Worlds Most Expensive Load vs something less expensive

[Gtr0]

I am trying to learn more about using my oscilloscope in amp circuits for biasing, troubleshooting etc.

Until now I have been using my Suhr RL+IR as a load… it’s a reactive load box for 150W @ 8ohm. But I have also read that a static dummy load is better to use in this situation… I also read that it only matters for wanting to calculate power at the output. And with everything else, I believe very little to none of what I read on the internet from unknown sources until I learn more.

This is the only thing that I use this rather expensive 700€ box for… a load for when I want to play around with my oscilloscope, which is not as often as I’d like.

I see some amp repair guys, like Steve Fryette for example, use these huge wire wound in aluminum housing resistors as a static dummy load, but those huge things seem to elude me on these internets. I bought some (4) 8Ω 100W resistors, but they are a bit smaller than I’d expected …



I know they need a heat sink to be able to soak up more than a few watts, and I did buy 4x of these supposed 100W 8Ω resistors to run in series/parallel to get more wattage out of them. I also have one of these larger Hammond project die cast aluminum boxes. The one I have is about 25cm x 6cm x 4cm… about 10" x 2.5" and 1.5" high … very roughly speaking.



This box is definitely big enough to mount the four resistors inside comfortably - using perhaps some CPU heat sink paste, I wonder if this is enough? In the meantime I am on the lookout for something more substantial, but until then, I love little building projects so I figured if it’s good enough to handle my amps, then great.

I have a few Marshall "style" DIY amps and a few real Marshall amps… 100 watts and 50 watts, a few JTM45s… I know a 100 watt Marshall will put out a lot more than 100 watts at full tilt…

So the question becomes.. will these types of resistors handle this kind of power when mounted in this type of aluminum box? Or do I need something more substantial?

Thanks as always!

[Jon Snell]

They should be fine until the aluminium heat sink burns a hole in your bench, like mine did.
The dummy load withstood the heat but my bench still bears the scar!
If you place a 24volt 5Watt vehicle side lamp in series with a monitor speaker, that will allow you the option of winding the amplifier that is being tested up without annoying your neighbours or damaging your hearing.

[Gtr0]

haha... thanks for the info. I am having a chuckle because when I first started doing this, in around 2003 or so, I did some crazy experiments in my amp to which I hooked up some similar resistors and they left burn marks in my chassis stand after 15 minutes of use.

FWIW, Mouser used to elude me with shipping everything from the US and Belgium has all of these crazy import duties and fees on top of that.. BUT as I see having tried again after living here in BE for about 15 years, Mouser has become more international friendly... so I just ordered two of these.... 200W 8ohm ... seem must more robust.

https://www.mouser.be/ProductDetail/...pdpudKxA%3D%3D

I just ordered the resistors first as a test... I want to make sure I don't get the custom fee smack down... if it all goes well, I can get something more "fitting" for a housing or some type of heatsink... the heatsink made for these resistors are 80€ each with a long manufacturing wait time

I do thank you for the info!!!!

[Jon Snell]

Mouser are pretty good and Fedex or UPS are used to deliver.
They state, on their invoicing, that all import duty will be met by Mouser and purchasing components from them, using my business account, I have never been charged duty on any imports from USA or EU. They can be a tad on the pricey side though!
Happy days.

[Gtr0]

I don't mind a bit extra expense for being able to widen my available spread of products to buy... not being tied to TADoctor, Tube Town, Banzai Music etc is great because they have a very limited range of goods for this type of work. I haven't tried Mouser since I first moved here from the US and things have drastically changed for the better in terms of product availability, delivery, etc... So, I did see that Mouser covers all of the fees etc... awesome!

