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Old 02-03-2013, 04:42 PM   #1
Lucky13!
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Broken Tube Amp Advice

I have recently run into massive trouble with my Marshall MA100c. If anyone could offer any advice it would be greatly appreciated.

The problem: The amplifier has almost no output volume, except if I turn it up to 10, It isn't deafening at that volume and if you strum hard it sounds like a speaker is shredded. If you turn off standby mode is makes a loud hiss for about 5 seconds that fades out and then it is perfectly quiet. The same hiss happens when you flip the standby switch to turn it on again.

I had this problem about 6 months ago where some days it would work and some days it wouldn't. I took it to a tech then but he couldn't find anything wrong with it. After that it worked flawlessly for six months with regular use until today at band practice.

It is worth noting that if i plug my guitar directly into the return socket of the effects loop none of the controls on the front work but the amp is quite loud and sounds fine. (Guess this is because I am bypassing the pre-amp part of the amp).

I am very tired and frustrated with this problem so if anyone could give me any advice or at least point me in a direction it would be greatly appreciated.

Thanks in advance
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Old 02-03-2013, 04:50 PM   #2
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I take it you havent put new tubes in it?
Do that first
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Old 02-03-2013, 05:03 PM   #3
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Well I changed all the pre-amp tubes but not the power tubes because it seemed to me that the problem wasn't with the power amp
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Old 02-03-2013, 05:12 PM   #4
R45VT
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Quote:
Originally Posted by Lucky13!
Well I changed all the pre-amp tubes but not the power tubes because it seemed to me that the problem wasn't with the power amp


It sounds like a bad coupling capacitor. Take it to a tech and get it over with. Should be an easy fix.
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Old 02-03-2013, 05:14 PM   #5
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Put some new powertubes in it. If the sound is dying out then it is most likely in the power section. The MA amps are not shipped with very good tubes so that would be my first place to start.
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Old 02-03-2013, 05:23 PM   #6
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Originally Posted by Robbgnarly
Put some new powertubes in it. If the sound is dying out then it is most likely in the power section. The MA amps are not shipped with very good tubes so that would be my first place to start.


Normal volume when plugging in FX return is not power tube.

Noise when on/off standby can be capacitor charging/discharging.

His problem is in his preamp- before the PI.

No signal from guitar sounds like a coupling capacitor when you combine the noise. Of coarse it could be many thing but that is what I would look for.
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Old 02-03-2013, 05:43 PM   #7
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Quote:
Originally Posted by R45VT
Normal volume when plugging in FX return is not power tube.

Noise when on/off standby can be capacitor charging/discharging.

His problem is in his preamp- before the PI.

No signal from guitar sounds like a coupling capacitor when you combine the noise. Of coarse it could be many thing but that is what I would look for.

I didn't catch that he had went into the FX return and the problem was still there
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Old 02-03-2013, 05:47 PM   #8
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Update: I removed all the power tubes, and took the amp out of the cabinet to inspect. I didn't see anything that looked out of place so I put it back in the cabinet and plugged in all the same power tubes. Turned it on and now it works... This is very suspicious.

Maybe being shaken around a bit caused the problem to go away, but I didn't see any lose connections so I am not sure.

If it is this coupling capacitor you are talking about how easy would it be to fix? and is there any way of testing it without replacing the part?

Thanks for the fast replies
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Old 02-03-2013, 05:58 PM   #9
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Quote:
Originally Posted by Lucky13!
Update: I removed all the power tubes, and took the amp out of the cabinet to inspect. I didn't see anything that looked out of place so I put it back in the cabinet and plugged in all the same power tubes. Turned it on and now it works... This is very suspicious.

Maybe being shaken around a bit caused the problem to go away, but I didn't see any lose connections so I am not sure.

If it is this coupling capacitor you are talking about how easy would it be to fix? and is there any way of testing it without replacing the part?

Thanks for the fast replies


Could be a bad solder joint as well.

For testing the capacitor you would need to have the chassis out of the amp and have it turned on- with the speaker hooked up.

That is VERY dangerous.

You them use a non electrical conducting device- such as a chop-stick to tap on capacitors. If you hear a noise it's faulty.

Again- 1 mistake and you are dead. I would take it to a tech- they have the proper equipment to diagnose it. That tapping method is a poor mans way of testing.
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See, it's important that people clarify when they say "metal", because I pretty much always assume they are a Cannibal Corpse fanboi.

