Good designs for dummy loads?

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Mark
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Good designs for dummy loads?

Post by Mark »

I'm wishing to build a dummy load that can take the brunt of a SVT. I want the load to at least be able to handle 4 ohm, 8 ohm, and 16 ohm outputs.

Thanks for your assistance.
Yours Sincerely

Mark Abbott
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nworbetan
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Re: Good designs for dummy loads?

Post by nworbetan »

Are you looking for a purely resistive load for testing and troubleshooting purposes, or do you need something reactive to use as an attenuator?

I personally have 500 watts worth (10 x 50 watts each) of 4 ohm resistors that I've been needing to buy/build heat sinks for but haven't gotten around to yet. I have an Ampeg V4B and a Marshall Major clone on my bench, not quite as big as an SVT but close enough that I'm very interested in this thread too. :)

Edit: I'm not 100% sure this link is going work but here goes nothing...

https://www.mouser.com/ProductDetail/Vi ... mggw%3d%3d

or search for NH0504R000FC02 on mouser.com

The catch is that they're derated to 25 watts without additional heatsinks, which can be as simple as a 1 sq ft aluminum sheet (see datasheet for thickness). But 10 sq ft of aluminum is kind of unwieldy no matter how cleverly you arrange them, and I haven't made it a priority to decide on more compact heatsinks to do the job yet.
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martin manning
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Re: Good designs for dummy loads?

Post by martin manning »

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xtian
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Re: Good designs for dummy loads?

Post by xtian »

Look at these—only $15 for 1000w!

https://www.parts-express.com/4-ohm-100 ... r--019-015
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sluckey
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Re: Good designs for dummy loads?

Post by sluckey »

xtian wrote: Mon Jan 08, 2018 4:02 pm Look at these—only $15 for 1000w!

https://www.parts-express.com/4-ohm-100 ... r--019-015
That's only 100 watts. Here's a 1000 watt...

https://www.ebay.com/itm/1000W-2-4-8-16 ... QncCRlqFcw
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xtian
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Re: Good designs for dummy loads?

Post by xtian »

Oops, you're right. Eyes blurry in the morning...
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pompeiisneaks
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Re: Good designs for dummy loads?

Post by pompeiisneaks »

I did a dummy load based on schematics on aiken amps site here:



There's a typo I think on the layout I did on my site, I put one of the wrong resistances but basically the idea is very simple.


~Phil
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TUBEDUDE
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Re: Good designs for dummy loads?

Post by TUBEDUDE »

Im using 100 watt resistive loads llike the type at parts express. I saw a vid somewhere of a lad that used dryer heating elements. Just cut to the measured resistance.
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R.G.
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Re: Good designs for dummy loads?

Post by R.G. »

A typical hair dryer is about 1200-1500W. At 120Vac, that's 10A - 12A, and 12 to 9.6 ohms. So the coil in there is in the right range for making a approximately 8 ohm 1kW load. Since dead or dying hair dryers are basically free, it's easy to pick up enough hair dryers and mess with them to make a 4, 8 and 16 ohm many kilowatt load. And cheap.
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martin manning
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Re: Good designs for dummy loads?

Post by martin manning »

How stable would the resistance be with applied power? Wouldn’t you need to have the fan blowing air over the element to use it’s rated power?
R.G.
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Re: Good designs for dummy loads?

Post by R.G. »

The resistance does change a bit, but not as much as, for instance, a tube heater or a light bulb filament. The wires inside hair dryers are almost certainly nichrome, and operated below the temp where they glow, as the air coming out can't really be allowed to hit the ignition temp for human hair. :shock:

Nichrome has a tempco that's at least an order of magnitude less than most metals, including copper and tungsten, the light bulb filament metal, so it's not doing dramatic changes. A quick google yields nichrome's tempco as about 0.00014. And remember that nichrome is the wire most used in wirewound resistors, anyway. So - not too bad unless they're glowing, and then you have a different problem than resistor tempco. :D

Yes, to hit full dissipation, the wires need a fan. But there is a fan included in each hair dryer, again for free. You might need to get extra hair dryers to get working fans, but the price remains about the same. Or, you could use a small external fan, added on. A possible fanless solution is to get yet more hair dryers and do the series/parallel thing and make nichrome resistors from the heating coils that don't need fans to hit kilowatts of dissipation. Again, much the same price. A quick trip to Good Will might turn up some candidates with working fans.

I'm a big fan of cross purposing gadgets. Sometimes I walk through Home Depot, where I have found that I can award myself extra points for thinking of a completely non-obvious use for some device in there. :lol: One other non-obvvious device use along the hair-gadgets line is the heated styling curler. These are handles with a line cord and a round hair brush on the end. They're mini-hair dryers that pump heated air out through the styling brush, and are often discarded when the brush gets dingled up.The heated-air handle often still works, and without the brush attachment, makes a slow but effective heat gun for heat shrink tubing, much more controllable than a pistol-styled heat gun that can also remove paint and melt solder.
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martin manning
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Re: Good designs for dummy loads?

Post by martin manning »

Thanks, R.G. I've never taken a hair dryer apart, but might do just that if the opportunity comes along. For dummy load purposes 300W would be plenty, so maybe there wouldn't be a need for forced air cooling. Speaking of Home Depot, a hot water heater element would be a good option too. I see on their site a 3800W, 240V element for under $10. That would be 15.8 ohms at rated power. (edit: et voilà: https://www.prosoundtraining.com/2010/0 ... ummy-load/)
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drew
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Re: Good designs for dummy loads?

Post by drew »

^ The next logical step in cross-purposing would, I think, be to put together a peanut oil filled unit that can simultaneously serve as a dummy load and a turkey deep fryer.
R.G.
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Re: Good designs for dummy loads?

Post by R.G. »

... and that was my next caution: water heater elements can only support their rated power when submerged in water. In air, they will have a dramatically lower power rating.

Still, maybe several hundred watts.

How hot something gets under internal heating depends mostly on how much heat is generated inside, the total amount of surface area, and the thermal efficiency of the surface. Lots of square inches makes for easier heat transfer; it's why heat sinks have fins. So does high thermal efficiency, which is best done by bathing the surface in a moving fluid.

Hmmm: that made me think of cartridge heaters. I ran into these when tinkering with 3-D printers. These are little 1/4"-ish by 2" stainless cylinders with two wires coming out. They're nichrome inside, and designed to get HOT. They come in various voltage and power ratings and are CHEAP on ebay. A few of these inserted or clamped onto a heat sink in series/parallel to get the right resistance would also make for a dandy multi-kW dummy load.
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Re: Good designs for dummy loads?

Post by SoulFetish »

R.G. wrote: Thu Jan 11, 2018 10:09 pm A typical hair dryer is about 1200-1500W. At 120Vac, that's 10A - 12A, and 12 to 9.6 ohms. So the coil in there is in the right range for making a approximately 8 ohm 1kW load. Since dead or dying hair dryers are basically free, it's easy to pick up enough hair dryers and mess with them to make a 4, 8 and 16 ohm many kilowatt load. And cheap.
Damn! what a great idea. I wish I had this tip when I built mine a couple of weeks ago. But, if I need a kilowatt load in the future, I'm going to give this a go.

Here is a simple way to switch your 4,8, and 16Ω in parallel for 2Ω (2.2875Ω precisely) if you need it and want to save some space.
You can use toggle switches, but this makes for a simpler and more elegant interface using a 4P4T rotary switch, IMO. (Just make sure to use a switch which can handle the load current.)
Rotary_Switch_for_parallel_2ohm_load.jpg
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