Configurable Dummy Resistor Load Box for Amp design/repair

[knausz]

What do you guys think of this dummy load I built for testing my amps? It's configurable for 4/8/16/32 ohms using just switches and has all the connectors. It's in an instrument box that should be easy to cool if I add a fan. It's about time I built one, it's making my life much easier. Any other ones out there?

link to the complete documentation on my website including the schematic:

Dummy Load for Amp Testing | knausz.com

-Imre

[drewl]

hey that's nice.
I just have a 4, 8 and 16 ohm 100w power resistors mounted in seperate boxes that were RF amp module covers.

[captntasty]

There is an article and schematic for a number of dummy loads over at aikenamps.com under Tech Info/Advanced... one of these days I'll get around to it!

[hasserl]

I just have 2 200 watt 8ohm resistors. I parallel them for 4 ohms, series them for 16 or just use one for 8. At 200 watts ea they're beefy enough to handle anything I'm working on. I have them mounted to a board with the wires attached to bolts with wing nuts. It's kind of crude I suppose and not as fast or easy as flipping switches, but it's not difficult to remove the winguts and move wires around as needed to get the desired load. Since they're mounted in the open no fan is needed, they dissipate to the atmosphere just fine.

[tcio]

I am rebuilding my tube amp to go into a dummy load and from there into a line level device. So far though every schematic I have come I across that have dummy loads go nowhere but end there. The line out goes to a speaker but the dummy load goes nowhere and has no output. What does it take to go from a resistive dummy load to a line level in order to plug straight into a line level device?

Thanks in advance

[Chuck H]

For starters... This is called "hijacking a thread" and is generally frowned upon. You really should start a new post if you see another post that raises a question of your own. Nuff 'bout that.

Look at the line level on any number of amp circuits. There is no reason any speaker driven line out circuit wouldn't work as well off a dummy load. You should be able to figure it out from there.

Back to the original question...

My load box doesn't accomodate a 32 ohm load. But I've never seen a guitar amp that would either. So I built mine for 4/8/16 ohms. It uses a single standard on-off-on toggle switch to save $$$ and is about as economical a solution as I've seen for an "all in one" load box. It also has a low level output that can be plugged into a cab for audible monitoring or Oscope reads (this output doesn't really care what impedance speaker you use +++). I used the aluminum housed resistors mounted with heat sink paste to an aluminum chassis (the 50W units can be puechased at Mouser for cheap). It's good for well over 100W at any impedance and I've had no problems so far without a fan. I use it all the time and find it indespensable.

Chuck

[Steve Conner]

Getting back to the original topic... Nice lab Mr. Knausz

I owe you one, because I was looking for a distortion analyzer for ages, and I didn't even know the Tektronix one existed until I saw it on your gear page. Then I looked around and it turned out that a friend had one for sale for $200, problem solved!

On the dummy load front, I always used a selection of power resistors screwed to a piece of sheet aluminium, hooked up to the amp with alligator clip leads.

As regards getting a line-out from a dummy load, there are hundreds of ways to do it. I feel that an isolating transformer is important to break ground loops, unless you're using it with a piece of kit that has a balanced input. Perhaps the easiest way is just to get a DI box, most of them can cope with speaker level and have two jack sockets wired in parallel, so you can plug the amp into one and the dummy load into the other.

Here's my own contribution to the confusion: A simple speaker simulator

[tcio]

For starters... This is called "hijacking a thread" and is generally frowned upon. You really should start a new post if you see another post that raises a question of your own.
Chuck

Sorry…. I have never posted on this forum. Perhaps I shouldn’t have added the word “Tube amp” in and just asked “Does anyone know how to put a line out on one of these?” I did an extensive search for this answer on dummy loads through Google and on this forum. Actually I was at it altogether last night for 5-6 hours just to find a solid clear answer. I thought this would be the thread to post this type of question rather than getting my head ripped off for starting a topic that has already been started and being too lazy to type the word “Dummy Load” into the search box. I found nowhere on this forum an answer for adding a line out to a dummy load. I am just a simple friendly guy looking for a simple answer and a friendly forum. I apologize if I have disrupted this forum in any way shape or form. No worries. I won’t be posting here again. Thank you everyone for your time and the info you provided

[Chuck H]

Take it easy. It's not "beat up on tcio time" or anything. I wasn't even ripping, just pointing out a small matter. The new thread could be titled "adding a line out to a dummy load". Then you have your own audience and discussion started on your particular needs. See, I was trying to be helpful.

In fact, I did address your question and if you look at my load box you'll see it does have a low level output on it. As Steve said, it may be a good idea to isolate grounds to avoid loops and hum. I haven't used my box as a feed for another amps input so I can't report about ground loop hum.

Chuck

[tcio]

I was not going to post again but want to say thanks for that Chuck. I totally took it the wrong way. Every now and then on a forum you run into a sour puss and although I have done a lot of searching on this forum I have never posted and am not familiarized with the members or personalities yet. It seems on many forums members get very aggravated when starting too many different threads related to the same topic. I thought I’d play it safe and not do that.

I surely do appreciate the info you and Steve provided. Great info! Perhaps I will take your advice and start a different thread on dummy loads for my specific needs seeing that seems to be encouraged here as I am very fond of this forum and have found many answers here

[Jazz P Bass]

Speaker to line level

Here is a link:Speaker signal to line level

[tcio]

I saw this circuit a few times during my zillion searches but didn’t know at that time it was as simple as throwing a line out circuit onto the dummy load. I believe I am all set from here!

Thanks everyone for your patience and help

[knausz]

I may as well weigh in. I'm glad this topic came up, I think it's a good thing to add to a load, be it for recording or debug - nice circuits Chuck and Steve. The Jazz Guy posted a circuit that's as simple as it gets. I hacked up that drawing and attached it here. I thought it would be useful to add some adjustability on the output level and also some output clamping to protect some devices with sensitive inputs. I think typical line out voltages are around 1Vrms, so this circuit will clamp at that voltage assuming the forward voltage is 0.7V which is typical for a silicon diode. I think I'll add this to my load as soon as I get a chance!

Thanks for the positive comments everyone.

[jrfrond]

I've used the same setup since 1980, to wit, two Dale 4Ω/250W resistors on a heavy-duty rotary switch to configure for 2/4/8, and the same with 8Ω resistors for 4/8/16. I built a load switch boxes with the rotary switches (for L/R use on power amps), toggle switches for load/no load/speaker, input jacks and output jacks to the test equipment, loads (which are bolted to the bench rear for heat-sinking) and speakers. I constructed these for three of our six benches and they work very well, save for the fact that I must replace the switches every so often. On the bench for my main power amp tech, we have 1000W Milwaukee loads for burning in power amps. We fan-cool those because they can get hot enough to cook on!

[tcio]

I was going to start another thread but seeing I basically already know where to go from here and everything is still being discussed on this one, I hope you all don’t mind if I jump in again.

Some real fascinating work and ideas shared here. I like that diode thing for what I am currently working on. I also stand corrected. I just noticed the posts under my “join date”. I guess I have posted on this forum before (man I gotta take a break from this project and get some real sleep!)

Anyway, I have been playing around with some of these schematics also. What if this simple circuit was to shift over towards something like the Weber Load Line circuit (only 8 Ohm for me though) and adding those diodes. Not to make it more complicated (like that’s all my small brain needs lol) but I would really like to see how well that tone section works in regards to getting back some highs after the load. Please bare with me as I am not too experienced at drawing out schematics.

Would something like this work?