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Old 02-03-2013, 06:11 PM   #10
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Originally Posted by R45VT
Could be a bad solder joint as well.

For testing the capacitor you would need to have the chassis out of the amp and have it turned on- with the speaker hooked up.

That is VERY dangerous.

You them use a non electrical conducting device- such as a chop-stick to tap on capacitors. If you hear a noise it's faulty.

Again- 1 mistake and you are dead. I would take it to a tech- they have the proper equipment to diagnose it. That tapping method is a poor mans way of testing.

Actually, it's most tech's first point of call... got a few chop sticks in my tool bag but I do have other, more sophisticated, methods too
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Old 02-03-2013, 06:24 PM   #11
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Originally Posted by GABarrie
Actually, it's most tech's first point of call... got a few chop sticks in my tool bag but I do have other, more sophisticated, methods too


I use metal screw drivers. I do have life insurance though.
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Old 02-03-2013, 06:24 PM   #12
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Quote:
Originally Posted by R45VT

That is VERY dangerous.

You them use a non electrical conducting device- such as a chop-stick to tap on capacitors. If you hear a noise it's faulty.

Again- 1 mistake and you are dead. I would take it to a tech- they have the proper equipment to diagnose it. That tapping method is a poor mans way of testing.


Okay, I am quite used to work with electronics. But I understand if that capacitor discharges on me I am done...

Is it possible that moving or shaking the amp could get the capacitor working or not working? I am just trying to figure out why the problem comes and goes like it does.
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Old 02-03-2013, 06:26 PM   #13
R45VT
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Quote:
Originally Posted by Lucky13!
Okay, I am quite used to work with electronics. But I understand if that capacitor discharges on me I am done...

Is it possible that moving or shaking the amp could get the capacitor working or not working? I am just trying to figure out why the problem comes and goes like it does.


It is possible.


Do you know what the coupling caps look like?? I don't want you to tap/touch on anything your not supposed too...



Also if you feel inclined to do this- no guitar plugged in- volume up a bit. You want to hear the noise come through the cab.

And with the amp live you are looking at 450v or more.... Again I stress am abundance of caution. If you don't feel comfortable with lethal voltages take it to a pro.


It does not sound like anything in you power supply either- don't bother with the big electrolytic caps.(filter caps)
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Last edited by R45VT : 02-03-2013 at 06:37 PM.
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Old 02-04-2013, 04:33 AM   #14
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Not trying to be harsh, but with the questions you're asking, I SERIOUSLY doubt you know enough to be pokiing around in there. The first poke you make could be your last. This isn't like the inside of a computer or pedal or anything, even when the amp is unplugged, it holds enough charge to fry your ass.

Take it to a tech.

Again, I'm not trying to be an ass, I would just rather you DIDN'T kill yourself out of overconfidence.

I mean, if it's a money issue or a patience issue, is it really worth being dead over your amp not working? I think not.
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Old 02-04-2013, 04:42 AM   #15
R45VT
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Originally Posted by Blktiger0
Not trying to be harsh, but with the questions you're asking, I SERIOUSLY doubt you know enough to be pokiing around in there. The first poke you make could be your last. This isn't like the inside of a computer or pedal or anything, even when the amp is unplugged, it holds enough charge to fry your ass.

Take it to a tech.

Again, I'm not trying to be an ass, I would just rather you DIDN'T kill yourself out of overconfidence.

I mean, if it's a money issue or a patience issue, is it really worth being dead over your amp not working? I think not.


He knows more than the average bear, and wants to diagnose it himself. A non conductive probe such as a chopstick will limit his potential for harm. People bias there amps all the time, this is a similar danger- less current and voltage probe. He sounds like he has done his homework.

More than most. Maybe he can learn something here. The big question is if he finds something can he fix it himself? It's a PCB so soldering on it takes more skill than say in a guitar. You can really mif things up with too much heat. Nothing like lifting a current track up.
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Old 02-04-2013, 05:21 AM   #16
Lucky13!
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Thanks for all the replies. I am not going to do anything rash and dangerous, I am always very cautious when it comes to anything electrical.

Even if I can identify the problem as the capacitor I am not to confident with my soldering skill to replace it. I will probably take it to a tech as soon as possible. I just wanted to have an idea of what the problem could be and how I would be able to solve it.
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Old 02-04-2013, 05:31 AM   #17
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Okay, fair enough.

Again, I didn't want to come of as condescending or an asshole, but I would rather seem like a dick than have not said anything and have someone get injured or worse because of it.
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