[Chuck H]

I use those aluminum housed resistors. I'm using the 50W models and double them up. At the time I bought them two 50's was more affordable than single 100's. I have mine mounted to the inside/top in a similar chassis to the one you've shown. I used thermal paste for this as well. My wiring is differen from yours now but I did have them in standard series/parallel as proposed for a while. I never had any trouble running my Marshall 100W into it. It did get pretty hot (but held up fine) when fixing a big (200W) Kustom head I worked on. I'm careful to not hammer the thing for too long at a time under such conditions.

[catalin gramada]

Just sink it in a bucket with cold water. I.ll bet you may dissipate 200w by far on a regular 50w resistor. Just keep it cold.

[tedmich]

I bought a few Ohmite Tap600 planar resistors off Ebay for cheap
https://www.ohmite.com/assets/docs/res_tap600.pdf

Theyre good to 600W and 150C, I have them on a Al plate and could drop them into an oil bath as needed

[Helmholtz]

Theyre good to 600W and 150C,.

Not quite, as the rated power at 150°C is zero.

[g1]

Just sink it in a bucket with cold water. I.ll bet you may dissipate 200w by far on a regular 50w resistor. Just keep it cold.

Tap water is conductive.

[glebert]

I use old space heaters as resistive loads, the kind with wire heating elements and fans. I get them at thrift stores for a few bucks. I've found that a 1500W heater will give about a 10 ohm load,if I want to get to 8 ohm for sure I put a 50 ohm wirewound in parallel with it and mount that to the metal shell of the heater. I leave the AC line hooked up to the fan to use to cool down the element. I have a couple of these loads that I can put in parallel if I want a 4 ohm load.

[Helmholtz]

Tap water is conductive.

Yes, but as always, my question is "how much".
So I filled a large plastic bowl with tap water and measured.
Results are rather strange (and interesting).
1) Using an analog Ohmmeter I found a resistance around 25k between the immersed probes. Varying probe distance didn't make much difference.
2) But with different DMMs results were very inconsistent, typically above 1M (!).
3) Finally I used an LCR meter to measure AC resistance at 120Hz and 1kHz.This gave me 1.6k@1kHz and 3k@120Hz.??

I think, only the results with the analog Ohmmeter are reliable.
The digital meters use a pulsed test voltage/signal and synchronized current evaluation.
My guess is that the ions in the water travel quite slowly causing a delay between test voltage and current, thus upsetting pulsing/gated meters.

Nevertheless AC resistance could be lower than DCR.
More research needed!

[Chuck H]

My only complaint would be that dunking my wired up resistors in a bucket of water is messy and probably bad for all components involved in relatively short order. There are a couple of threads here where guys that need to test enormous amplifiers have come up with creative alternatives for resistors. My favorite so far has been water heater elements. They are inexpensive compared to resistors (per watt) and if you ARE going to do a water bath this would seem like the way to go.

[nosaj]

Yes, but as always, my question is "how much".
So I filled a large plastic bowl with tap water and measured.
Results are rather strange (and interesting).
1) Using an analog Ohmmeter I found a resistance around 25k between the immersed probes. Varying probe distance didn't make much difference.
2) But with different DMMs results were very inconsistent, typically above 1M (!).
3) Finally I used an LCR meter to measure AC resistance at 120Hz and 1kHz.This gave me 1.6k@1kHz and 3k@120Hz.??

I think, only the results with the analog Ohmmeter are reliable.
The digital meters use a pulsed test voltage/signal and synchronized current evaluation.
My guess is that the ions in the water travel quite slowly causing a delay between test voltage and current, thus upsetting pulsing/gated meters.

Nevertheless AC resistance could be lower than DCR.
More research needed!

Add some salt to it if you want to change the resistance.
hams call it a saltwater dummy load, they just add salt till it gets to 50 ohms.
nosaj

[Leo_Gnardo]

Think oil, not water. I've been offered a hinky looking load resistor "sculpture" used for high power testing, a bunch of big ol' wirewounds bolted together like something from Dr. Frankenstein's lab, and intended to be immersed in a barrel of "transformer oil." This one just wasn't for me, I gave it a pass.