Wirewound Resistors Dummy Load question

[Chuck H]

Ok... I spent a little time rethinking my own load box. I made some improvements and I think this is good stuff.

If you use the Arcol aluminum housed resistors from Mouser the 50W model is disproportionately affordable compared to the other wattages. $3.19 each. Also, I don't know why but the 1% units are cheaper than the 5%. These resistors are the best deal going so that's what I designed with. I had to upgrade the switch to a DPDT and add one resistor so that added to the expense. This box can still be built for about $50 (chassis and shipping charges included).

The impedances are now accurate and it's good to 200W at any impedance setting. I also changed the resistor arrangement so that there is a better likelihood of keeping some sort of load on the amp if a resistor should blow. I guess I just wasn't happy with my old design. But I did build it from parts on hand. So it wasn't so much "designed" as worked out

[hylaphone]

Anyone used these?
Resistors & Pots | MPJA.COM

[Leo_Gnardo]

The impedances are now accurate and it's good to 200W at any impedance setting.

Please remember Chuck and all others, fan cool if you're running serious power, or at least make sure your load box isn't going to roast things nearby. I mentioned months ago I smoked my work-surface carpet with a "portable" 200W load box running only 50W thru it. Melted the rubber feet off & melted the insulation off the interior wiring besides. Theres a big black scorch mark on my workbench rug to remind me every day. Lucky I was downstairs making lunch and smelled something burning that wasn't food. Could have been worse. Safety first PLEASE!

[Chuck H]

+++10

I know! I've had the unpleasant experience of realizing that tube amp voltages under OD conditions can create A LOT more wattage and much higher voltage spikes than one might expect by looking at the circuit.

These aluminum cased resistors are really great! I've never run my load box more than 20 min. at it's limit but it has handled it with excellent integrity. I have them mounted in a standard .044 ga. Hammond box with heat grease and the worst it's been is "oooh! That's almost too hot!" Actually OVER it's rating at times, but that was a solid state amp Can't compare with the abuse of a clipping tube amp!!! There are reasons for this that don't require qualification for the initiated. But I just can't say enough about the durability of these aluminum cased resistors. Tough X 10! And the fact that the 50W units sell for so cheap is a bonus to all of us. Get 'em while they last!

Sorry for the burned carpet. What weight do lessons carry without scars? I haven't melted the feet off my load box yet... But I hope to!

[MarkusBass]

I fix mainly bass amps with power rating from 300 to 1000W. My dummy load is 800W but without a fan it starts melting. So I use two 24V fans when I test a bigger amp. Yesterday I was checking Ampeg SVT6Pro - 600W into 8 Ohms and the load was getting really hot (even with fans). And, of course, I do not put the load on a carpet :-) .

Mark

[Mick Bailey]

That's why I have two 4" fans fitted. An Amcron 5000 amp is a good test of a dummy load.

[km6xz]

I could not resist joining the fun by putting the reactance and wire wound resistor debate. Wire resistors can have measurable inductance, particularly low power /high resistance types but high power resistors have very few turns of resistance conductor so at normal test frequencies of 1000 hz and lower a 250 watt edge wound will have only a couple uh inductance. Far less that a speaker. The actual impact on measurements will be in the 0.1-1 % range or a little less than your test leads.

True rms meters are not less accurate measuring distorted than pure sinewave signals. Regular ac meters which simply rectify a sample of the voltage do not read accurately with distorted signals.
How is power measured? Power it the work performed in a unit of time so any measurement that does not measure work performed is a derived calculation. For frequency and waveform independent measurements we use temperature rise. Submerge the load in a water filled insulated container and measure temp change. That is how accurate measurements are done. You can use a thermometer or thermocouple. From dc to microwave that is how power is measured directly. The rms value of a signal is the same numeric value as a pure dc voltage heats the same load. So a amp at a clipped signal is generating a power level the same as, say, a 30 volt battery would, is generating 30 volts rms.
In other words, in practice, the accuarcy of a reading across a edge wound power resistor is about the same as the accuracy of any other elenemt in the measurement path. It is just anal retentive arguments that say otherwise. BS is bs.

[Chuck H]

I only do guitar amps. Nothing over 100W. Putting my puny dummy load on an Amcron 5000 would be like inflating a water balloon with a fire hose!

