Dummy load for dummies...

Paul-in-KC · Post by **Paul-in-KC** » Wed Jun 27, 2012 7:49 pm

All,

I recently completed my third build (it's a CeriaTone JMP-50 Plexi-Lead) and thought that I had a problem in the amp, only to find that the problem was in some new power tubes that were purchased specifically for that amp.

Which leads me to the understanding that I need to do a bit more burn-in or torture testing of chassis' before declaring them finished.

So, I need a "dummy load" for quiet testing.

I have ordered 4 wire-wound 68 ohm, 100 watt resistors that I will wire in parallel for a nominal 17 ohm load.

I plan to mount them in a ventilated metal enclosure with a fan for maximum (or at least adequate) heat dissipation.

I think that's all simple enough.

I'm sure this next part is simple also, but I'm not certain on the details.

(which is to say that I am more simple than the question)

I'd like to have a small speaker as part of the load box that could be switched on/off. Not to use for playing, but just a momentary monitor to hear what the testing "sounds like".

I imagine that I can just place the speaker in series "behind" the resistors. Or even place it "behind" one of the resistors.

I'm not sure how much signal will be left after passing through the main resistors. I should be able to have a pot parallel to the speaker to control the volume, but not sure how many watts the pot or the speaker would need to be.

Can anyone help get me in the ballpark for values (for speaker and volume control)?

Thanks!
-Paul

Reeltarded · Post by **Reeltarded** » Thu Jun 28, 2012 2:28 am

Hmmm.. 1st, don't add anything in series. If it fails your load fails.

Answering a question, your load is a virtual 4-12 cabinet. Depending on the resistance of your speaker thing, the speaker could end up with a load half, 1/4, or equal... meaning it would be like adding a cabinet to a half stack.

Pretty loud.

You could tie a rheostat on there to knock off a bunch of volume, paralleled with the resistors at the end of the line, then if it fails you still have nominal 16ohm load.

Hmm..

I wondered where you were off to, ya crazy gun slinger!

Paul-in-KC · Post by **Paul-in-KC** » Thu Jun 28, 2012 3:52 pm

...slinging guns of course

and a soldering iron here and there.

Okay, how about this...

Parallel to my speaker load, I have a higher value resistor (let's say, 1K). So, a small fraction of the signal uses that path (I think it should be 17/1000).

So, I think a 2 watt, 1K resistor in parallel to the load resistors should "skim off" about 1-2% (1.7) of the signal.

Then I think I should be able to add a pot for additional resistance (less signal) and then a speaker?

Yes/no?

How far off am I?

-Paul

Reeltarded · Post by **Reeltarded** » Thu Jun 28, 2012 4:04 pm

You could just build a simple line out thingy like that and it would work.

Hmmm google L-Pad. I am pretty iffy on dreaming up great ideas.

katopan · Post by **katopan** » Thu Jun 28, 2012 10:22 pm

Incorporating an L-Pad into the ground connected end of a resistor load circuit would work fine. Use a standard pot and unless it was big and you were tapping off a very small amount of signal, you'll probably blow it.

Standard sort of setup for this is to have four resistors, two parallel pairs in series (like a 4x12 is wired). But then replace one of the bottom resistors with an L-Pad and take that signal off to the speaker. You can then dial anything from zero to quarter power. 8 ohm L-Pads are most common. If you wanted an 8 ohm load it'd be 3x 8.2 ohm resistors with the 8 ohm L-Pad as the 4th. For a 16 ohm load you could go 3x 15 ohm resistors and then for the last leg you'd series up a 8.2 ohm resistor in front of the L-Pad so that leg would match the others.

Kagliostro · Post by **Kagliostro** » Thu Jun 28, 2012 10:25 pm

Give a look here

http://www.tube-town.net/diy/tt-pos/tt-posx-en.html

[img

765]http://www.tube-town.net/diy/tt-pos/pos50-01.JPG[/img]

K

Structo · Post by **Structo** » Fri Jun 29, 2012 1:12 pm

A while back I bought one of these. I use 8 ohm loads.
I mounted it to an aluminum bar for added heat sinkage.
And soldered the wires to a jack.

[img:400:188]http://www.parts-express.com/images/ite ... -020_l.jpg[/img]

http://www.parts-express.com/pe/showdet ... er=019-020[/img]

Reeltarded · Post by **Reeltarded** » Fri Jun 29, 2012 1:14 pm

Structo wrote:A while back I bought one of these. I use 8 ohm loads.
I mounted it to an aluminum bar for added heat sinkage.
And soldered the wires to a jack.

http://www.parts-express.com/pe/showdet ... er=019-020

Fixed link I think. Delete thismif it works?

Kagliostro · Post by **Kagliostro** » Fri Jun 29, 2012 2:27 pm

http://www.parts-express.com/pe/showdet ... er=019-020

http://www.parts-express.com/pe/showdet ... er=019-030

http://www.parts-express.com/pe/showdet ... er=019-015

http://www.parts-express.com/pe/showdet ... er=019-025

K

Paul-in-KC · Post by **Paul-in-KC** » Fri Jun 29, 2012 2:53 pm

Thanks guys - as always, you're the best.

I had already ordered the resistors for the project, so I will proceed with those parts.

The posts and discussion here confirm to me that I was/am on the right track.

I'll post some pics of the gizmo that I put together.

-Paul