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Tube amp crisis???


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Hello,

 

I was playing my guitar through my tube amp tonight (via an attenuator/load box) and into Logic and then all of a sudden all the bottom end went out. Now the amp sounds very very thin. The amp is only two months old and I purchased it brand spanking new. It's still under warranty obviously, but, how could I tell if it's a broken tube or something else? I replaced the stock Chinese tubes the amp came with with some brand new JJ Electronics tubes...both preamp and power tubes...about a month ago.  

 

Here's the amp:

 

http://www.jetcityamplification.com/5012c

 

Thanks so much. 

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For starters, you have to go easy with attenuators.  Not all attenuators are created equal, and the less expensive ones can be very hard on your Output Transformer.

 

The fact that your amp sounds "thin" leads me to think that you may have blown one of your two Output Tubes.

 

When you use an attenuator, your drive the Output Tubes the same as if you were playing at much higher volumes.  This, in turn will cause them to burn out faster.

 

Also, if you didn't re-bias the amp after replacing the tubes, they may have been idling too high (which will also cause them to burn out faster.)

 

If you don't know what a Filter capacitor is, and you don't know how to discharge them, you shouldn't stick your hand in the amp.  But since you said you already replaced the tubes..

 

1.  Remove the attenuator from your chain

 

2.  if you can figure out where they are, check all of your fuses (although I think your problem is an Output tube, sometimes an Output tube will take a fuse with it when it burns up.)

 

3.  Pull one of the output tubes (be careful, they could be hot) and see if you get any sound out of the amp.  If it sounds the same as it did before you pulled the output tube, the tube you pulled is the culprit.

 

4.  If you have no sound after pulling the output tube, then the other output tube is the culprit.

 

5. If the output tubes aren't the issue, then get a new pre-amp tube, and start replacing one tube at a time, starting with the first pre-amp tube, to see if your problem goes away.

 

6.  If none of that helps, check your Speaker cone to make sure it's not torn.

 

And when you take it in for warranty, I probably wouldn't mention that you were using an attenuator.

 

Lastly, be careful where you get your tubes from.  A lot of tubes are coming from China these days, even the well known brands like Groove Tubes.  So the tubes you used to replace the stock may have been Chinese as well.  

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Great response. Very informative. Thanks! I did not know that about the attenuators. I actually had the attenuator cranked and the amp sounded fantastic. I'll pass on draining the filter capacitors. I know how to do it but I'd rather have a tech do it if it comes to that. I assume by output tubes you mean the power tubes? The 6L6's. I still have the stock 6L6's I replaced with the JJ's. Why not put those in and see if all is okay?

 

Thanks!

 

By the way, how long should power tubes typically last on average? These JJ's are only a month old! Must be the attenuator driving them too hard? If that's the issue.

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Great response. Very informative. Thanks! I did not know that about the attenuators. I actually had the attenuator cranked and the amp sounded fantastic. I'll pass on draining the filter capacitors. I know how to do it but I'd rather have a tech do it if it comes to that. I assume by output tubes you mean the power tubes? The 6L6's. I still have the stock 6L6's I replaced with the JJ's. Why not put those in and see if all is okay?

 

Thanks!

 

By the way, how long should power tubes typically last on average? These JJ's are only a month old! Must be the attenuator driving them too hard? If that's the issue.

Yes, Output tubes = Power tubes.  Just another term used for them.

Perfect, you can use the stock 6L6's to trouble shoot.

As far as draining the caps, that's to save YOUR life.  The caps can store 700 volts for weeks after you unplug the amp.  So BE CAREFUL poking around in there.

Gerald Weber used a great analogy.  Power tubes are like tires on a car.  The harder you drive them, the faster they wear out.

The attenuator allows you to drive them hard, to the point where they start to break up and sound awesome.  That's a dual edged sword.  They sound awesome, and wear out at the same time.

I would say, if you play hard one hour a day, the Output/Power tubes will last roughly 3 to 6 months.

If you play at volume 5 (without the attenuator) only 4 hours a week, one to two years.

Let me know if you have any more questions.  Good luck

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Fixed!!! It was a bad preamp tube, thankfully. I pulled the chassis out and swapped all of the JJ Electronic preamp tubes with the stock Chinese preamp tubes the amp shipped with. I didn't feel like testing each tube individually. The stock Chinese preamp tubes the amp shipped with sound brighter and much more alive than the JJ's did last night. I think one of the preamp tubes was on its way out possibly? I was having trouble dialing in a good tone last night and then that's when all this began. Amp sounds absolutely fantastic now.
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Fixed!!! It was a bad preamp tube, thankfully. I pulled the chassis out and swapped all of the JJ Electronic preamp tubes with the stock Chinese preamp tubes the amp shipped with. I didn't feel like testing each tube individually. The stock Chinese preamp tubes the amp shipped with sound brighter and much more alive than the JJ's did last night. I think one of the preamp tubes was on its way out possibly? I was having trouble dialing in a good tone last night and then that's when all this began. Amp sounds absolutely fantastic now.

Great work.  

I would hang on to all of the JJ's, you always need  spares.  

Obviously one of the 12AX7's went bad, the only way to know for sure is to get a new 12AX7 and go through one at a time until you find the culprit.  But you have time, since your Amp is back up and running.  

I found a website called "Doug's Tubes" where the prices were reasonable, and Doug will answer your questions in search of a good fit for your situation.

Groove Tubes used to have an excellent reputation, but there's been debate in recent years.  You need to head over to the Marshall or Fender forums and see what guys recommend for 6L6's.

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You get what you pay for, right? :lol:

 

Amp quit on me again. :)

 

This time is a bit different though. Last time when I was playing through the amp all the bottom end went out, this time I was playing through the amp and then all of a sudden everything went silent.  All I can hear now is the 60 cycle hum. However, if I really crank the master output (6-7) on the amp I can just barely hear the guitar playing in the background. I am plugged straight into the amp…no pedals, no attenuator. 

