Bench testing high powered amplifiers?

[in a vacuum]

So, I have a 1500 watt power amp that seems to be working, but I want to run it through its paces for a while to make sure it is now solid. My 100 watt 8 ohm power resistor seems a bit puny for the task and I don't think the neighbors would put up with a bank of PA speakers being run by an amp producing 70% power for four hours. What routines are you using to test these amps before putting them back on the street?

[Chuck H]

1500 watts??? Really, or is that some "peak" power rating? If true then it sounds like a good reason to build a big strappin load box. Mouser sells 50 watt aluminum housed resistors for about 4 bucks, that's pretty expensive by the time you series 30 or 40 of them. But I've read that some guys use those spiral burner elements that you can buy at the hardware store. They read about 8 ohms, take something like 1200 watts and sell for about 12 bucks. A pair of those seems reasonable. There was some question about how thier resistance might change when heated though. I haven't seen any difinitives on that yet, but it may be worth a try.

Chuck

[g1]

Like Chuck said, heater elements are cheap & simple. I have a couple from baseboard heaters that measure 8 ohms each and have a moveable clamp-on tap. Not sure if they were 120 or 240V but good for at least 1800W each. If anything I expect the resistance to increase when real hot but I'm not doing precise measurements so works for me.
If you are doing high power stuff a lot you probably would want a fancy load box with all the switching options (2,4,8,&16R) and a built in fan & meter.

[defaced]

Did a little digging on the resistance vs. temperature thing. From AZOM (AZoM™ - The A to Z of Materials and AZojomo - The "AZo Journal of Materials Online"

A marked increase in electrical resistivity is observed with increasing chromium additions. An addition level of 20% chromium is considered the optimum for electrical resistance wires suitable for heating elements. This composition combines good electrical properties with good strength and ductility, making it suitable for wire drawing. Commercial grades include Nichrome and Brightray. Small modifications of to this composition may be made to optimise it for particular applications.

...

The 80/20 Ni/Cr alloy is often used for wrought and cast parts for high temperature applications, as it has better oxidation and hot corrosion resistance compared to cheaper iron-nickel-chromium alloys. This alloy is highly suited to applications that are subject to oxidation.

Sooooooo.... assuming an 80/20 alloy, the following site gives some resistances as a function of temperature:
Electrical Resistance Wire - RW80

So, if the element reads 8 ohms at room temp, you can expect it to read about 8.5 ohms at 600C (~1100F).

[J M Fahey]

I do use spiral heater elements, but consider they go *way* up in resistance when heated.
If allowed to get red hot they may have up to 5X the cold resistance.
I put them in an *enameled* oven pan, full of water (no kidding). You can also use some Pyrex one but not aluminum or stainless, because it will short them..
Anyway the water starts boiling after 10 to 30 minutes, depending on power, but that's more than enough.
¿4 hours? Not even the manufacturer tests them that long.
*Maybe* the FTC does, considering they have nothing else to do, and specially having all that taxpayer's money to burn on "Mil Spec" $800 each platinum wire resistors.

[tedmich]

Carborundum resistor can be had pretty cheap in surplus sales, Nebraska has a ton but as usual their prices are pretty high (storage costs in Nebraska are apparently inconsequential) The key is getting the units in a low viscosity oil bath of decent volume. As an example these 25 ohm ~40w units
RF Dummy Loads and Microwave Delay Lines
could be run 3 parallel to get 8 ohms and probably +1000w in a 10gal pail of mineral oil. Do the calculations as oil can ignite!

A tech I knew used to test Phase Linear Amps full on with an 8 ohm dummy load in a bucket of water (DI would be best!) but they weren't 1.5kW...

[jrfrond]

We have a bank of four 1000W Milwaukee 8ohm load resistors and 10AWG cable that we use to test the big beasts, plus 300A service and 220V lines. The loads we actually got from Crest in the OLD days when they were in NJ. They had rooms full of them mounted to metal racks for heat-sinking during amp burn-in. Shoulda seen the size of the variacs they were using too! We got one of those as well.

No doubt about it, large power amps present their own issues when testing and burning in. BTW- those loads LITERALLY can fry an egg when the amp is burning in. Lends a new meaning to "cooking"! We fan-cool the loads, and in the winter, that means free heat!