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Post subject: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 4:40 am
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Recently another user wanted tips on building a dummy load for amp repair.
So I took a couple hours and built one as a tutorial (can't have too many tools)
Much of this is just scrap, a piece of Oak 1x8 (3/4 x 7.75) You can use ANY wood or Phenolic or whatever but it has to be able to take HEAT and for this design non conductive would be good.
a couple Vishay Dale 8 Ohm 300W wire wound resistors.
These are 1-1/8" in diameter and 8-1/2" long... the Through Bore is 3/4"
I work on Amplifiers putting out as much as 300W and occasionally an 8ohm 600W load could be needed. and 4-8-or 16 Ohms might be necessary...
So I had a 2' section of 2"x2"x1/8" Aluminum Angle on hand as well as some
1-1/2x1/4-20 Brass Machine Screws, Some 3/4x1/4-20 Brass machine screws, Brass Nuts for those, Two Each, 1/4-20 Wing Nuts, 12AWG wire, 12AWGx1/4" Lug Ring Terminals these are Tyco Brand, I use non insulated Crimps and then use Heat Shrink on my wires to make it neat. Also a MUCH more secure termination. and last but not least a Switchcraft 1/4" TS Jack, Same as I use in Speaker Cabs.
Bill of Materials
2x Vishay Dale 8 Ohm 300W Wire Wound Load Resistors
4x 3/4x1/4-20 Brass Machine Screws
3x 1-1/2x1/4-20 Brass Machine Screws
14x 1/4-20 Brass Hex Nuts
3X 1/4-20 Wing Nuts
2x 3/8-16x10 Carriage Bolts
2x 3/8-16 Locknuts
3' 12 Ga stranded wire
6x 12G Non Insulated 1/4" Ring Lugs
1' Heat Shrink (3/8")
Aluminum Angle
8 1/2 inch Square board (not important)


I used Red Wire going to/From the Resistors and Black going from the binding posts to the Jack, No requirement to color code but it makes the explanation easier.

So I cut the Aluminum Angle to the length of the width of the board.
Clamp the two pieces of angle together back to back and drill the 3/8" holes for the Carriage bolts that go through the resistors
Drill the 3/8" hole for the Jack wherever you wish it to be. I placed it dead center in the "Front" I am going to add some Banana Jacks so I can connect a DMM and a BNC so I can connect a scope.
Bolt the angle together on each end of the resistors, drill and screw the angle to the board.
Drill 3 1/4" holes through the board and counter bore them from the bottom so the screw heads will settle in below the surface.
put 1 1-1/2" screw through each of these holes from the bottom. and secure with a brass nut (do not tighten this yet)
You may tighten the CENTER nut down to secure that screw.
Install a 3/4"x14-20 screw in the lugs on the resistors (you MAY need to drill these out be gentle)
secure these screws with one nut and tighten these down.
Cut 4 6" pieces of 12 Ga Wire and terminate with a ring lug on each end.
Cut an 8" piece of 12 Ga Wire and terminate with a ring lug on one end.
Cut a 4" piece of 12 Ga Wire and terminate wiht a ring lug on one end.
Strip and Tin the end of the 8" and 4" piece of wire.
Secure a single 6" double lug wire to each resistor terminal with one 1/4-20 Brass nut.
Secure the 8" and 4" to opposite END screws that are in from the bottom of the wood.
Now you may tighten those nuts...

Solder the 8" and 4" wires to the Jack (polarity is not important)

Install the jack in the "chassis"

Now if you connect BOTH resistors to the End Lugs of the center bolts you will be parallel and 4 Ohms...
secure with wing nuts
Image
Connect one resistor to those lugs and you have 8 Ohms
Image
if you connect one end of opposite resistors to the end lugs (jack) and the free ends to the CENTER lug you will have 16 Ohms...
Image


Resistor NOT connected is 63F, Resistor IN Use for about 3 minutes at 100% Volume at 2Khz connected to a Marshall 100W amplifier, shows 20F temperature increase to 83F.
Image
Image


Here are some pics.
Hope it helps someone!


