mod kit works! I am thoroughly impressed with the quality of sound now coming from the EVJ, to the point of I would use this amp to record with now. The instructions for Stage 2 were a little confusing at first -- but reading carefully, I was able to follow them well enough to get it installed and working on the first try.

I especially like the tone of this amp with the boost knob pulled out at the 6:00 position, volume at the 6:00 position and the toggle switch set to "Whoa" (highest). The TRIO mod, as anticipated added some brightness to the amp overall, much of the gain "noise" has been tamed and the result on the higher settings is very rich, creamy and smooth. With a little reverb added, this amp could do a pretty fair Pink Floyd to Santana type tone. The sustain is now much better at the higher settings, with a very controllable and musical feedback coming in. The cleans at the lower volumes have improved as well, but not as dramatically as the high gain stuff.

I really didn't find that much of a difference between the "Low" toggle switch setting and the "Mo" setting, especially when the boost knob was pulled on. But flipping it in "Whoa" (highest), really does bring this amp to life. If I were to gig with this amp right now, with only this mod installed -- I would throw a Marshall Guvnor or other overdrive pedal in front of it for a pure metal sound. When I ran it full blast on everything, a very fat Mesa Mk 1 type of tone came out of it on the neck pickup -- I was pretty surprised.

For the price -- this mod kit from BitMO makes a huge difference and is well worth the time, effort and cost to implement -- converting this amp into a real musical unit, very nice indeed.

Here are some more samples -- note that the syntax is Mod, Volume, Boost and Toggle. So the name of Mod2-1200-B1T1 will translate to Mod-2 (TRIO), Volume at 12:00, Toggle in the Low (1=Low, 2=Mo, 3=Whoa) and Boost on (1=on, 0 =off) position. A total of 6 new random samples have been added with this mod.