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post #1 of 41 (permalink) Old 03-02-2004, 06:51 AM Thread Starter
 
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JCM2000 DSL100 volume swell problem

Hi, I have a Marshall JCM2000 DSL100 along with a Marshall 1960A 4x12 cabinet.

I bought both units 2nd hand (came together), and it's in mint conditions. When I bought it, it looked brand new except for a few marks on the cabinet.

Not long after I bought it, I noticed that an weird sound would be coming out of the speakers (regardless if I was plugged in or not) - an airy sea shell kinda sound, with a smooth light crackly sound aswell...

When I was plugged in and playing, I also noticed that the volume would swell. Sometimes it would get louder, and then sometimes it would go softer.

I didn't do anything about it until it become REALLY noticable, and the swelling was unbearable..

I talked to an amp specialist and he recommended I get the amp reconditioned. So I got all valves replaced, balanced etc.. this solved the problem...

The amp has had very little use since, until recently.. It has been about 6 months since the valves were replaced and just the other day I noticed that the swelling has come back.

there is not sea shell sound (which is different from last time), but there is definately volume swell happening... and it is especially noticable at loud volumes..

it does not matter which guitar I use (I have 4).. the swelling is there...

is this a commen problem?
has anyone experienvced this problem before?
would replacing the vavles fix this problem? (I hope this is not necessary, as they are only 6 months old - with very little use)
could it be a balancing issue?
what else could cause this problem?
if I don't fix the problem, will it get worse?


I first noticed the volume swelling after I picked up my amp from my friends house, which is where I was storing my amp over the past 4 months...

I would use it about 2-3 times a week.... and I never noticed any problems at all..

It was only until I took it back to my house where I noticed a problem (volume swelling)

The swell might take 1 minute.. normally playing softer for the most part of its cycle... so in other words, it gets loud for about 5 - 10 seconds and then goes soft again...

I noticed when I was transporting my head/cabinet, that when I went over bumps, I would hear a metal rattling sound coming from the head..

I was wondering if the rattling set something out of balance.. because I remember my amp specialist telling me that he carefully balanced the valves...

I don't know the effect of unbalanced valves.. or why valves need to be balanced..

I am not familiar with the technical sides of valve amplifiers.. but I love them because they are so powerful and awesome sounding..

Perhaps when I transported it, I bumped the bias out?

what normally causes this problem I am having?
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post #2 of 41 (permalink) Old 03-02-2004, 10:41 AM
 
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The rattling you hear over bumps is the reverb tank. It has long thin springs inside that will bounce around and hit the top of the tank, which is just a metal cover. It would be impossible for the springs to do damage to anything other than themselves and in that event would have no effect on anything other than the reverb.

Having your bias set properly or improperly would not cause the problems you are having I wouldn't think. I think a more likely cause would be some resistors or capacitors somewhere inside the amp have gone bad and are not behaving consistently. Your tech should be able to walk you through the process of checking this thing over...it's not just a matter of throwing new tubes and bias job in there. Techs love that because it takes like 5 minutes and they get to charge a butt load of money for it because everyone thinks it's so hard. Literally it takes 5 minutes to do a full tube swap and a bias adjustment on a DSL or TSL.

I would have him reverify the bias settings, then go in and check the values of the critical elements on the circuit boards...he needs to trace things back and discover the bad component. If I were him I'd start from the output jack and work my way back inside the amp and check every component in the path.

Also, do you have the same problem when using different sources of power...try going somewhere else with the amp and plugging into a different outlet. Sometimes a poorly grounded outlet or noisey house wiring can do weird things, and the JCM2000 seem to be pretty touchy about that stuff...at least mine was.
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post #3 of 41 (permalink) Old 03-02-2004, 01:31 PM
 
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I know with solid state amps that overheating will often cause a volume decrease, but a swell. Quite odd i would imagine. How much do you play? How much did your friend play it? Do you think your friend could have dropped it and have to replace some of the tubes?
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post #4 of 41 (permalink) Old 03-02-2004, 07:39 PM
 
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Hey there. I have the same marshall dsl100 watt head.

I too was having the same problems as you after 1 year of use. Exactly the way you were describing it. Eventually 1 tube blew. So I pulled the outer two tubes which made the amp 50watt, and fixed the problem...for a little while.

The problem came back after a week or so. (vol swell, that echoey sea shell sound even with the guitar unplugged, etc).

So I took it to a tech.
The biasing was way off. One side was set to around 15 (should be 7) and the other was just as far off.

We changed all the tubes, power and preamp. We then rebiased both sides of the power tubes. The problem has gone away for now (heavy usage for the past 2 months -> volume at 5 on both channels).

Do you use a powerbrake of some sort?

Take it back to your tech.
wilch is offline  
post #5 of 41 (permalink) Old 03-02-2004, 09:00 PM Thread Starter
 
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Quote:
How much do you play? How much did your friend play it? Do you think your friend could have dropped it and have to replace some of the tubes?
Since my tech replaced the valves, I used it about twice, and then sat for 6 months not being used. Then it got used for about 5 months at about 1 - 3 times a week.

I very much doubt my friend used it much, but it could be possible. The other thing that was really odd when I got my amp back frm the tech. He said he replaced all the valves in the amp, including the little ones. But, when I looked at them, all the big ones looked brand new, but, one of the little ones looked new and shiny, and the other two little ones looked dustry (just like the old ones did)


Quote:
Do you use a powerbrake of some sort?
do you mean like a powerboard (with multiple power adapters)?

Quote:
do you have the same problem when using different sources of power
Actually, it's been tested only once at home (so, since I noticed my problem it has not been tested again)

when I was using the amp, I was using an extension lead and a double adapter pack (for the power).

