Guitar Amp "power Soak"

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Introduction: Guitar Amp "power Soak"

Guitar players generally love tube amplifiers, especially when they are driven hard to get that nice, creamy tube distortion. Problem is, the amp will be friggin LOUD - even a small 15 watt amp can be too loud in certain applications. So, how do we get that overdriven tube sound without rattling the windows or destroying the ear drums of the people in the first row at our gig? We use a "power soak" device. This is essentially a load that is placed between the amplifier output and the speaker that reduces the signal to the speaker, but lets the amp drive hard. The trick is that the amplifier wants to see a constant impedance on it's output - in many cases 8 ohms - in order to transfer the correct amount of power. Just adding a simple resistor in series with the speaker won't work.

This Instructable will show you how to build a simple power soak for your low-wattage tube amp - basically any power level up to 25 watts into a 8 ohm load. Total parts cost is around US$15, and all parts can be ordered online from one source. I assume that you have basic soldering and wiring skills, and the tools to go along with that work. 

Here we go...

Step 1: Tools and Parts


You will need basic soldering and wiring tools such as:
  • Soldering Iron
  • Solder
  • diagonal wire cutters
  • wire stripper
  • long nose pliers
  • small wrench
  • screwdriver
The parts you need are as follows, and can all be sourced form Parts Express:
  • Mono L-Pad, 50W , 8ohm, p/n 260-255
  • 1/4" mono phone jack, p/n 090-321
  • 1/4" mono phone plug, p/n 093-140
  • Zip Cord wire, 18 gauge, 2 feet max length (it is sold by the foot), p/n 100-050
You will see that Parts Express sells (as of the date of this writing) three different L-Pads, a 15W, 50W and 100W. My particular application is for a Fender Pro Jr. amp, which can deliver 15W into 8 ohms. I chose the 50W L- Pad so that I can be sure it will never overheat. In general, always use power components rated at least twice what their actual power dissipation will be. (I learned this many years ago designing military grade power supplies)

Step 2: A Note About Zip Cord...


Zip Cord is two conductor wire where the two conductors are connected together by a thin strip of the PVC coating - they can be easily separated from one another like a "zipper", hence the name. In the photo you can see that one of the wires is marked with a red stripe to easily indicate which wire is "positive" or "signal". The unmarked wire is generally called "negative" or "ground".

I'm using #18 AWG wire since this is a low power application and the wire lengths are short. You can use 16 or 14 AWG if you want (the lower the number the thicker the wire) but thicker wire is harder to fit inside the jack and plug housing.

Step 3: Locate the L-Pad


I installed my power soak in one of my Fender Pro Jr. amplifiers (I have two of them - for large gigs I bring both and run them in stereo). I found a space on the back panel of my amp that had just enough room on which to mount the L-pad, and I only had to drill one hole to do it. If your amp doesn't have the space, you could easily mount the L-Pad in a separate metal chassis.

1. Mark the hole to mount the L-Pad shaft
I marked the location of the 3/8" hole on strips of masking tape, 3.5" from the left edge of the back panel (as viewed from the rear) and 1.5" up from the bottom edge.

MAKE SURE YOUR AMPLIFIER IS TURNED OFF and UNPLUGGED  from the AC MAINS for at least 15 minutes BEFORE REMOVING THE BACK PANEL! There are dangerous high voltages inside that WILL KILL YOU if you touch the wrong part while it is plugged in. If you choose NOT to unplug your amp then you have chosen instead to vie for a Darwin Award. Good luck, but don't blame me.

2. Remove the back panel and drill the mounting hole
You will need a 3/8" drill bit for this, but I suggest that you drill at least one pilot hole first

3. Countersink - or chamfer -the back side of the 3/8" hole.
I found that the L-pad has a bushing surrounding the threaded shaft which prevents the L-Pad from mounting flush to the back side of the panel. This also reduces the number of threads available on the front side and makes it difficult to tighten the attachment nut. Use an exacto knife for this (turns out that the back panel of my amp is not solid wood, but more like press board)

Step 4: Wiring It Up


The three terminals of the L-PAD are numbered 1, 2, and 3 (the numbers are stamped into each of the terminal lugs). As you can see on the photo, I chose to mount the L-Pad to the panel first, then wire it up so I added masking tape with numbers so I wouldn't make a mistake. As an alternative, you could wire it up first, then attach the L-Pad. (note that the L-Pad comes with mounting hardware, a knob and a faceplate.)

