Dummy Load

[martin manning]

I picked up one of the 16-ohm lug resistors Allyn offered recently for use as a dummy load (thanks Allyn!). After some thought I arrived at the attached scheme- simplest, cheapest way I can think of to wire the thing up for 4, 8, and 16 Ohms.

Cheers!

Edit: this dummy load has evolved... it's a little smarter now. See new .pdf below... Monitor section added to simple, cheap version.

Edit: See new .pdf below... scope out divider modified to get accurate 1/10 of load resistor voltage and a few edits to the description.

[Andy Le Blanc]

I use a couple 300w ohmite 8 ohm fixed, series parallel etc..., even they can get hot with 100w applied to them.

[M Fowler]

I looked down towards the bench the other day, measuring in an amp, and saw my dummy load chord had come unplugged from the amp.

Whew the OT didn't get hurt, thankfully.

[Andy Le Blanc]

I tried to burn a hole into the top of my bench once with an undersized dummy load.
A cerment 10 or 20 w, I think, TUBE POWER ye haw...

[Cliff Schecht]

I looked down towards the bench the other day, measuring in an amp, and saw my dummy load chord had come unplugged from the amp.

Whew the OT didn't get hurt, thankfully.

Was the amp SE or PP? I've always been of the mindset that SE transformers are gapped so they don't saturate as quickly and therefore can handle a static DC current without saturating and heating up. PP transformers aren't meant to handle any static net DC current and therefore saturate much quicker when the secondary is unplugged. I've had a few instances where I flipped on an amp without a load and it seems like the PP stuff makes all kinds of sounds whereas the SE stuff doesn't seem to flinch.

I tried to burn a hole into the top of my bench once with an undersized dummy load.
A cerment 10 or 20 w, I think, TUBE POWER ye haw...

I've done this with a few switchers I was testing. Never fun trying to dig ceramic resistors out of a plastic bench.

[M Fowler]

This was a PP amp and no sound, funny thing is I usually just plug in a speaker rather than my dummy load.

[martin manning]

Improved Rev 1 schematic posted. See above.

[Smokebreak]

Thanks a bunch Martin. Looks like it's time for me to actually build one

[gktamps]

Thanks for updating this, Martin. I like the scope output.

It's much nicer than this much more crude version I built using a surplus heat sink and 100w resistors, with 1/4" plug.

The other boxes are safety load boxes for my groups of amp heads. I have sometimes forgotten to have a head plugged into a cabinet, since I've got many more heads than speaker cabs. I keep all heads plugged into a safety load box, except for whichever is plugged into a speaker cab.

Cheers,

Greg

[martin manning]

Of course the monitor section could be added to whatever you have, and any available load resistors could be used. Originally I was looking to get several load resistances without the need for an expensive switch.

Side note: As I was fooling around with it, driving the amp from my phone, it occurred to me that I could easily go wireless by connecting a bluetooth receiver to the amp input. A small USB powered one I use for playing music through monitor speakers worked great.

[Stephen1966]

Hi, I'm going to build a dummy load from Aiken's schematic.

This is for the 4, 8 and 16 ohm 100W (version 2)
https://www.aikenamps.com/index.php/dummy-loads

Here's the BOM:
Mouser No
71-RH50-8.0 Vishay 3x Wirewound Resistors - Chassis Mount 50watt 8ohm 1%
71-RH50-4.0 Vishay 1x Wirewound Resistors - Chassis Mount 50watt 4ohm 1%
502-12A Switchcraft 12A Phone Connectors 2 CONDUCTOR 1/4"
546-431611 Hammond Enclosures, Boxes & Cases ExtrudedAlum Sinkbox 4.92x4.2x2.36" Black
5164-1202BA Davies Molding Knobs & Dials 25/32"D X 5/8"H

(Mouser didn't have the rotary switches but I had one from Tube Town from a previous build)

Tube-Town Store - Rotary Switch 1 pol / 3 pos - 6 A Impedance / Voltage Selector
https://www.tube-town.net/ttstore/rotar ... ector.html

Edit: Here are the specs of the rotary switch if you can't find the Alpha type Aitken used.

4080_1.jpg

I will post some photos of the build when the components arrive.

Stephen

[Stephen1966]

So as promised, here are some photos of the Aiken 100W dummy load build.

Here are the parts...

Parts.jpg

To this, I also added some thermal compound I had left over from an old computer build...

Resistor prep.jpg

After, figuring out the layout, the resistors were mounted on the steel plate. The plate was just a bit of mild steel I had lying around, that I cut to shape and drilled. I used 3.5mm self-tapping screws throughout. The resistors' mounting holes needed to be drilled out as they are about 3mm but very easy to drill and plenty of flange left over.

Resistors.jpg

Then, before soldering any parts, testing the circuit. The jumpers proved to be somewhat high impedence so the readings were high at this stage. In this photo, you can also see the box which also needed to be pre-drilled to accept the resistor's mounting plate. The box is a good fit for the version 2 dummy load components, but you might struggle if you wanted to add the extra resistor included in version 1.

Test circuit.jpg

I used 18AWG cable left over from a speaker installation for the leads, you might use lighter I guess, and solid core would have been easier to work with.

Assembly.jpg

With the circuit leads in place, the readings for 4, 8 and 16 ohms were all within 1%

You can find a photo of the finished article on the 'Oscilloscopes' thread, here: https://ampgarage.com/forum/viewtopic.p ... 86#p417186