What do you use for a load on large power amps

[BrianS]

Hello folks,

When repairing/testing high wattage (800 to 1000+ watt) power amps, what do you guys use as a dummy load?

I know you don't need to hook a solid state amp up to a load to measure the output signal, but there's got to be times when it's important to hook these babies up to a proper load.

I do repair in my house in a residential neighborhood, so running a 1000+ watt speaker system is not an option!!

I'm wondering what techs actually use, not what could be used.  I see there are big power resistors available, which is probably what I'll end up getting, but still curious as to what other techs use.

Thanks!!

[g1]

  Big power resistors are about the only option.  Most load boxes have them wired up in multiples to get the power rating, and in different combinations with switching for choice of load impedance.  Also fan cooling when dealing with so much power.
  I lucked out with a couple old heating elements from baseboard heaters.  The are wirewound, about 8 ohms each, and good for at least 1KW each.  Because they are wirewound, you can tap off at any point for lower impedances (but lower power handling).

[Roly]

This;


...only bigger, like this;

(tip find, about 30kW from memory)

[BrianS]

I lucked out with a couple old heating elements from baseboard heaters.  The are wirewound, about 8 ohms each, and good for at least 1KW each.  Because they are wirewound, you can tap off at any point for lower impedances (but lower power handling).

g1, can you tell me a little more about the heating elements?  I may have a couple different sources for something like that.  Was it luck that they ohmed out correctly, or lucky that you found them?

Interesting pics, there, Roly!

[J M Fahey]

Just do the Math

Suppose 1000W 120VAC water heaters.

W=V^2/R so R (at full power) = V^2/W=120 * 120 / 1000=14.4 ohms.

"Cold" (ambient temperature) may be different , of course but Math is accurate at full power ... which is what you want.

So 2 of these in parallel means slightly over 7 ohms , consider them 8 ohms, and *real*  2KW dissipation .... as long as they are inside a bucket of water.

FWIW I always test my amps at lower than nominal impedance, because :
a) in fact speakers have DC resistance about 80% of nominal, your amp *must*  be able to drive that or it will not survive in the real world.
b) speakers are reactive, not resistive ... real bad boys.
FWIW I test mine using 5 ohms instead of 8 and so on.

[g1]

g1, can you tell me a little more about the heating elements?  I may have a couple different sources for something like that.  Was it luck that they ohmed out correctly, or lucky that you found them?

I was lucky to find them for free, and lucky they were 8 ohms.  I'm not 100% sure they were base board heaters, but was told they were.  I believe they were industrial as they came out of a college. 
  They are about 2 feet long, with ceramic squares stacked and a single thick wire spiraling along the length.  Any modern elements I see pics of are the finned type, which would not allow changing impedance.
  I will try to post a pic, maybe someone will recognize the type.
Meanwhile, thinking about this has made me wonder about stove-top electric elements.  Anybody try these or know if they would be a viable load?

[J M Fahey]

They are viable, but remember that resistance goes up a lot when hot, specially if red hot, so put them in some kind of water vat to limit temperature rise to 100 deg C tops (boiling water) which is acceptable.
Solder wires to the ends and cover the connection with Silastic or similar which is heat resistant and will protect it from corrosion.