Terminal #1 is "ground". The ground pin of the 1/4" phone plug AND the ground pin of the 1/4" inline jack connect to this terminal.
Terminal #2 connects to the "positive" terminal of the female inline jack (using the red striped wire).
Terminal #3 connects to the tip of the 1/4" phone plug. (this plugs in to the output jack of the amplifier)


Step 5: Oops.. Minor Adjustment


When I first put this all back together, I didn't like where I had located the L-Pad terminals - they were facing the left side of the amp, when viewed from the rear. I loosened the L-Pad and rotated it 90 degrees such that the terminals were facing up toward the amplifier chassis. This made it easier for me to access the amplifier output jack. NOTE - wrap the L-Pad terminals in electrical tape to prevent accidental short circuits. I also added a cable strain relief to make sure the wiring wouldn't get pulled apart when wrapping up the power cord.

Step 6: Finished!


See the photo for the finished product. I can now run my amp with the volume cranked to 11 and the guitar wide open to get that sweet tube overdrive, but with a volume level that won't disturb my neighbors. The L-Pad gets slightly warm, but nothing to worry about. With the L-Pad control set fro maximum volume I did not notice any change in tone or loss of signal (I didn't expect any), but I verified that with my Tektronix audio analyzer. There was no measurable difference between having the L-Pad connected or plugging the speaker directly into the amp as before. 

I will be bringing this to my next "acoustic" gig and using it with my electric resonator guitar to get a nice and dirty delta blues kind of sound.

Some useful links:
http://www.bcae1.com/lpad.htm

http://www.parts-express.com/wizards/searchResults.cfm?srchExt=Cat&srchCat=196


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Would you be able to advise on how to build a power soak for a 40W combo amp?

Sorry for the delay. This is easy to do, just use a 100W L-Pad. The circuit stays the same.

http://www.parts-express.com/speaker-l-pad-attenuator-100w-mono-1-shaft-8-ohm--260-265

I have this L Pad. But it is a linear attenuation. So all of the attenuation is in the first 1/10 of a turn. Dang. Really wanted this to work. Any thoughts for a mod? Be nice to have a nice gradual attenuation throughout the range.

Hmm, something isn't right. Most of the attenuation happens when the Lpad is near the minimum point, It's normal to notice very little attenuation at the beginning of travel. Remember that we're really attenuating the loudness, or volume, of the amp which we measure in dBs - they're logarithmic. UNtil you get to about half power, your ears won't notice much.

If you're getting all of your attenuation right away, there might be a miswire. No mod is needed, the circuit works as described.

The wiring diagram is the same, but instead of the inline 1/4" jack (that connects to the speaker) and the 1/4" plug (that connects to the amp output) you will use two 1/4" panel; mount. Everything gets installed in a box - make sure there are some vent holes to let the heat out! You will then need two cables, one from tehj amp to the attenuator and one from the attenuator to the speaker cabinet.

I have an amp head and speaker cab. Can't I just make a box with the l-pad in it and have in and out jacks to put the box between my amp and cab? If so, could you please provide a simple wiring diagram? thanks

Can't wait to try this. I have the mono L-pad, but I'm wiring an Ampeg that has the speaker wired directly to the amp with no 1/4 plugs. The problem is that I can't seem to tell which wire to the speaker is grounded. If I hook up the multimeter to either contact on the speaker, it says there is no resistance. I'm stumped!

the output from your amp is ac, there is no polarity. Don't worry about which wire to hook up where. Just hook one speaker wire to the 'ground' and split the other with the one from the speaker connected to 2, and the amp output going to 3

Sometimes there is a "+" and a "-" on the speaker terminals. You can also check the polarity by attaching a battery to the speaker terminals - if the cone moves away from the speaker that'll tell you which terminal is positive.

Very good info here! I´m glad I find you nice and friendly people her. Inspire me to build things I even don´t use :) I´m going to follow you, r3son8tr!