Dummy 8 ohm load - how to?

[cannikin]

I want to use my guitar head with a Palmer gear Junction DI
http://www.palmergear.com/pdi09.shtml without a speaker.

traditionally you use this DI: amp Head to junction DI to speaker cabinet the DI gives you a 600ohm output so you get the flavor of your amp head.

but I want to use it without the speaker attached.

So the question is how does one create an 8 OHM dummy load box to make the head "think" it has a speaker attached and not blow up my amp head.

 

[solder_city]

http://www.aikenamps.com/DummyLoads.htm

 

[cannikin]

it really this simple:

here

 

[cannikin]

[quote author="solder_city"]http://www.aikenamps.com/DummyLoads.htm[/quote]

awesome thanks :thumb:

 

[BR]

Hey David,

What is the power rating on the head? 100W?

check out these guys:
http://www.action-electronics.com/resist50.htm
or
http://www.action-electronics.com/resist25.htm

Wouldn't a series-parallel arrangement of some of these work?

Hey, I thought this link bellow would be cool to determine precise gain values. what do you think?
http://www.action-electronics.com/resistor.htm#Decade

 

[cannikin]

its small 18Watts

 

[BR]

I guess you can just get a 5, 2 and 1 ohm 50W and put it in series and be done with it.

Or just get a 25W 8.2ohm..

Funny that it looks like the same resistor from the previous link.

 

[pstamler]

I found some 1 ohm 25W resistors at a surplus store, mounted them on a chassis, and wired up some binding posts so I could use either 4 or 8 of them for 4 ohm 100W or 8 ohm 200W. Most surplus stores have small-ohmage high-wattage resistors someplace around.

Peace,
Paul

 

[PRR]

> it really this simple:

Oh, jeez, it is way simpler than that. Computer drafting and sharp dividers..... so-so-bo-gus.

> its small 18Watts

Grab a half-bad speaker cord. Cut it. Figure which half is bad.

Go to Radio Shack. They have an 8 ohm 20 Watt resistor for 2 bucks.

The hardest part is getting the bubble pack open. Then strip the cord leads, twist real darn good to the resistor leads. Plug, play.

A hard-played "18W" can get that resistor hot enuff to char tissue paper. If I drive my "16W" amp into continuous gross clipping (about 25W total heat) while the resistor is on plywood, after about 5 minutes I smell hot wood. I don't think any string-plucker can hold a steady max-power signal like my oscillator can. But if you need to be sure, tap two nails in a board and hang the resistor up off the wood.

This will prevent amplifier over-voltage from lack of load (as long as your connections are good.... sometimes i solder).

It will not emulate the unique interactions between a naked-plate tube amplifier and a lively loudspeaker. There is no perfect way to emulate that except.... a lively loudspeaker. There are commercial hotboxes with fake speaker impedances inside. Just as any two speakers sound different (in part because of different impedance curves), these don't do the exact same thing as any specific speaker, just a general bass-bump and top-rise. If you are not distorting, you can EQ it in the mix. If you are distorting the plates and speaker, you need a real speaker and darn the neighbors.

 

[mhelin]

[quote author="PRR"]If you are distorting the plates and speaker, you need a real speaker and darn the neighbors.[/quote]

 

[thermionic]

I thought the classic trick for someone who doesn't want to spend the earth on a stack of dummy loads was to buy a high-wattage wire-wound resistor and keep it in a bucket of water.

Once you've finished testing, you can pour a cup of tea with the water!



Justin

 

[bcarso]

Beware of d.c. though if more than a few volts---you'll get electrolysis of various metals if in contact with the water, which you probably won't want in your tea.

 

[cannikin]

I figured I would follow-up on what I did....
You guys made me nervous with the heat consideration so I went a little more robust.

Got this thing here: parts express

 

[solder_city]

it never hurts to uprate.
nice job.

 

[BYacey]

For load testing the "Big stuff" , I bought some ceramic pedestals that are used in pottery kilns for holding up shelves inside the kiln. These can be had in various lengths, and the ones I got were about 1.5 inches square. I simply wound some 18 guage nichrome wire until I had an 8 ohm resistive load, terminated onto a 1/4 inch bolt on each end. I had to drill the holes for the bolts very carefully with a masonry bit. I also made some at 4 ohm.

These will light up bright orange with about 1500W input, and I have yet to burn one up. I never did check the exact impedance, but it probably isn't far off from a speaker coil.

I thought I would add this as 1500W resistors generally aren't available from most electronic wholesales.

 

[Guest]

David;
I have a big box of 20 Ohm wirewound resistors. Gimme your address. Also, I can ship you a 12V fan so you may use it to cool your dummy load. You can power the fan from the same amp's output through a diode bridge. How much power do you need to dissipate?

 

[Freq Band]

I went hunting the surplus store, and somehow managed to create either 4 , 8, or 16 ohms with these non-inductive resistors, depending on the wiring. Mounted on a heatsink.

20 ohm
6.4 ohm
6.4 ohm
16 ohm
(the resistors aren't exact, but it's their error that makes it work, + wire resistance)

=FB=

 

[Guest]

I decided to build one for myself, so I'll post my pictures soon. :grin: