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)

Comments

[stonykill]

tell me about the longevity of this. Does it hold up? Or are you replacing it s lot? If yours has held up well, I plan on building one!

[r3son8tr]

it has been holding up really well. I use it a lot, mostly for prctice, and have been setting the output level to about 5 watts - with the amp wide open so the L pad is soaking up about 10 watts. It gets warm after a while, but never too hot to touch so all is well.

[reidhowland]

My order from Parts Express arrived today, so I'm looking forward to putting Jr. through some paces this weekend. I'm confident this is going to work well for me. That said, out of sheer curiosity...

What exactly is the difference between something like this and say, a THD Hotplate--(features aside, obviously)? I guess I'm wondering, given that Hotplates etc. are sort of pricey, why hasn't anybody thought of this before? If I get a hundred-watt L-pad, will it tame my AC-30, or is that where the Hotplates etc come into their own?

Thanks for sharing your expertise with a bunch of guitar players!

[r3son8tr]

I haven't torn apart a Hotplate yet (mostly cuz i don't want to pay $350 for one just to take it apart) but from what I've read the main differences are:

1) Hotplates are available in different impedances (2ohm up to 16ohm) whereas L-Pads are only 8ohms (although you can build a custom attenuator at any impedance, it just won't be a rotary knob)

2) Hotplates offer a Line out so you can literally turn off the speaker and run direct into a mixing console.

3) Hotplates claim to be frequency-compensated so that at greater attenuation levels (lower volume) the highs and lows are boosted - or really just attenuated less. This compensates for the change in sensitivity of the human ear, NOT the effect of the attenuator circuit.

For single speaker applications, #1 isn't really an issue as pretty much all guitar speakers are 8 ohms. It's only when you get to cabinets with 2 or 4 speakers do you see different impedances. Issue #2 could be important to some folks who record their tube amps at home. This wasn't important to me. Issue #3 I don't think is a major factor for the following reason - you can simply boost the treble and bass on your amp to compensate for the lack of sensitivity of your ear.

My goal was to design a circuit to reduce the volume of my amp to allow for at-home practice at reasonable levels and small-gig performance when mic'd through a PA, all at low cost.

Good luck modifying your Jr, and have fun!

[bmasschelein]

Hi. Interesting instructable. I'm wondering if I could apply the same for my Peavey Vypyr 30W:

http://www.peavey.com/products/instamplifiers/guitaramps/vypyr/index.cfm/item/116751/Vypyr%26reg%3B%26nbsp%3B30.html

It's not a tube amplifier, but I have the same problem: when I enable distortion, it's hard to put the amp at an enjoyable sound level.

I guess I would need a higher-watt L-Pad, but would the overall idea work on this non-tube amp?

[r3son8tr]

This concept will work just fine with your amplifier. The 50W L Pad will suffice with a 30W amplifier even though it "violates" my power derating rule of thumb by a little bit. If it makes you nervous, then by all means use the 100W L Pad from Parts Express - the circuit is exactly the same.

The only thing I noticed when I looked at the Peavey website was that the speaker wires are hard-wired to the amp (most solid state amps are built this way) so your task is only slightly more complicated. You can either buy crimp on lugs (males and females) and a crimp tool from Radio Shack or Parts Express, or solder everything together. Personally, I would go with the lugs so that if you ever want to remove the L Pad you can do it easily.

[jimmyjive5]

Hi,
While this will work fine, solid state amplifiers don't get the same sonic benefit from being driven hard that tube amps do. If it is sounding better at high volumes it may be more related to how the speaker itself reacts to the output then a factor of the amp being driven hard. Since the speaker will "see" the power that it does at lower volumes you may not notice a benefit.

Still for $15 it might get you what you want...

[bigkidalex]

I have heard that this kind if setup is hard on tubes. Is that true?

[r3son8tr]

Not at all. The amp still "sees" the same impedance regardless of the L-pad setting. It is no harder than if you just run the amp wide open into a speaker load.

[SideburnsBrent]

I definitely need to make one of these for my setup. (Hopefully) quick question:

How would someone go about making one of these for a 300 watt 4ohm head ala. Ampeg SVT Classic?

[r3son8tr]

Interesting question. First issue is that you wouldn't be able to make a rotary variable power soak, because - as far as I can tell - L Pads aren't available at the power level you need. There are commercial devices such as the Marshall power Brake and the Dr. Z Air Brake, but they also only go up to 100W levels.

For your application you will need to come up with a fixed (non-variable) attenuation level that you like. For example, let's say that instead of 300W out, you only want 75 W max into your 4 ohm cabinet for small club gigs. This happens to be a 6dB attenuation level. (I chose this as an example because the component values work out to be standard values...)

It turns out that to build this fixed (non adjustable) L Pad, you would add a 4 ohm resistor in parallel with your existing speaker, then also add a 2 ohm resistor in series with this set. The 2ohm resistor would need to handle 150W, while the 4 ohm resistor handles 75W. For reliability, we would spec a 300W 2ohm resistor and a 100W to 150W 4 ohm resistor. These kinds of components are available - I used to use high power load resistors when I designed power amps at Crest Audio - but they aren't cheap. If you are interested, I can do some research for you if you want to find prices. I can send a schematic as well.

I hope this answers your question, let me know if you need more info.

[SideburnsBrent]

Thank you r3son8tr! If you wouldn't mind me tapping your knowledge to be able to build one of these that would work for my setup I would be so grateful!

[r3son8tr]

Sure - stay tuned....