It would be awesome if this was the problem (much easier and cheaper)
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post #6 of 41 (permalink) Old 03-02-2004, 09:34 PM Thread Starter
 
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Quote:
Do you think your friend could have dropped it and have to replace some of the tubes?
no, he would not have moved it, so I highly doubt he dropped my amp.
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post #7 of 41 (permalink) Old 03-02-2004, 11:51 PM
 
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A powerbrake is a device used to lower the output volume of the amp, it goes between the amp and the speakers.

I think I'd probably seek out another tech or hang around while this guys works on your amp. When they change out tubes, always ask for the old ones back. Again, I don't like to see folks throw money away, so I'd advise learning how to bias this amp yourself...it is so very easy to do with the JCM2000 series amps. All you need is a digital multimeter that you can pick up at any good hardware store, and the millivolt bias settings for your particular amp which can be gotten if you call marshall customer service.
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post #8 of 41 (permalink) Old 03-03-2004, 12:40 AM
 
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and provided the expense of dealing with tube amps, sounds like he just changed a few of the tubes and ripped you off for the others
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post #9 of 41 (permalink) Old 03-03-2004, 10:54 AM Thread Starter
 
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Quote:
millivolt bias settings
can you please further explain this?

and also, can you tell me where I could find information detailing the process of balancing the valves, etc, and anything else I would need to know in order to maintain my amp.
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post #10 of 41 (permalink) Old 03-03-2004, 06:56 PM
 
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Ok, it may be easier with pics but I'm at work at the moment. When I get time I can take some pics if you like.

Firstly to bias the amp you need a voltmeter (which can read millivolts).

1. Take the cover off the back of the DSL 100
2. Locate 3 small metal prongs (pins) that are just below the power tubes. They will be pointing straight out at you horizontally.

3. Attach the negative terminal of the voltmeter to the centre pin.
4. Look to the left and right of these pins and you will see two small white screw head type things. This is where you can change the bias on the DSL

5. Power the amp on. **edit: (ON and NOT in standyby mode)
6. Attach the positve terminal of the voltmeter to the left side and read off the settings, with the DSL 100 it should be around 7.

7. If not, adjust the screw to lessen or raise (the higher the number the hotter the tube will run).

8. Repeat for the right side.

9. Left and right should be as close as possible.

Hope that helps.

wil..
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post #11 of 41 (permalink) Old 03-04-2004, 02:30 AM Thread Starter
 
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Thanks a lot for that info, I am about to perform a simple test (try a differernt powerpoint, and also not make use of a possibly fault powerboard i was using)

As a matter of further interest wilch, and anyone else using the same amp head/cabinet as me.. how have you configured the connection between the head and the cabinet?

I am currently using the 16 ohm power output from the amp head, into the 16 ohn power input on the cabinet.

It is my understanding that there are multiple correct configurations which can be used, and also multiple incorrect configurations (which will damage the amp head)

Once again, I have a Marshall JCM200 DSL100 amp head, with a Marshall 1960A 4x12 cabinet
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post #12 of 41 (permalink) Old 03-04-2004, 03:02 AM Thread Starter
 
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I just tried it then. The problem is still there. It seems it is regardless of the powerpoint which I use..

also, this time I noticed that the crackle is there... like the first time I experienced this problem...

I think the next thing I am going to try is re-balancing the valves..

Whilst it was nice that last time my amp tech replaced my valves, perhaps all was needed was them to be balanced?

I won't know until I try it myself this time.

Under what circumstances do the valves need re-balancing?

how often should they be checked/adjusted?

how often to the valves need replacing?

is it easy to replace them?

I reckon my amp would have had about 50 hours of use (over a one year period) since the last replacement of valves.
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post #13 of 41 (permalink) Old 03-04-2004, 06:11 AM Thread Starter
 
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What I might do is take a few photo's of my valves in place, and maybe you guys can get an idea of the state in whcih it is in.

How can I go about troubleshooting this problem?

I have lost trust in my amp tech guy....

How can I issolate the bad valve (regardless of if the valve is bad because it needs replacement, or just bias adjustment)?
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post #14 of 41 (permalink) Old 03-04-2004, 08:02 AM
 
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hey there joshka,

ok, with the connection between the cab and head you can plug your 16ohm cab into the 16, 8, or 4 ohm output without doing any damage to any of your tubes. Basically what you're doing by running your cab at 4ohms is giving your cab alot less than it can handle (so things will be "quieter").

NEVER, plug an 8 or 4ohm cab into the 16ohm output of your head. This will damage the tubes. Also never plug a 4ohm cab into the 8ohm output as it will do the same.

In regards to issolating a bad tube... very hard to do.

Basically the way to tell your tubes are "bad" or have "gone bad" is something won't sound right. You'll know when to replace them when they become microphonic, or you start getting crackling and popping.

There could be one of a few things that could be wrong with your amp:

* The tech replaced the tubes, but didn't rebias them properly (they were running too hot).

* The tech only replaced the power tubes when the problem was the pre-amp tubes, or visa versa.

* You got bad tubes.

* There's something wrong with your amp other than the tubes.

From the noises it's making however, it sounds like you're having the same problems I had when my tubes went bad. The crackling, and airy noise, then it goes away... then the volume swells, then there's some crackling, the tone kinda changes, then it goes away... etc etc.

wil..
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post #15 of 41 (permalink) Old 03-04-2004, 07:49 PM
 
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Since the problem was fixed for a while after re-tubing, perhaps the bias is just so far off that the amp is eating tubes real fast. Bad tubes have caused the same problem in my Marshall. One time it was because of a bumpy road and the amp tipped over in the car. Another time the tubes were just old. In both cases, replacing the power tubes solved the problem.
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