 

Any ideas? This sounds to me like another tube issue, but, I am just guessing.

 

Thanks. 

 

 

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Isn't there a trick to checking tubes by tapping a pencil on them or something like that? I'm just looking for the easiest way without having to pull the chassis out every time. 

You are correct.  But that's more to isolate a microphonic tube.

 

I searched all over the internet for a "Tube Tester."  They simply don't exist.  

 

I thought you could hook a meter up to a tube, take a reading, and see if it's bad.  Without a Master's Degree in Electronics, I have been unable to figure that one out.

 

The only "Tube Tester" I can find, is my Amp.  Plug a tube in, if it works it's good.

 

The filaments in a tube are like the filaments in a light bulb.  They become brittle with age, and can break with rough handling when hot.  That's why after you play a gig, you're not suppose to bounce your amp around when you pack it up.  Need to let everything cool down.

 

A very important rule of thumb is to NEVER run your amp without a load.  The "load" on your amp is the speaker.  If you were to run your amp without the speaker attached, you'd blow the Output Transformer and ruin the amp.

 

My suggestion would be:  Take the amp apart again, and figure out a way to hook it up with the speaker connected, and the tubes exposed.  

 

Check the fuses again.  This time, whatever went may have blown a fuse.  You'll need to google around if you don't know where they are and how many the amp has.

 

Then go through the checklist I sent you earlier.  

 

A smart mechanic told me once, that it's usually "the simple stuff" that goes, but people try to overthink and get lost.  You ran the amp hard with an attenuator.  The caps, resistors, and wires were all designed to handle a maximum load.  The speaker (because of the attenuator) was actually operating below it's capacity.  So what is left?  The tubes, which were being asked to run at full power.  

 

Once you get it working again, leave it in your troubleshooting configuration for a day, and play it that way, just in case another tube blows.

 

Hang in there, you'll figure it out.

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Dropped the amp off at the shop. Will have to wait and see now what exactly is wrong with it. 

 

By the way, how are the more expensive attenuators different than the cheaper ones? Everyone I have spoken with has suggested that I am most likely driving the amp too hard with the attenuator I now have (pictured in an above post). 

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Dropped the amp off at the shop. Will have to wait and see now what exactly is wrong with it. 

 

By the way, how are the more expensive attenuators different than the cheaper ones? Everyone I have spoken with has suggested that I am most likely driving the amp too hard with the attenuator I now have (pictured in an above post). 

It has to do with the ability to match impedance.  The Transformer steps down the impedance to match the speakers.  The Attenuator steps in between the Output Transformer and the speakers, and doesn't absorb impedance fluctuations as well as speakers do.  The cheaper ones are more prone to this than the expensive ones.  

When the impedance that the Transformer expects to "see" isn't there, it's hard on the Transformer, and can cause it to burn up.  

 

At least, that is my understanding of the how the whole thing works.  I have a high end attenuator, and a vintage 100W Marshall.  I've been too chicken to use it much.  

 

There isn't too much clear information on attenuators,  either in the Tube Amp books, or online.  I called the manufacturer of my Attenuator, and was told that it does a good job of matching impedances.  Not sure if I understand how or why.

 

I do know, that all attenuators will run your tubes hard, and cause them to burn out sooner.  Tubes are easily replaced.  Output Transformers, not so much.

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Speaker loads are reactive because the voice coil has both resistance and inductance.

 

Resistive Attenuators are mostly a fixed resistive load.  

 

Reactive Attenuators attempt to mimic the speaker's reactive load.  

 

So in theory, a Reactive Attenuator is much better for the amp.  Although, from what I've read, they can't completely mimmic the speakers load.  As such, even the reactive attenuators can harm the amp due to  extended use with a mismatched load.

 

I was researching this a little more tonight, and I came across an attenuator call the Weber Mass, which is actually a speaker motor that moves to attenuate sound.  Sounds like that's a better idea than just using a resistor.  You might want to look into it more closely.

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Thanks. I did check with the maker of the attenuator I currently I own and am using with my amp. He said an attenuator would be hard on the power tubes but not on the preamp tubes. I'll ask him about the output transformer as well. Also, I saw the Weber attenuators when I was looking for an inexpensive but good one. Unfortunately, they are not sold in stores like Guitar Center. I had to buy an attenuator from Guitar Center...no choice.  
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Re-soldered a preamp level pot. Under warranty so I didn't have to pay anything. 

 

What's a preamp level pot, by the way? And what would possibly cause that to become loose, most importantly? 

 

Thanks. 

A "pot" is short for potentiometer.  Basically a volume or tone knob.  You should find out which one they re-soldered, just so you know in case there are problems when your warranty expires.

As to what caused it to come loose, could have been a cold solder joint or the result of RoHS compliant solder, which has been in use since around 2006.  I'm no expert, but I know guys that are experts, and they swear the new RoHS compliant solder is useless.  

Soldering is an art, and when you attach something with solder, you can't move it while the solder cools.  If you do, you get what the call a "cold joint."  It's usually not shiny, and is subject to corrosion and problems down the road.

It's possible that when you swapped out the tubes, the joint re-connected temporarily.  But as you used the amp, the vibration broke the connection.  

Glad they fixed it for you.  

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Forgot to mention, I was talking to the amp tech when I picked up my amp the other day and he said that the JJ Electronic tubes are not that reliable anymore. Much to my surprise, the 12AX7 tube that broke in my amp was actually not one of the stock unbranded Chinese tubes that came with the amp...it was a brand new JJ Electronic tube. The tech said Electro-Harmonix tubes are very good. 
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