Dummy Load

Dummy load boxes are decent choice for several applications:

  • Use a small tube amp into a dummy load, tap the signal out and re-amp it with a higher power amp.
  • Crank the heck out of the amp and re-amp it with either a headphone amp or a small amp to get it to sub ear-bleeding level.
  • To be able to use effects with amps that generate most of the distortion in the power amp (such as AX84 Firefly). Such amps won’t benefit from having the FX-loop because effects will be distorted too, and that’s not a good thing for delay, reverb and other time based effects. One of the solutions is to drive the amp into a dummy load, tap the line level signal off, sent to effects and re-amp it.

When re-amping, secondary amp doesn’t have to be a tube amp at all, as long it’s not overdriven. The idea is to have it as clean and transparent as possible, to preserve the tone of the primary amp.

When constantly moving and playing late it’s not easy to carry a cab around and to crank it. In the previous couple of months I’ve been building a gear around idea to have either just a preamp or a whole amp with a dummy load, use my Palmer PDI-09 as a speaker sim and plug that into Boss Micro BR recorder. That setup allows me to change amps/preamps and still be able to easily record stuff, use my headphones and play over backing tracks.

The idea for this particular dummy load came from Palmer PDI-03 which is similar to my PDI-09 just with couple of extra switches and dummy load. It uses three 22 ohm resistors paralleled for ~7.3 ohms of resistive load in series with a 100uH air core inductor. Guys at SLOClone forum reverse engineered one of these and measured the inductor to be 150uH, so I was shooting for something in between. The original inductor was wound with a 1mm wire, but since it’s a 100W dummy load and I don’t have an amp over 5W I settled with #22AWG magnet wire from Radio Shack. They have a magnet wire set with three different gauges and #22 being the thickest.

Below is a plot for 100uH inductor in series with a 7.5ohm resistor (thanks FredB!). It shows that impedance increases after 1KHz or so. For purely resistive load it is constant for any signal frequency.


When playing with a 9W resistor as a dummy load, it would get very hot even with my 2W AX84 4-4-0 amp. I got some higher power resistors just to keep everything cooler. Two 15 ohm, 17W resistors in parallel yield 7.5 ohm load with total power handling of 34W which is way more than I need. I wound the inductor on the bobbin from Radio Shack that wire was originally wound on. I wound leftover wire on top of the other gauge from the set. It wasn’t as easy as I was hoping for because #22 is a bit thick and it’s not easy to keep the windings tight and close to each other. I used the Pronine inductor calculator to determine number of turns and it was just over 3 full layers of wire. Each layer of wire is coated with nail polish to fill the gaps and keep everything in place. When it was done it measured 110uH on my DMM.
Resistors are bolted to the chassis tightly and the inductor is mounted on the top. That makes the box filled very tightly on all axis. That could be a problem with heat, but I hope that 34W don’t even get warm with a cranked 2W amp.


To be honest, I was expecting that inductor will make a bigger change, but just can’t tell the difference between the resistive load and this device. To test this, I recorded a clip of clean guitar, played it from my laptop through the amp and recorded on Boss Micro BR. One clip is recorded with a resistive and the other with resistive-inductive load. I ran spectrum analysis in Audacity for both clips and put them on top of each other to make them easier to compare. Contrary to my expectations, inductive load has a bit higher treble response starting from 6KHz, but again, it’s not something I can hear. Other than that, both look pretty same (see below – red area is for resistive and blue for resistive-inductive load, purple is where they overlap). Good thing about this is that bigger resistors get just few degrees hotter than room temperature, but sonic difference doesn’t seem to justify the hassle of making an inductor :(. The next thing I’m gonna try is to gut out a speaker and use it’s motor as dummy load. Stay tuned…


Click on an image to see more details.

