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Post subject: How can I tell which preamp tube is bad?
Posted: Tue Jan 27, 2009 8:35 pm
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I have a Super-Sonic 112 combo. I have reason to believe that I have a bad preamp tube. I play at very low volumes in my home so I hardly push the power tubes. Lately I've been cranking both gain stages of the burn channel and playing for hours at a time. The last time I plugged in I noticed that the amp sounds weak and completely lacking the sustain and general tone that it had before. I could be wrong in my theory that a preamp tube needs replacing. If, however, I'm right, is there something I can do to figure out which tube it is? ...without having to buy more tubes first? This amp has so many preamp tubes I don't know where to begin.


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Posted: Wed Jan 28, 2009 7:59 am
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I`m not familiar with your amp, but are you able to bring it to a music store for them to look at so you don`t have to buy tubes you don`t need ? Push them all in and make sure their seated right. Sometimes when you move the amp around a tube can shift and cause problems you didn`t have before.

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Posted: Thu Jan 29, 2009 2:22 pm
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It may be hard to tell by looking at them. Sometimes a bad tube will get the plates glowing red, or it will flash. You could buy one new 12ax7 and place it in the first position and see if that fixes the issue. If not put the original 12ax7 back and replace the next tube with the new until you find it. Generally the first tube in the signal chain has the greatest affect on tone

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Posted: Fri Jan 30, 2009 9:44 am
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Put the volume on your amp really loud without connecting a cable.
GENTLY tap the pre-amp tubes with a pencil or small screwdriver.
If all hell brakes loose that's the faulty tube.
Works for me.

Peter


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Posted: Fri Jan 30, 2009 10:51 am
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Thanks for all the great suggestions. I just did the "tap test" with the end of a pencil. All hell did not break loose on any of them, but I did notice that V1 and V2 made more noise than any of the others when tapped. Also... one of the two 12AT7 tubes (not the reverb, but whatever the other one is for) sounded much different than all the others although not loud or alarming... just different. So for you experienced folks out there what does all this information possibly mean? Do you think I may need to replace both V1 and V2? My previous amp had only 3 preamp tubes, and when I changed one I just changed the whole set, but with this amp having 6 12AX7s and 2 12AT7s I'd rather narrow it down. Thanks again for everyone's help.


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Posted: Fri Jan 30, 2009 11:22 am
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It could be V1 and V2, take a new tube and put it in V1, listen to it.
Sounds okay then try V2 etc.
Btw the driver pre-amp tube, in your case probably V6, should also
be checked. Whenever changing the output tubes also change the driver.
If you play your amp every day for hours output tubes will last you for
about a year and pre-amp tubes 2 year.
In reallity they last longer in my experience but i don't earn a living
playing tube amps lol. :lol:

Peter


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Posted: Fri Jan 30, 2009 11:51 am
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lostindesert wrote:
It could be V1 and V2, take a new tube and put it in V1, listen to it.
Sounds okay then try V2 etc.
Btw the driver pre-amp tube, in your case probably V6, should also
be checked. Whenever changing the output tubes also change the driver.
If you play your amp every day for hours output tubes will last you for
about a year and pre-amp tubes 2 year.
In reallity they last longer in my experience but i don't earn a living
playing tube amps lol. :lol:

Peter


The power tubes in my amp are about 8 years old, and the preamp tubes are original. Still sounds good. I did replace power tubes once and they sounded ok, but started burning fuses. I bet it would sound better and louder with replacement. Mesa recommends changing power tubes annually as they say they can damage your amp when they go totally bad. I say play em until they die.

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Posted: Fri Jan 30, 2009 6:11 pm
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lostindesert wrote:
It could be V1 and V2, take a new tube and put it in V1, listen to it.
Sounds okay then try V2 etc.
Btw the driver pre-amp tube, in your case probably V6, should also
be checked. Whenever changing the output tubes also change the driver.
If you play your amp every day for hours output tubes will last you for
about a year and pre-amp tubes 2 year.
In reallity they last longer in my experience but i don't earn a living
playing tube amps lol. :lol:

Peter

Thanks. I was shopping around for tubes and I noticed that Eurotubes sells sets with a "matched phase inverter". If I am buying individual tubes from someone other than Eurotubes do I have to find a specific "matched phase inverter" for my V6 slot?


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Posted: Fri Jan 30, 2009 6:29 pm
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That matched phase inverter talk is new to me.
But you need a good 12AX7 tube in V6.
Check the specs on that tube, if it's a 12AX7 replace it with a quality one.
Better not a Chinese tube.
Btw maybe a visit to a qualified amp tech isn't a bad idea.

Peter


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Posted: Fri Jan 30, 2009 6:38 pm
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lostindesert wrote:
That matched phase inverter talk is new to me.
But you need a good 12AX7 tube in V6.
Check the specs on that tube, if it's a 12AX7 replace it with a quality one.
Better not a Chinese tube.
Btw maybe a visit to a qualified amp tech isn't a bad idea.

Peter

I did some checking. It seems it is benificial to have a matched phase inverter. Here is a quote from thetubestore.com
"For guitar players: if your amp has a "phase splitter" (or "phase inverter") it is important to have matched triode sections in this tube location because of how the tube is used in the circuit. For example, Fender amps often have a 12AT7 that is used this way. For other parts of the pre-amp stage in guitar amps this service is not necessary."
So I learned something new. BTW, my amp uses a 12AT7 in this position.


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Posted: Sat Jan 31, 2009 7:09 am
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Try this. http://www.youtube.com/watch?v=QM2qWZYb ... depot.com/

Claude.


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