Wirewound Resistors Dummy Load question

[soundguruman]

Might be okay with a 10 watt amp, but for anything more, I'd space it well off of a piece of metal. I would not depend on the mounting brackets to dissipate heat.

Mounting brackets are intended just for that purpose.
That load in the picture cranks 300 watts, and the outside of the box remains room temperature.
Never even gets warm on the outside.

[kg]

edgewound is intended to reduce inductive reactance.

of course! my cheeky reply was insinuating that since no real load has minimal inductance, making it a design goal for a dummy load is a bit funny.

[soundguruman]

Lower inductive reactance = more accurate wattage calculation.

We calculate wattage ignoring power factor...so, it's not really accurate.

[Enzo]

OK Guru: How much inductance does the average "normal" round wound resistor have? And then how much does your edge wound one have. SO we want to know not only how large a reduction we are talking about but also how much inductance there is in the first place. One microhenry is twice the inductance of 1.2 a microhenry, for example, but neither will have much effect on our work.

[kg]

Lower inductive reactance = more accurate wattage calculation.

We calculate wattage ignoring power factor...so, it's not really accurate.

what is a watt that appears on a purely resistive load but not on an actual load worth to a musician?

[bsco]

Hey SoWhat....nice picture......I will soon have to assemble one of these. I have to ask you...Is your wooden jig adjustable so the ends can be moved in or out to acomodate different sizes of chassis, or is it fixed???

[soundguruman]

OK Guru: How much inductance does the average "normal" round wound resistor have? And then how much does your edge wound one have. SO we want to know not only how large a reduction we are talking about but also how much inductance there is in the first place. One microhenry is twice the inductance of 1.2 a microhenry, for example, but neither will have much effect on our work.

Don't know.
We ignore Power Factor in all wattage measurements, so we are not really measuring watts.
We are measuring VA.
We just call it watts.

[The Dude]

Don't know.
We ignore Power Factor in all wattage measurements, so we are not really measuring watts.
We are measuring VA.
We just call it watts.

This is "much ado about nothing". As with many numbers, the value of the number is comparison more so than complete accuracy. When you buy something that costs 99 cents, you naturally assume that it's going to cost a few cents more including sales tax. Is it relevant? In most cases no. The value of pricing is to compare the cost of one thing to another. To argue over a minimal percentage of change in a wattage calculation is just idiotic. For God's sake, post something productive.

[g1]

We ignore Power Factor in all wattage measurements, so we are not really measuring watts.
We are measuring VA.
We just call it watts.

Hilarious, Enzo asks you about your inductance statements and you start rambling about wattage and power.
And you have it completely backwards, VA is apparent power and takes power factor into account. Measuring watts is measuring watts. We don't measure VA.

[bsco]

I could be wrong here but I thought Power was Volts times Amps. Not meaning to stick my neck out and ruffle somebody's feathers but is really that much of a difference in the inductance of the different resistors to make a big difference on the power measurements?? I am not trying to be funny here guys ....I am just asking a question.....I do not have the vast experience that you guys have........Can I use wire wound resistors for dummy loads and will that really affect the measurement of the output wattage on an amplifier?? Or are there other types of resistors that are more suitable for the application.....and if it is not that big of a deal then I can use the wire wound ones that I already have....or do I need to buy another type of resistor??

[The Dude]

I could be wrong here but I thought Power was Volts times Amps. Not meaning to stick my neck out and ruffle somebody's feathers but is really that much of a difference in the inductance of the different resistors to make a big difference on the power measurements?? I am not trying to be funny here guys ....I am just asking a question.....I do not have the vast experience that you guys have........Can I use wire wound resistors for dummy loads and will that really affect the measurement of the output wattage on an amplifier?? Or are there other types of resistors that are more suitable for the application.....and if it is not that big of a deal then I can use the wire wound ones that I already have....or do I need to buy another type of resistor??

You're resistors are fine. The sideline banter was simply one man's hot air.

[bsco]

Thanks Dude.........Appreciate it!!

[Enzo]

There are laboratory measurements and then there are practical guitar amp measurements. A plain resistor has no reactive element, voltage and current track. This is fine for most purposes. What we are all yammering about is the reactive component of a speaker, mainly the inductance of the voice coil. Now we are talking about impedance - the AC version of resistance. Impedance is the sum total of the resistance and the reactances. The reactances come at a phase difference from the resistive. So curent peaks in the inductance at a different time from the peak in the resistance.


When we are talking tone, we see this being cared for in some attenuators, like the Weber Mass. BY adding some inductance and other tweaking, they make what would have been a plain resistor react more like a real speaker. Some folks find that desirable, but to others it doesn;t make so much difference. But tone is a subtle thing.

Now we get to our dummy load. A claim was made that non-inductive - well actually LESS inductive - resistors would be "better." I asked how large an effect would resistor inductance have, and the person who brought it up has no idea. I suspect it has minimal effect. But that person who had no idea what effect it might have still felt comfortable telling us all we should seek specific parts because of it.

[bsco]

Thank you Enzo....So for all intents and purposes I can use pretty much any type of resistor I like as long as it can handle the power and is of the correct values for the loading of the amplifier output....and I take it the measurement that will be obtained doesn't have to be exactly 100% accurate....as long as it is fairly close to the specs...if it is off by a couple of watts then no big deal....I'm happy with that......Thanks man!!
Cheers,
Bernie

[Sowhat]

Hmm. I was informed it would be ok? Well should I then find a piece of metal or heatsink material to mount them to instead? Not sure if the mounting brackets on the resistors will dissipate the heat?

Yeah resistors get hot, I think I learned that back when I was not paying attention as a kid. But this max wattage thing is getting pretty blown out of proportion. Maybe everyone else does but I, only on special occasions crank an amp to the max and let it sit there. Most time when using a dummy load I have just a small fraction of output power going to the loads, after all I'm repairing the amp, not trying to see if I can melt it down. Also, when putting a signal through an amp I really never let it go into distortion, I like a clean sine wave throughout, otherwise my readings become nonsense. This brings your output wattage way down, in some cases by more than 50%. Back in the stone age I used to just hang a 250 or so ceramic watter from my bench not mounted to anything but test leads... it worked fine for me at the time as well as all the other techs I worked with. You can put a 400 watt load on a 800 watt amp and it will work just fine, just not for a real long time. There's a window of time it takes the resistor to heat up which is usually long enough to do your magic.