EVJ Mod-3 (Optional)

I elected to continue on with the BitMO products for this installment; and decided to build the BitMO 10-Uator. The BitMO 10-Uator is a low power rated attenuator, the first question that many may have is: "Why build an attenuator for such a low power amplifier?"

The answer is simple - the EVJ with the previous mods installed is very loud, pushing 115db through 2-12" speakers, is about the volume to practice with a drum kit and bass present. In order to get that tube saturation, the volume knob must be cranked up - the BitMO attenuator solves the problem of high volume in a bedroom or home recording studio by allowing you to get that power tube saturation without rattling the windows.

Some other features of the 10-Uator is a cabinet voiced line level out with separate volume control, to take the speaker signal direct to mixer or multi-track device with the speaker as a low level monitor or no speaker at all. In addition, the price is also attractive to get this functionality.

4.1 What is the 10-Uator?

The BitMO 10-Uator is very simple, there is no schematic supplied since it is basically wiring some jacks together along with an L-Pad, power resistor and some capacitors to create a 'load' for power dissipation as the output is directed away from the speaker cabinet itself.

All tube powered amps MUST have a load device attached to the power output or run the risk of frying a transformer when the amp is powered up. Normally, this is accomplished by a speaker(s) - with the 10-Uator, power not directed to the speaker load, is compensated for by directing the remaining load to components capable of providing power dissipation, which means this unit will run warm. Consider if you will, an attenuator as a device that provides a volume control for the speaker itself. When full power is not reaching the speaker, the remaining power is dissipated through the L-Pad and the power resistor inside the device itself.

An L-Pad is simply two rheostats wired in parallel, a rheostat is basically a heavy duty power rated potentiometer - the way an L-Pad works though, is no matter where in the wiper/rheostat is rotated to, the load remains constant - as in, if wired for an 8 Ohm load - at any position for volume, the load will always be the same. This is true for the 3 common load ratings this unit will support - 4, 8 and 16 Ohms respectively. I chose to build the 8 Ohm version in this installment.

The BitMO 10-Uator comes in two versions, one is a standalone box and the other is a plate mount to the back panel of the EVJ itself. I elected to go with the standalone box version, which will allow me to use this neat little gadget with other low power amps as well.

The BitMO supplied kit contains everything you need to build the 10-Uator; including power resistor, L-Pad, wire, jacks and kit box for mounting (if you choose the standalone version).

4.2 Building the 10-Uator

What is there really to say? Just follow the simple wiring diagram, mount the parts in the box, and you are done. A total of 14 solder/wire connections are really all it takes to build this handy little unit and the documentation supplied is crystal clear. I was able to complete the entire kit in 45 minutes, and that included a stretch from the work bench. See Figure 4-1 for a completed 10-Uator.

Figure 4-1. The 10-Uator Completed

4.3 Using the 10-Uator

Before hooking this unit up to an amplifier - double check or even triple check your work!
An incorrectly wired attenuator can fry a transformer!

As with any attenuator, the more power you divert to them, the more highs that are robbed from the signal. To compensate for that, I chose to wire the attenuator in the 8 Ohm configuration and plug it into my 4 Ohm output from the amp, since most all amps can handle a 2:1 load output, and using an 8 Ohm load will brighten the output signal somewhat. The speaker output of the 10-Uator is connected to my 8 Ohm load speaker cabinet as well. There is little need for concern on this project with a 2:1 load rating; given this is a low power application anyway.

The unit has 3 jacks; 1-Line Level Out (controlled by the small pot), 1-Speaker Jack (Controlled by the L-Pad), and 1-Input Jack (source signal from amplifier). The BitMO instructions provide usage instructions fro a no speaker application - with a speaker, operation is simple - plug the 10-Uator in between the amp and the speaker cabinet, adjust the volume on the amp for gain and adjust the L-Pad for volume of the speaker.

For line level control, I had to play around with the line level pot, starting at a low setting - to find the best signal output to my un-powered mixer. Depending on what you are running the line level signal into; you will always want to start with a small output and work your way up, especially with a mixer, to keep from possibly overloading/blowing the input on the mixer. On the sample below, I re-plugged the 10-Uator to the 8 Ohm tap/output of the EVJ.

You can also use the line level out jack as a source for another power amplifier; or into the power section of another amplifier through the effects loop return jack on the target amp. This will essentially turn the EVJ into a preamp - but with a fully processed output signal, instead of like most preamps, which only send a signal from whatever preamp tubes are being used.

4.4 Summary

Using the line level control - I was able to record direct, through my mixer to the computer - the capacitor used in cabinet voicing was quite adequate and I found the unit to be pretty musically useful. Very handy for a late night session where you may need to unplug the speaker. I have included a sound clip of the line level output being usedu the clip in the player is suffixed with an "LL" to denote it is a line level signal used on the clip.

I was summarily impressed with the attenuator function of this simple little unit, having built a few attenuators and dummy load boxes - the pure simplicity of this unit is a real plus, and the results are very good. In another clip, suffixed "ATT" - I ran the amp at full blast, and turned the speaker volume down below normal voice conversation level, around 60 db - the tube saturation comes through very well and mic'ing the cabinet was still easy, given the a quiet room environment. If you have the 10-Uator, you can get a very wide variety of clean/dirty tones out of the EVJ at various volume outputs - great for recording or even mic'ing it up on stage.

The sample clips this time around were done with a Strat, by request, to hear the bridge pickup through the EVJ. I fudged together two quick clips - the first one has the Strat playing a clean rhythm through its rhythm pickup with the lead played/overdriven using the bridge pickup. This combination was a DI from the Line Level output, into the mixer and then into the computer. The voicing using the Line Level only is darker than using the attenuator through a speaker cab/mic'ed.

The second clip was overdriven at high volume for both the rhythm and the lead tracks, the speaker was mic'ed about 10 inches away - since the output of the speaker cabinet was very low volume - you can plainly hear how a Strat interacts with the EVJ and the brightness of the overall sound coming from a speaker cab - the EVJ really likes single coil inputs in its tone stack.

Overall - the BitMO 10-Uator is a great optional addition to the EVJ mod project - it gives the amp recording, preamp and low volume versatility, perfect for the at home bedroom player. My only suggestions for this mod would be to use a tin/metal box instead of plastic (if using the box version) and including a passive tone control on the line level out. Neither of which is critical - but if running direct to a power amp, it would be nice to have at least one more level of tone control on the line level signal.

Of course this kit is optional, but I have to say at this point in the project, with mods from the previous installments done, and hooked to the 10-Uator, the EVJ is pretty darn musical at this point, and extremely flexible. For a total investment of $236.85 to this point, given the features, one would be pretty hard pressed to find a 12W tube amp capable of what this amp is now capable of… at this price!

4.5 BitMo 10-Uator Wrap Up

Materials: BitMO 10-Uator Kit
Total Cost: approx. $69.95
Time to complete: approx. 1 hour
Total Investment: Amp + Mod1 + Mod2 $166.90 + 10-Uator $69.95 = $236.85