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> How Do You Check For Bad Tubes?
dcz702
post Feb 5 2015, 05:33 AM
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I have a mesa boogie mini rectifier. The sound is cutting out should I just go ahead and replace pre amp and power tubes? It's a more than a couple years old and I don't know how to check for Wich tubes are bad. The manual says its a fixed bias.
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klasaine
post Feb 5 2015, 06:52 AM
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'Cutting out' could be many things, tubes and/or - ?

That has two El-84s, correct?
First just change the EL-84 power tubes. Pre amp tubes, unless they are defective or actually break, will last for years (potentially even decades). If those power tubes have a rating number on them (usually between 3 and 7), try to find Mesa labeled power tubes that have the same rating.
*Fixed bias (in Mesa's case) means that they've set the bias. It can be changed but it's relatively involved if you don't know how to work on tube amps. And if you don't, remember - a tube amp can kill you if you make a mistake.

This post has been edited by klasaine: Feb 5 2015, 06:54 AM


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fkalich
post Feb 5 2015, 08:18 AM
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Get some cleaner spray. Radio Shack has that, "Electronics Cleaner". It is not a rare thing for these problems to be due to bad connections in the tube sockets, very common. Spray the terminals of the tubes, all of them, and work that around in the socket some. Hold the power tubes by the base, not the glass. But also clean the pre amp tubes, they may be the problem.

The problem can certainly be in the pre amp tubes though. You can find out which does what, and move them around. Some of the sockets may be more critical than others, e.g. one may be dedicated to reverb or something else that will not cause the amp actually fail if the tube is dicey.

I get my tubes from Eurotubes.com. You need a matched set of power tubes if you need those. You can do that and if you still have issues get the pre amp tubes. I probably would just get the entire set though, they are not that expensive.

Regarding bias. Well I set my own. But I have worked with electricity since I was a kid, and know the basics, the risks, and how to be safe. I even wear electrician gloves and use an electrician screwdriver. I also work with one hand, keeping the other free. There are just some basic things you do to be safe, it is possible to screw up, so you have to follow the protocols to be safe. The filter capacitors can store a serious charge. I have heard that modern amps drain fairly quickly after you turn them off, but I am not sure about that. I am always very safe with electricity, do 3 or 4 things that each should in themselves keep me safe, but that gives me layers of safety should I screw up somehow. And that can really happen, you can just screw up, or get forgetful.

QUOTE (klasaine @ Feb 5 2015, 12:52 AM) *
'Cutting out' could be many things, tubes and/or - ?

That has two El-84s, correct?
First just change the EL-84 power tubes. Pre amp tubes, unless they are defective or actually break, will last for years (potentially even decades). If those power tubes have a rating number on them (usually between 3 and 7), try to find Mesa labeled power tubes that have the same rating.
*Fixed bias (in Mesa's case) means that they've set the bias. It can be changed but it's relatively involved if you don't know how to work on tube amps. And if you don't, remember - a tube amp can kill you if you make a mistake.


This post has been edited by fkalich: Feb 5 2015, 08:59 AM
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jstcrsn
post Feb 5 2015, 02:11 PM
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+ 1000 , they can kill you if there is current stored in the tubes that is not dispersed, usually unplugging it before turning it off will do the trick, but be careful


This post has been edited by jstcrsn: Feb 5 2015, 02:14 PM
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dcz702
post Feb 6 2015, 01:28 AM
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cool, thanks guys.
of course i would never open any electronic equipment with it powered up or plugged in.
i think instead of trying to isolate which tube has gone bad im just going to replace them all, and replace the pre amp 12AX7 tubes with spax7. thats ok right? says so on this website. http://www.humbuckermusic.com/mesa-boogie-...retube-kit.html
hopefully the noise cutting out is a bad tube, but i will see after tubes are replaced
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fkalich
post Feb 6 2015, 01:43 AM
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you might check these guys out, a lot of people buy from them

http://www.eurotubes.com/store/pc/home.asp

They will have different options specifically for your amp, and they test everything themselves.

But first I would clean terminals and sockets with the spray. Really, you will be surprised how often that is the problem, the corrosion on the prongs or sockets.


QUOTE (dcz702 @ Feb 5 2015, 07:28 PM) *
cool, thanks guys.
of course i would never open any electronic equipment with it powered up or plugged in.
i think instead of trying to isolate which tube has gone bad im just going to replace them all, and replace the pre amp 12AX7 tubes with spax7. thats ok right? says so on this website. http://www.humbuckermusic.com/mesa-boogie-...retube-kit.html
hopefully the noise cutting out is a bad tube, but i will see after tubes are replaced



The tubes don't store a charge themselves. The culprit's are the filter capacitors. those are the large cylinder capacitors, sometimes you have 4 or 5 of them. Their purpose is to get rid of 120 cycle hum. Your amp takes the 60 cycle input and converts that to 120 cycle signal, but without the capacitors you would have a terrible hum. Although if you listen to a tube amp output speaker even with the volume turned off and nothing plugged in you can still hear a little hum, I have never heard any tube amp get rid of all of it.

I know that old amps could store that charge in the capacitors like a battery even with things turned off or unplugged. Perhaps newer amps may discharge it a lot more quickly, but I would not bet my life on it. Which is what you would be doing if you grounded yourself and touched a connection that lead to those capacitors.


QUOTE (jstcrsn @ Feb 5 2015, 08:11 AM) *
+ 1000 , they can kill you if there is current stored in the tubes that is not dispersed, usually unplugging it before turning it off will do the trick, but be careful

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dcz702
post Feb 6 2015, 05:20 AM
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QUOTE (fkalich @ Feb 6 2015, 12:43 AM) *
you might check these guys out, a lot of people buy from them

http://www.eurotubes.com/store/pc/home.asp

They will have different options specifically for your amp, and they test everything themselves.

But first I would clean terminals and sockets with the spray. Really, you will be surprised how often that is the problem, the corrosion on the prongs or sockets.





The tubes don't store a charge themselves. The culprit's are the filter capacitors. those are the large cylinder capacitors, sometimes you have 4 or 5 of them. Their purpose is to get rid of 120 cycle hum. Your amp takes the 60 cycle input and converts that to 120 cycle signal, but without the capacitors you would have a terrible hum. Although if you listen to a tube amp output speaker even with the volume turned off and nothing plugged in you can still hear a little hum, I have never heard any tube amp get rid of all of it.

I know that old amps could store that charge in the capacitors like a battery even with things turned off or unplugged. Perhaps newer amps may discharge it a lot more quickly, but I would not bet my life on it. Which is what you would be doing if you grounded yourself and touched a connection that lead to those capacitors.

hey fkalich,
thank you brother i just pulled all the tubes and blew them off, made sure the terminals were clean and then re seated them, and one of the preamp tubes was sitting sideways to, so maybe got moved out somehow been playing for a hour or so with no issues so i guess my amp is fine, im gonna order a full set of tubes anyway just to have them.
thanks guys

This post has been edited by dcz702: Feb 6 2015, 05:23 AM
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