200 Watt dummy load

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alf
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200 Watt dummy load

Post by alf » Fri Feb 27, 2009 5:34 am

My Marshall is cutting off when I try and push it over 5 or so. I was planning on using a dummy load I got so that I wouldn't break eardrums in my house while I try to dope out what is wrong. This is what I have.

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

My question. My Marshall is 100 Watts. The load is 8 ohm 200W. Is this going to be ok? Or will I be making the tubes try and dump a lot more power into the load than they are supposed to because of the mismatched wattage? I have heard that spkrs rated under wattage isn't as big a deal as those OVER the rated wattage of the amp. Is this the same thing w/ this load?

I've never used one of these before and don't want to damage anything because of ignorance. I can get a 100W one for something like 13 bucks, so it's no big deal if I have to.

Thanks!

bluesman
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Post by bluesman » Fri Feb 27, 2009 7:09 am

I wouldn't use an inexpensive dummy load for fear of damaging the amp. Check out some of the popular high power amplifier attenuators like the Marshall Power Break, Sholtz Power Soak, etc. I have used a Power Soak with a 100 watt Marshall with pretty good results...its also good for heating your room! Keep in mind that part of the rockin sound comes from the speakers being driven to a pleasant level of distortion...power attenuators do lower the volume, but don't sound nearly as good as the rockin real deal. A better solution might be a low power tube combo like a Peavy Bravo...don't laugh, these things really do rock, tube type amp, with chanel switching, select the clean chanel & turn it all the way up. Its only 10 to 15 watts but it sounds great, especially when properly mic'd & recorded...absolutely huge sound....best of all, they are really cheap..at least until the secret gets out.........doh! :biggergrin:
You can also try throwing a moving blanket over the speaker cab. I have tried this myself after seeing it done in a night club. Mic the amp as always, then cover both speaker cab & mic with a moving blanket...worked like a charm...big ass overdriven Marshall sound & the sound man had total control in the mix.

best of luck!
"The digital future sucks the boils off my white ass." McHugh

bluesman
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Post by bluesman » Fri Feb 27, 2009 7:26 am

after re-reading your post, I realize that I didn't answer your question as all...so here is a better response...
No, I wouldn't use a dummy load for troubleshooting your amp. The moving blanket over your speaker cabinet will work much better & give the amp a proper load. What are the symptoms...does is sound mechanical in nature, does it cut out suddnly or fritz out? Does this also happen when the amp is not sitting on top of the cabinet? How long has it been since you replaced the power tubes? This could possibly be a power tube that is shorting under load. I have had this happen before & a new set of power tubes fixed the problem. I later isolated the bad tube & kept the three good ones as spares.
Tell us more about the problem.
"The digital future sucks the boils off my white ass." McHugh

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suppositron
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Post by suppositron » Fri Feb 27, 2009 7:56 am

Actually, using a dummy load is just fine and in fact a lot of repair techs use them. The 200w load should be fine. Just don't use anything under 100w.

nclayton
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Post by nclayton » Fri Feb 27, 2009 9:15 am

I agree. Technically a dummy load is actually BETTER for your amp than speakers since its impedance is constant. It will be fine as long as your marshall has an 8 ohm output setting.

I can't figure out if you're saying the amp blows a fuse when you push it above 5 or if the sound just cuts out and then presumably cuts back on again when you go below 5. It will make a big difference as far as things to look at.

Ned

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Post by bluesman » Fri Feb 27, 2009 10:15 am

I'll say this & let it rest. A dummy load will not let you listen to the amplifier. Many problems can be found by removing the chassis from its cabinet, hooking it up to the speaker cabinet, turing it on with volume up & tapping all wires & components with a non conductive stick while listening to the amplifier's response. Flakey components, cold solder joints, microphonic tubes, bad jacks...etc can be found this way. In the absence of more sophisticated test equipment, this method works well, but only if you can hear whats happening with the amplifier.
I will add the usual disclaimer about potentially lethal voltages existing inside any amplifier & if you are unsure about what to do, it may be best to err on the side of caution & take the amp to a qualified tech.
"The digital future sucks the boils off my white ass." McHugh

alf
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Post by alf » Fri Feb 27, 2009 2:36 pm

I am familiar w/ the voltage cautions. I really want to learn to fix this stuff myself and have been doing decently.

