Question about Marshall MG100HDFX 100-Watt Amplifier Head

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I was playing through my Jvm410 it was sounding great then the volume got quieter then no sound all lights work whats the problem power tubes maybe?

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Yes, you MAY have destroyed the output tubes...

When replacing, make sure your MATCHED pairs go into V1 and V4 and the other into V2 and V3 respectively. Many don't realize the proper placement of matched pairs.

Open unit and if you find the tubes with the plates glowing red OR purple haze down between the elements, the tubes are gonzo.

Check the bias at connector CON1, Between pin 1 and 2 is a 1 ohm resistor in the cathode of the V1 and V2 side. Adjust the Bias1 for the number of milivolts matching the required bias current for two of the output tubes. Do the same for pins 2 and 3 of CON1 for the second side using the BIAS2 control. Fir your amp, you will want each of these set to 70millivolts.

It is possible (hopefully) that one of the smaller tubes burned out. These are much less expensive. EACH of these will have TWO points of light visible when they are operating. If you only see one, then 1/2 may be burned out. When turbes burn out, they cool down and sound will fade out.

There are other components that could cause your problem but those are not DIY unless you are adept at electronics.

Posted on Jul 14, 2010

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Maybe try adjusting the MASTER level?

Good Luck

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Do you turn on standby every time and wait for the tubes to heat up first? If you don't, it can severely damage the tubes.
But a common cause of this could be just old tubes. Like any other piece of technology, tubes get old. If you like to play your amp LOUD then they wear out a lot quicker. You have to replace tubes every now and then. Because I'm not there to check out your amp, I can't give you an exact answer but it sounds like that's the problem. This isn't always a bad thing. You can make the sound of your amp exponentially better with nicer, new tubes. Your amp has 12AX7 tubes. Very common tubes to use actually. I'm not sure what kind of music you play, I'm assuming mostly rock and blues. But here is a list of tubes and reviews on them.
http://thetubestore.com/12ax7review.html
This is how to replace a tube:
http://www.ehow.com/video_4438285_change-pre_amp-tubes-guitar-amps.html
Upkeep on a tube amp:
http://www.ehow.com/video_4438295_replace-tubes-guitar-amp.html
A note he doesn't make in the video is to dust out your amp. I use the compressed dust spray for the inside of computers.
So overall, stop turning up your amp to 11, make sure it's well ventilated and dust it out from time to time. Hope this helped.

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After the volume drops off, let the unit sit idle for a minute and then try it again. If the volume has restored for another brief period, suspect the filter caps are weak and voltage drops when you start driving the unit. In any case you need to have it serviced. Unless you can see the heater in a tube going out with vibration this may not be a DIY repair. If you see purple glow in a tube, replace it.

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Ah yes: complete catastrophe. Usually when everything is not working, it is a single reason and when that single reason is fixed, so is the amp.

Based on what you tell me, I am going to say you have a bad power tube. And I will even go as far to say that you buy a set of tubes and play on them as long as you can. If so, then you not only burned up the tube, but weakened the power section and may have a slooow tube frier. I always tell customers: modern tubes have an effective life of less that 1200 or so hours and that varies. Replace the tubes once per year if you gig with the amp: period. Otherwise, guys like me (doing repairs) stay busy and the tube companies get rich selling tubes to amps with weakened power sections.
It sounds like you burned a power tube and blew a fuse. First, you need a new set of power tubes and some extra fuses (go to Radio Shack and set the value you need in SLOW BLOW fuses).
Then, replace all the power tubes. Don't worry about biasing yet, we are just seeing if there is a problem. Next, replace the fuse.
Turn on the amp and play on it at various volumes and settings. If all is well, take the amp to a tech and get it rebiased. If you can afford it, pay to have the grid/plate and other resistors changed so the power section will be like new (clean slate with new tubes). Your amp will love you for it.
Almost all of the amps I have worked on for performance problems (cant keep tubes to stay alive for very long) are directly related to end user use. When you use a tube until it blows it ALWAYS TAKES SOMETHING WITH THE TUBE WHEN IT GOES (like the power section components). The compents will be weakened and the tubes will 'wear' at different rates that can even move them out of the 20% tolerence they must be within to sound good. 99% of the time a board repair with a retube after a catastrophe fixes the amp until the next time it is 'run into the ground'. Tube amps are NOT invinceble: they are weak compared to solid state and expensive to own. But we love tubes because they sound great. I have solid state to knock around on, when when it counts, I play only with tubes. I have spent hundreds on good tubes because you do get what you pay for.

Hope this helps!
-mike

Mar 14, 2009 | Music

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