13 Responses to “Dummy Load”
  1. thedudebro says:

    Hey man, I’m down to the wire on this subject, basically I just want to do what the Palmer PD-03/5 does with the tapping (Dummy Load)
    In the 1st or 2nd paragraph I got excited when the theory about using a separate power amp to suck the power from the speaker out of the “thru” from an attenuator, but then that brings the fact that the power has to go somewhere.
    I’m all cool with buying a Behringer or some other cheap attenuator. My goal is simply to record onto my interface from my Carvin X100B without using speakers because I live in an apartment building.
    I’m fairly competent in circutry if I’ve got a clear-cut guide, I can carry out a project to emulate the PD-03, I think it’s nuts that they want almost 700 bucks and nobody rivals them!
    The Tru-Load idea is something I read about, looks like it’s all done for you with no attenuator necessary but it still uses speakers. I might just do that if i can make a soundproof enclosure for them, otherwise I really just want to make something and rub it in Palmer’s face.
    I’m kind of confused about the end product of what your post actually is, maybe it’s exactly what I’m talking about haha. But that SLOClone forum is done for, hope you can get back to me 3 years after this got posted lol.

    • Bancika says:

      This is only the dummy load part of the palmer PDI-03, but not the main part of the beast – the cabinet simulator. I have the cheaper palmer PDI-09 which is exactly that, but without the dummy load. It costs 80 euros in Europe/150$ in US. It’s worth it IMO. Just bear in mind that dummy load has to be rated for at least twice the power of your amp. You can just get resistive or reactive dummy load from weber, they are not expensive https://taweber.powweb.com/amptechtools/truload.htm

  2. denny says:

    Are you talking about running a dummy load, instead of a speaker.?
    Doesn’t that present the next amp with a lot of current and voltage at the input.?
    Is the dummy in series or parallel with the speaker jack.?
    Seems like you need some type of line out or something.
    How does this work exactly.?
    Thank You So Much

    • Bancika says:

      Yeah, if you want to tap the line-level signal to feed the other amp you’ll need some sort of line out circuit. I was using this with palmer PDI-09 which does that.

  3. John Good says:

    Need some help on a line out box for a dry/wet rig. There are different variations out there (see links below) but all seem to be lacking. I built them all for a wet signal. They seem to add undesirable artifacts or deficiencies to the waveform. Can you provide the best schematic and instructions for a line out box tapped from the speaker out of an amp (no load).


  4. David says:

    Have you tried one of the weber speaker motors? Are thy safe to use alone?

    • Bancika says:

      I have one but didn’t get to use it. I’d be conservative with power rating as there’s no much movement as in the real speaker, most of the power will be dissipated as heat.

  5. Riccardo says:

    Interesting article! I wanna try to so the same for at least a 100w tube amp
    What components would you use for a 16 ohm dummy load? Is the inductor just an isolated coil? do you wire it after the resistor and feed back in the jack connector? I guess you had to apply nail polish on every turn of the wire…


    • Bancika says:

      Actually, it’s taken from a circuit intended for 100w (palmer pdi-03), so just grab a bigger resistor and you should be set. As for the inductor, it’s a simple air-core coil. I used nail polish between each layer just to pot it. For 16ohm you’d probably need to scale the inductor, I think by diving with 2.

  6. Shelby Smothermon says:

    Hi. One of my favorite bass players, Geddy Lee, uses a SansAmp RBI running into a Palmer PDI 05 and then direct into the PA for his live sound. I love the simplicity of this set up. Here’s my question, is it possible to build a dummy load with a line out, like the Palmer, that can handle 450 watts continuously?

    • Bancika says:

      Sure, just make sure that you get resistors capable of dissipating at least 600W or so. Weber has 200W resistors for 15$, getting 4 of them will enable you to get whatever impedance you need by wiring them series/parallel and give you nice 800W total rating to be safe. Here’s link https://amptechtools.powweb.com/truload.htm


  7. BluesXL says:


    I’m would like to feed the signal of an Epiphone Valve Junior Head into a reverb unit (or other post power-amp FX) and then into a second EVJ;

    Would this setup work:
    – speaker out of first EVJ to Palmer PDI-09
    – Palmer PDI-09 speaker thru to dummy load (16 Ohm / 50 watt)
    – Palmer PDI-09 (set to ‘normal’) line out to Palmer re-amping box
    – Palmer re-amping box to second EVJ

    What does the cab simulator of the PDI-09 do to the signal?

    I would highly appreciate your advice!


Leave A Comment

  • About

    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.