Guitar Amp "power Soak"




About: I'm a guitar playing electronics engineer...

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 for 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:



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43 Discussions


2 years ago

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

1 reply

Reply 2 years ago

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


2 years ago

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.

1 reply

Reply 2 years ago

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.


2 years ago

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.


2 years ago

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!

2 replies

Reply 2 years ago

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.


3 years ago on Introduction

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!


3 years ago

bugger that... if it has an "effects loop", just stick a volume pot spliced into a guitar cable thru it

acts as a master volume multiplier, sitting between the preamp and the power it retains 100% of preamp distortion and tone, albeit losing a bit of that power amp sag (which you never really get anyway unless "master" [marshalls] / "post gain" [peavey] is cranked to homicidal neighbours levels)

250k works wonders for a 5150/6505-style amp. rolls off the 120w monster to whisper quiet easily and effortlessly, no more searching with a micrometer for that "sweet spot" between postgain 1.0 and post gain 1.1 lol


3 years ago on Introduction

I am more interested in the workings of L-PAD itself. It appears to be three pots ganged on one shaft. If possible and not too much trouble, could you please let us know what is what on the internals?



4 years ago on Step 1

I need a 'Power Soak" that can handle 100 Watts and have a safety factor, like using a 100watt P.S. for a 50 watt amp. Is there any resister that I can put in series with the L-Pad ( figuring using a 8 ohm L Pad and 8 Ohm resister to match the 16 ohm impedance load) ? or would I have to use 2 L-Pads in series with only 1 of them adjustable with the other not mounted on the "face" of the P.S. ? I'd appreciate any ideas that won't roast the output stage of my amp, since my amp also acts as a space heater for my bedroom. LOL


4 years ago

To the OP, this is great! A friend and I recently came across info on power soak and was looking into it and I found your project. I've got an Egnater Rebel Mark II 30 head and a Tourmaster 212 cab. I just ordered the parts, I ordered the 100w L-pad just to be safe. I can't wait to do this project, I'm going to mount it in a pedal size box. Good info here, glad I read about the other post of using speaker cable. Thanks for he details!

I recently built an attenuator box from an L pad. I notice a faint sounding reverse feedback under the tone. It kinda of sounds like a tape being played backward under the note; very prominent when bending. The box is in between the 8 ohm input jack of my amp and my 8 ohm 1x12 greenback. The impedance matches, no extra parts, just a couple mono jacks and some 18awg wires and the Lpad.

Any ideas? It's driving me crazy!

2 replies

Sorry for the delay...
That's a very unique effect you're hearing. The Lpad shouldn't be causing this, but without hearing it - or putting a scope on the output - it's hard for me to tell what the cause may be.

I have heard a similar effect when I use my ElectroHarmonix Hum Debugger and bending notes, especially above the 12th fret. It's kind of a comb filter, in some ways like a slight chorus effect, but it's only noticeable with overdrive or distortion.

sigh... I designed mine in a box with in and out 1/4 in jacks. And connected everything up with patch cables, not speaker cables! I figured it out, it was a good learning experience. Thanks for repsonding.


4 years ago on Introduction

I'm looking to build an L-Pad for my Silvertone 1484 (which is insanely loud for it's size and age).

From what I can tell it's a 4 ohm head fed into two 8 Ohm Jensen 12"s wired in parallel.

Would I still be able to use an 8 Ohm L-pad in parallel with the existing two speakers?

This option wouldn't leave me with a consistent 4 Ohm load and I can't think of how to achieve it.

As an added side note, I'll be building this in a metal project box with 1/4" in's and out's I don't have to physically modify the cab or head. I don't think this will change anything as far as the wiring.

Any help would be greatly appreciated. I need to tame this beast!!


4 years ago on Introduction

Hello r3son8tr
I would like to ask you about building fixed level attenuators. The attenuator is to be used on a festival stage with shared cabs and personal heads. The attenuator would need to be able to accommodate common head wattages 50W, 100W, 200W as well as associated Ohms, 4ohm, 8ohm, 16ohm.

The level of attenuation only needs to be slight -3dB, then -6dB and then maybe a combination of to make 9dB if feasible. I envision a large box with simple flick switches and chains of ceramic/ sand fixed resistors found in pa speakers.

would I be right in thinking the box would need potentially 9 sets of resistor chains for just -3dB at all options and then another 9 resistor chains for a further -3dB?

how would I go about calculating the resistors I would need at the many different requirements?


5 years ago on Introduction

This is a good topic- been thinking about buying a new amp that has the ability to "soak" up power and play at reasonable levels. I saw a couple people asking about this for their solid state amps- I don't think a feature like this will do anything for the sound will it? The overall principle of having a power soak on your system is to be able to drive the power tubes very hard in order to get the tone and sustain that you want. With solid state amps, the tone is not affected by the volume control really- it's all pre-gain vs gain ratios. Maybe I'm wrong, but I think anyone putting this on a solid state amp will be dissapointed at the outcome..