Here's the problem in a nutshell. At first it would not give much signal, but when touching the metal knobs(no knob covers) the signal would hop back up to normal.

I checked and found a few squirrelly solder joints and reheated them. No more problem. But when the amp was turned up about 1/2 way the next time, after an hour or so's playing at low volume, it cut out and I smelled something. The tubes seemed to be too hot. I haven't opened the chassis yet. Prolly tonight or tomorrow.

I don';t know if tubes could cause this but I am going to get a matched set soon and see if replacing them helps.

Thanks for the help!

alf
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Post by alf » Tue Mar 03, 2009 8:08 am

Ok. So I have found a bad input connection. I know this could cause the volume cut out problem, so I'm gonna replace that.

Does anyone know if this would cause the amp itself to stop passing signal and maybe begin overheating? I thought it was ok to have an amp w/ nothing in the input, or rather that it was not a damaging thing. But if the tubes are really being used hard and the sound is up and then an intermittent comes into play, is it possible this might make the tubes freak when the input signal keeps dropping in and out?

Going to crank it today and see if the other input does the same thing.

Oh and I used the blanket trick, but even w/ a bunch the muffled noise was still LOUD. Esp. the bass freq's.

Thanks again guys!

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Post by muttonhead » Wed Mar 04, 2009 7:49 am

What Marshall do you have, TSL,JCM...? Is there an effects loop? Are the plates of the output tubes turning orange? Have you poked at components in the amp? You can use a sharpie or a chopstick.

alf
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Post by alf » Fri Mar 06, 2009 6:12 am

Marshall JMP Mark 2 Master Model Lead 100W.

The tubes all seem to be glowing the same, nobody is red. But there is a blue spot that develops pretty quickly on the 3rd tube from the right(from the rear) and it gets bigger as time goes on.

I am going to try biasing it.

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Post by nclayton » Fri Mar 06, 2009 10:02 am

Hi. It's still unclear to me what you mean by "cutting out". You've never clarified whether that means the audio goes away completely, partially, or whether than means you blow a fuse. The problems are completely different.

But either way, I don't think biasing your amp is going to solve the problem. It sounds like the tube that's turning blue is probably experiencing some kind of secondary leakage, in which case it's very possible it's just a defect in the tube and it needs to be replaced. (or you might just not be noticing similar blue glows inside all the tubes?)

You ought to swap that tube into a different socket and see if the blue glow follows the tube. It might, but potentially there might be something going on in the amp (other than bias) that's making that tube behave the way it is. It would be worth checking.

I'd still be interested to know exactly what happens when the amp "cuts out" and if by turning it down you can get it to cut back on. It doesn't sound like a fuse blows, which might mean it's going into severe HF oscillation, which could be a very complicated problem to track down if you don't know what you're doing.

Ned

alf
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Post by alf » Sat Mar 07, 2009 9:26 am

Thanks again for all the help guys!

Re 'cutting out': It was dropping volume at first, less than 1/2 vol and would come up to regular when I touched the metal knobs. I figured a ground short.

It has recently just stopped giving any sound when turned up past 5 or so. The 'cut out' I was referring to. No blown fuse.

HOWEVER, last time I ran it high there were no problems and that was when I noticed the blue glow. I figure swapping it with one of the others and seeing if the glow goes along w/ the tube or is from a problem with the socket.

I realize that these things can be tricky. But I am in the process of learning. I am shooting to become a repair guy. Everyone has to start somewhere right? If I just take it in to a tech, then I don't learn anything. I mean the techs have to start out somewhere right? Doing a LOAD of reading etc... right now.

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