I've read here that some guys use multiple 8 ohm kitchen stove elements submerged in a water bath for really big amps. There was a following discussion about resistance shifts at temperature and I think it was determined that it would be small enough not to matter for amp repair purposes. I guess if you know your volume of water a thermometer could be added to the assembly and Stan's suggestion for power calculation could be used

[Leo_Gnardo]

I've read here that some guys use multiple 8 ohm kitchen stove elements submerged in a water bath for really big amps. There was a following discussion about resistance shifts at temperature and I think it was determined that it would be small enough not to matter for amp repair purposes. I guess if you know your volume of water a thermometer could be added to the assembly and Stan's suggestion for power calculation could be used

There was another correspondent who was getting lots of load R suggestions last year, and one of them was heater-wire as in a space heater. I'm sure stove elements would be similar. One of the very super-smart guys here - IIRC it was Salvarsan - shot that down with mention of resistivity vs. temp. At that time nobody mentioned a water bath, and heck that's starting to get way out of hand just to measure power from tube amps topping out @ 100W.

Test of a Crown/Amcron 5000 would be a test of the AC supply line as well as the load. You could smoke either end with an amp like that.

FWIW my load panels are 1000W 3.75 ohm, close enough for jazz. And that scorch mark on the rug from a portable load serves me as a daily reminder to not be a dummy myself.

[Chuck H]

Just wanted to add this:

Camco 3500-Watt 240-Volt Screw-In Type High Watt Density Water Heater Element-15152 at The Home Depot

Should be about 16 ohms and designed to be submersible! Four of them costs less than $40 and good to 7000W @ 8 ohms and 14000W @ 4 ohms and 16 ohms!!! Actually since these ARE made to heat water you would probably need to spend another $100 on copper tubing and hose fittings to make a chiller for the water tub if you ever actually pumped over 5000W into them for any amount of time. Still... Screw stove elements. These are more commonly replaced so consequently cheaper AND submersible without corrosion concerns. Nifty screw flange for mounting too. Four of them screwed into a big aluminum plate about a foot apart flipped down into a big tub of water.

[bsco]

Chuck...I never thought about that....I have some heating elements that were removed from a couple of hot water tanks...(I hate throwing away anything)...They could be used to make a dummy load that can handle high power like the Peavey CS series power amps for example...or larger.......They would have to be submerged in order to stay relatively cool.....and i take it you can still add on the monitoring circuit that you used with your set-up...That might be another way to go as I am starting to get some high power amps for repair....I have 2 Peavey's here now....and I also get a few Yorkville's as well..(no all the time but every now and then).....I guess another thing I can do is to make a load bank for this high power stuff and then have something more suitable for medium power such as guitar amps and home stereos...Any idea of the power dissipation of these hot water elements???They must be able to handle pretty high power levels.....I think I would go with an 8 ohm load on that as every amp has this....If it works on that load when it is cranked up, it should work on other loads as well....Let me know what you think...
Cheers,
Bernie

[Chuck H]

Unless I'm missing something you can use Ohm's law to figure out the resistance. The power rating is what it is. If an element makes 3500W of heat energy with 240V across it that figures to be 14.58A across 16.45 ohms (Two of these elements in parallel being an 8.2 ohm load that'll take 7000W!).

Something like a 4500W/240V element would be 12.8 ohms. Not a real useful load for tube amps where a specific load is required for proper matching and power calculations. An odd load like that should be fine for SS amps where the power can usually be calculated from any load. Since most high power amps are going to be SS that might be fine for you. But for things like big tube bass amps that use tubes you'll be best off using the rated load for accurate calculations. So you're back to multiples of four.

My disclaimer is that I have no idea what the resistance value change might be at a given temperature. I remember a thread here where it was discussed and determined that it probably isn't significant. And, as mentioned, some guys use regular stove top elements effectively. Since the water heater elements would seem to be more temperature stable, because they operate in a more conductive medium than air, I would think that they would be more stable. That's a really macro view of it though.

Incidentally a 2000W/120V element would be 7.2 ohms. Since the resistance would probably increase with temperature this might be a good choice too because many higher power amps seem to be rated at 4 ohms. They can be had for about the same price and a pair could be arranged for 3.6/7.2/14.4 ohms. Possibly closer to the standard 4/8/16 ohms when hot. A pair of these smaller units wouldn't need as large a water tub and be good to 4000W at 4 ohm and 16 ohm arrangements and one element good to 2000W at 8 ohms.

[bsco]

Ok. No problem..Sorry about that.....Sounds like that idea in actually not a good one after all.....

[Chuck H]

Well... If you know the specs for your element you might find it useful. You can always put an ohm meter across it. An ohm is an ohm.

[bsco]

I thought I had 4 of them...turns out I only have one...I have 4 thermostats.....The element that I do have is 20 ohms...one is not much good to me....I' ll have to round up a couple more....and see what the resistance values are....but first I am going to put together a load bank for doing vacuum tube amps....once I get that done, then I'll tackle this.....
Cheers,
Bernie