Failing dummy load symptoms

[EtherealWidow]

Hey guys. I just build two dummy load DI boxes for the guitarists at my church. Tested both out on my amp for about 15 minutes each and everything works fine so far. I'm nervous about blowing my friends' amps up though. What are some symptoms of the dummy load failing and these guys' amps being run with no load? Would I be hearing some dramatic noises through the line out?

[Structo]

Can you post a picture of the schematic?

[Blackburn]

If your jacks are wired up securely and the wattage ratings of your resistors are high enough, I wouldn't worry about it. You can look at Parts Express for 100w resistors labeled as dummy load resistors. They're rated 100w and great. I ordered a single 8R and use it whenever working. $15 and a plug and you're set. But they are rather large..

[EtherealWidow]

Jacks and resistors are wired up securely. Just get nervous. I'd feel so awful if something happened to their amps because of me. Schematic is this
https://taweber.powweb.com/store/loadline_schem.jpg

[Structo]

So you are using the Loadline to plug the guitar amps directly into the sound board?

The only caution I would be concerned about is if the amp is run without a load.
For example if the person mistakenly has the switch set to speaker and there is no speaker attached, it can damage the output transformer, power tubes or both.

[EtherealWidow]

Right. I've told them it would be wise to always have their speaker hooked up as a precautionary measure. My concern is if they're using the load and the load somehow fails. I don't have any cold joints, but I just get nervous. I had to move a lot of components around to finally get the chassis closed and be a little rough. Don't think I wiggled anything too loose, solder wise. The other concern is the star ground. I used ring terminals on the ends of all the ground leads for the jacks and pots and attached those all to an aluminum screw in the chassis.

[Structo]

Yes, having the speaker still connected would really be wise.

Other than that I would make sure to use at least 18 ga wire.

For chassis grounds I like to use the ring terminals that have a toothed star pattern to dig into the chassis and also help lock the nut down.

[img:125:142]http://www.waytekwire.com/images/items/30086TH.gif[/img]

[Blackburn]

Ready to do battle, Tom?

It's my lug vs yours!

Fight!

[Leo_Gnardo]

Load resistors can get mighty hot so make sure you have plenty of heat sinking and load box placed so that you can't flambe' nearby objects or melt carpet or tolex. (That ain't incense burning...) I've made that mistake and lucky nothing major went wrong. But I now have a 4x6 inch black burn spot on my workbench carpet to remind me every day. Also melted the insulation off the wiring inside the load box. And that was with "only" 50 watts.

[rdjones]

Those load resistors are rated with a properly sized heatsink attached.
They are derated with a smaller sink and severely derated with none.

One thing I don't care for in a circuit like that is that during switching from load to speaker, and under the condition of no speaker attached and the load switch in "speaker" there is no useful load presented to the amp. (20k Ohms ?)
If it were me I'd be a lot less nervous with some damping resistance permanently tied to the input jack, say 50-200 Ohms.

Note that if these are solid state amps they are much more tolerant of being lightly loaded.

rd

[Structo]

Ready to do battle, Tom?

It's my lug vs yours!

Fight!

Hehehehe, that is the solder lug I was actually looking for.

Same as I use with one hole or two.

[Blackburn]

Ready to do battle, Tom?

It's my lug vs yours!

Fight!

Hehehehe, that is the solder lug I was actually looking for.

Same as I use with one hole or two.

CE has them. I bought a bundle a while back and haven't needed any more, still don't. I can't remember how many I got, but they're great and cheap. They solder beautifully.

[M Fowler]

I lucked out and found a bunch of those at my local surplus center since they didn't know what to charge I got them cheap.

Mark

[EtherealWidow]

Well thanks y'all for all your help. I used 18ga wire, and although the ring terminals I used didn't have teeth, I used a lock washer. That's about as good as I could do. Are you saying that the load resistors have heat sinks built in? If so, that would be very nice. There's not a lot of room in those boxes. :/ They are 16ohm, 50W. I put 2 in parallel to make the dummy load 8ohm, 100W. As soon as they were done I made them a nice little youtube video stating that it would just be wise to leave that speaker plugged in.

[Leo_Gnardo]

Are you saying that the load resistors have heat sinks built in? If so, that would be very nice. There's not a lot of room in those boxes. :/ They are 16ohm, 50W. I put 2 in parallel to make the dummy load 8ohm, 100W. As soon as they were done I made them a nice little youtube video stating that it would just be wise to leave that speaker plugged in.

No, if your resistors are the bolt-in type, with a wide flat surface, make sure you fasten them tightly to the inside of your metal box, with a layer of heat-sink compound between resistor & mounting surface. Then the box helps move the heat away from the resistors = heat sink. First run with them in actual use, check for heat production. If they feel burning hot after say 20 minutes to an hour of hard playing you might want to add finned aluminum heat sinks to the outside of the boxes. Most of all, nobody wants to see a fire get started. 50-100W in a small space can get hot enough to flambe' nearby items. melt carpet or tolex.

Nice of you to youtube your friends. These days, if it doesn't come through a little plastic box, seems it isn't worth paying attention. What an old geezer I am...