Guitar Amp "power soak"

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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...

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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!!

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?

PRSWILL1 year ago
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..
r3son8tr (author)  PRSWILL1 year ago
PRSWILL - you're absolutely correct, it will work from an electrical point of view, but there really isn't an advantage because as you point out, any overdrive you get from a solid state amp is all in the preamp. Pushing a SS amp hard to clipping will result in a pretty harsh sound unlike the softer "warmer/musical" clipping of a tube output stage.
No worries about the late response, everyone's getting the flu these days. It's much more clearer now; thanks for the help!
Forgive my ignorance, a little confused based on the last diagram. I understand how to wire this little gizmo together, and my understanding is that the phone plug connects to the speaker out on the back (for my amp, a fender super 210, that would be the 8 ohms jack.) I was curious what the phone jack connects to? Do I run an instrument cable connecting that to the output? If so, would that be power amp out? Again, forgive my ignorance on the topic; been wanting to crank my 60watter full blast for such a long time.

Thanks, and I can't wait to give this a shot!
r3son8tr (author)  jnatividad21 year ago
ok, the phone plug (male) plugs into the amplifier's output jack (marked 8 ohms, as you mentioned above). The speaker plugs in to the female jack that is part of the Lpad circuit we just built. I hope that helps, if not I can take more photos of my set up and point the parts out more clearly. Good luck.
Also, do you think a 100W L Pad would be enough for a 60watt amp?
r3son8tr (author)  jnatividad21 year ago
Sorry for the delay in responding, I've been fighting a cold or flu..

Yes, a 100W Lpad would suffice for a 60W amp, as I think I mentioned I shoot for 100% derating (so a 100W L pad would be used for a 50W amp) but that's just a rule of thumb. It should be fine.
drewfunk1 year ago
Hi, i followed your tutorial and used the L-Pad for my Pro Junior, but i get no sound out of the amp. I get hum out of the speaker but no guitar. I checked all the wiring and it should be good. The only difference is that i'm using a stereo L-Pad, but just using one side of it. Any help would be great!
r3son8tr (author)  drewfunk1 year ago
Sorry to hear that you are having difficulty with this. A stereo L Pad should be fine as long as you just use one set of terminal pads. Sometimes we focus on the complex issues and overlook the simple ones - double-check your solder connections, and make sure the volume is turned up on the amp and guitar (trust me, I've made that mistake in the past!) If that is all good, can you attach a photo of your project, maybe I will see something...
DTWGuitar2 years ago
I am needing to put a soak on a Marshall MA100H. Do you think the 100W L Pad will work for this application? I am only asking because of the comment from earlier about the 30W amp using the 50W or 100W L Pad. I would also like to know what the "power derating rule of thumb" that you use for this if you wouldn't mind.
r3son8tr (author)  DTWGuitar2 years ago
For the long run, I would use a 200W rated L-pad on a 100W amp just to be safe. That's my power derating rule of thumb, just double the power rating. That said, I have not found any L-pads with a power rating higher than 100W so for your application you would need to build this out of discreet (individual) power resistors.
stonykill2 years ago
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 (author)  stonykill2 years ago
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.
reidhowland2 years ago
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 (author)  reidhowland2 years ago
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!

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

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 (author)  bmasschelein2 years ago
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.
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...
bigkidalex2 years ago
I have heard that this kind if setup is hard on tubes. Is that true?
r3son8tr (author)  bigkidalex2 years ago
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.
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 (author)  SideburnsBrent2 years ago
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.

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 (author)  SideburnsBrent2 years ago
Sure - stay tuned....