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 7:47 am
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Very well done. Should work fine on your SUNN. Those ww resistors kinda look like um... an adult toy for women. :lol:


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 8:15 am
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BMW2002Ti wrote:
Very well done. Should work fine on your SUNN. Those ww resistors kinda look like um... an adult toy for women. :lol:

Those Tabs would be BRUTAL...

How about this.
100W amp.
800Hz test tone into the front end.
Measured AC Voltage across the Resistor is 37VAC on the CLEAN channel but with Volume and Gain on "10"

this is 4.625 Amps...
so the power is 171.125 WATTS from a 100W Tube amp.
on the OD2 channel it is 288W...


Last edited by kkroger on Tue Feb 25, 2014 4:37 am, edited 1 time in total.

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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 8:29 am
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Amazing. Which 100 watt amp is this? I know that rated "tube watts" maybe conservative. But, that is crazy.

The 171 watts is peak power? Not RMS. Right?

BTW... An RN would have no issues drawing blood from those hands. [my sig other is an RN, she had to point that out --- of all things] :lol: :lol: :lol:


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 8:48 am
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BMW2002Ti wrote:
Amazing. Which 100 watt amp is this? I know that rated "tube watts" maybe conservative. But, that is crazy.

The 171 watts is peak power? Not RMS. Right?

BTW... An RN would have no issues drawing blood from those hands. [my sig other is an RN, she had to point that out --- of all things] :lol: :lol: :lol:

Calculated Sustained Power, based on a True RMS voltage (meter is TRUE RMS)
I'll do it again this evening and post photos, I can measure Frequency and amplitude both RMS and PEAK...

Speakers are generally rated at "RMS", speakers are also inductors or chokes so they have a "Variable" resistance or Inductive Reactance.

"Wiggly" veins...


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 2:31 pm
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kkroger wrote:
...800k test tone into the front end...



800,000 Hz? :shock:

Surely you mean 800 Hz......right?

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---> "The amp should be SWITCHED OFF AND UNPLUGGED before you do this!" <---

Por favor, disculpe mi español, no se llega a la práctica con mucha frecuencia.


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Mon Feb 24, 2014 3:37 pm
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shimmilou wrote:
kkroger wrote:
...800k test tone into the front end...



800,000 Hz? :shock:

Surely you mean 800 Hz......right?


Yup 800Hz, Posting in a hurry from work... Ugh!


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Tue Feb 25, 2014 4:30 am
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Amplitude unknown on the signal generator but unable to HEAR it at this level without the amp. Failing of the APP is that it doesn't have an amplitude control.
Image

Output voltage True RMS... This was running the Signal Generator directly in the front of the amp not like I did previously. Initially I ran the signal through the guitar pickup.
When my real signal generator arrives I can utilize it a bit better.

Image


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Tue Feb 25, 2014 5:23 am
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You could check the input with your meter. Just a guess, but the sine input might be too high a voltage, and maybe should be in the low mV range.

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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Tue Feb 25, 2014 8:42 am
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shimmilou wrote:
You could check the input with your meter. Just a guess, but the sine input might be too high a voltage, and maybe should be in the low mV range.


I have a new App to try this evening, it has an OUTPUT level.
and is easier to use than the other.
I can also test the freq with my meter.


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Tue Feb 25, 2014 4:04 pm
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OK New App for a Signal Generator (Really hoping my REAL equipment arrives soon)
Expecting in a new O-Scope, and Signal Generator (Audio) and a Variac... As well as some other bits and pieces.
So according to the app, the signal is -20dB, generally "Close" to the output of a guitar... generally...

I measured it at 17.4-17.5 mVAC...
Image
Now this is at 2Khz if I make the Frequency 800Hz as before the output goes UP
Applying the test tone to the amp at -20 changes the complexion entirely apparently the other app was applying the tone at 0dB. a VAST difference.
Image
Now this is putting out LESS then one watt .8593 Watt to be precise...
RMS into 8 Ohms.
This is the MAX output at -20dB in...
Image


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Post subject: Re: Dummy (Dead) Load Construction (sort of tutorial)
Posted: Sun Mar 02, 2014 1:12 pm
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