Wirewound Resistors Dummy Load question

[Enzo]

I think it is starting to redplate...

[Tone Meister]

With one of these:




And one of these:



You could make a dummy load with an authentic moving coil that would provide true inductance. Available here: https://taweber.powweb.com/amptechtools/truload.htm


Photos property of Weber VST

[kg]

hey, a single bare wire has inductance too, you know.

that's why my amps don't have any wires in them. construction is a bitch, i have to admit.

[Mick Bailey]

I have a Marconi precision load that has switchable inductance ranges and accurately measures the signal going into a calibrated load. I used to use it out of a perverse notion of accurately matching the channels of Hi-Fi amps. Fools gold.

My conclusion with accurately measuring anything with amps is that it's futile. This is because there are so many other factors involved that cannot be controlled. In any scientific measurement the conditions for that measurement have to be standardised, and the measurement must be repeatable. The measuring equipment itself must be recently calibrated and and used under consistent conditions.

The phrase that crops up with measurement is "under standard temperature and pressure". How is 'accuracy' of a power output measurement meaningful if the next day the barometric pressure changes and the air temperature drops. Capacitors are subject to barometric pressure, the resistance of even the connecting leads changes, and the internal temperature of the amp isn't stable. Some components will exhibit PTC, some NTC. Shifting sand. And the mains supply varies all over the place. All of these factors also apply to the measuring equipment. A dummy load used for 30 minutes will have a different resistance to one at room temperature. I haven't even mentioned humidity.

A blacksmith bending a hoop will take the finished diameter and multiply it by Pi x D to get the length of bar needed. Or he may say to himself "three and a quarter - near enough". He could use Pi to 26 decimal places, but doesn't. In the world of science and measurement, we are blacksmiths bending that hoop.

So, if your imperfect load has some marginal inductance, and your leads too, and the whole thing reads a Peavey Classic 30 pushing out 29.8 watts after you've fixed it, that's good enough. Chances are the next day it will read roughly the same and you'll sleep happy and contented.

[soundguruman]

"Ah don't bother feeding the troll, Dude."

Yes Enzo, you should always lash out at those who think differently than you.
A true republican.
There's only one way, and that's Enzo's way.

Meanwhile the rest of the electrical world ...
Knows that VA does not = "watts." But he can keep on pretending.

[Enzo]

Actually SGM, while my lack of respect for you knows few bounds, I was considering the power wherein YOU called The Dude's perfectly respectable Sencore cap analyzer a Radio Shack toy. Had nothing to do with VAs vs watts. But then once again it is classic SGM bait and switch argumentation.

Fortunately I am not encumbered by any concern over what you happen to think.

[Pryde]

Hey Folks,
Got my WW resistors in and am going to build up the dummy load unit. I have 2 questions:

1. What type/gage wire should I use for this? (240w 8 ohms and 480w 4 ohms)

2. what is the best way to connect the wires to the tabs? (straight solder, moldex connectors ,etc)

Here are the resistors I bought:
New 120W Audio Power Tube Amplifier Dummy Test Load 16 Ohm Wirewound Resistor | eBay

I can just solder the connections but not sure how hot these will get and if that's the best way?

Thanks

[Leo_Gnardo]

Hey Folks,
Got my WW resistors in and am going to build up the dummy load unit. I have 2 questions:

1. What type/gage wire should I use for this? (240w 8 ohms and 480w 4 ohms)

2. what is the best way to connect the wires to the tabs? (straight solder, moldex connectors ,etc)

Here are the resistors I bought:
[ebay.com/itm/380391174612] New 120W Audio Power Tube Amplifier Dummy Test Load 16 Ohm Wirewound Resistor

I can just solder the connections but not sure how hot these will get and if that's the best way?

They look pokey enough to handle the job. I put a muffin fan behind my resistor panels but only switch it on when I know I'm going to seriously heat them up. Regular solder still hasn't melted after 25 years in use. To be super safe it wouldnt' hurt to use teflon insulated wire. Your runs are short so you can get away with 18 or 16 gauge. If you haven't any on hand I'd say check ApexJr - surplus dealer - allsorts of goodies at low prices.

[Regis]

I'm late to this party but here is a link to some cheap high power resistors:

4 Ohm 120W Power Resistor | MPJA.COM

[Chuck H]

Just wanted to share my design. Don't nitpick on the finite accuracy issues (that's happened here already). I'm using the inexpensive aluminum finned resistors from Mouser. It offers 4/8/16 impedance with a standard on/none/on switch. Very cheap to build and versitile.

Mostly I just wanted to show the low level monitor addition. When your probes don't happen to be on the test or you're not looking at your DMM or scope or you're off the bench looking for something you can still peripherally monitor what's going on audibly. When testing @ power this just seemed like a good idea to me.

[bsco]

Ok. Thanks Regis.....I'll check it out...I do have some resistors here which I was going to use....They will suit my purpose....but I will check out your link anyway....
Cheers,
Bernie

[bsco]

Chuck...thanks very much for that design...That was one addition that I so desperately wanted to add.....Sometimes I have to test amps while they are running and go do something else so it is impossible to listen to a tone at half volume for any length of time...and I can't always be glued to a meter or scope display either....so this would be great.....The 100 ohm 5 watt I will assume is a wirewound rheostat....
Cheers,
Bernie
P.S. I wouldn't be concerned about somebody pointing out design flaws or whatever....what you have is good....and what other people here had built for themselves is good as well...anybody starts that nit-picking shit just tell them where to go and how to get there....

[Chuck H]

The 100 ohm 5 watt I will assume is a wirewound rheostat...

Yes, it's a wire wound potentiometer. Mouser sells them. Made by Alpha. Cheap and the same size as a typical 24mm pot.

VW24F-10-19.5K-B100 Alpha (Taiwan) | Mouser

It's probably not a real necessity, but since I don't always want to undo testing conditions, I'm not always monitoring the test in progress and the test involves the amp making enough power to possibly burn something up, I just feel more comfortable knowing that if it starts to fizzle and pop I'll hear it instantly and have a better chance to avoid a problem.

The truth is that no problems have come up yet so this idea has really only succeeded in mildly annoying those close to my work bench (Wife: "Does that have to do that?!?")

[bsco]

lol......Ok then.... My wife doesn't mind....but it is nice to hear the sound so that you can hear if something goes wrong....basically that would be the reason for me to use it...
Thanks very much for the reply....It is much appreciated.....
Cheers,
Bernie

[MarkusBass]

This one: 5pcs 8 Ohm 8R 100W Watt Power Metal Shell Case Wirewound Resistor | eBay looks good. I use something similar (but not that nice as Sowhat uses).



Mark