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Eurotubes - FAQ - Frequently Asked Questions Page for guitar and bass tube amps.

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FAQ Page - Eurotubes Logo

Frequently Asked Questions.

Preface: Tubes are tubes, and it's not a matter of if they will fail, but when they will fail. They are magical little bottles of joyful tone that allow us to express ourselves in a way that no other device on the planet can. We abuse them by driving them into distortion until they are screaming their little heads off. We plug them into amps that travel down bumpy roads in the back of vans and pickup trucks to the gig from hell. Some of us put them inside of speaker cabinets (combo amps) strategically placing them within an inch or two of the speakers and then punish them relentlessly with back waves. Some of us use them until they are a thousand dog years old and then express disbelief when the die.

In general tubes are taken for granted. They are archaic little beasts by nature and the first time I witnessed the entire process a tube goes thru when they are born I was in awe as they seemingly battled to survive the almost medieval process! This page is here to answer some common questions about tubes and to explain a few things that will give your tubes a good start to their hard lives. Proper installation and handling is very important. A large portion of the phone calls and emails that we receive come from players who have purchased tubes from other sources and now that they have experienced problems they are contacting us because their source cannot help them.

At Eurotubes we are all players who understand amps and tubes, we are not refugees from the corner pizza joint, we're not suits behind a desk. We research what we know. I have personally spent thousands of hours researching every amp I can get my hands on, documenting what types of tubes an amp can safely use, what works for different styles, bias info, etcetera. Now that Eurotubes has grown I thankfully have help with this as we continue the research daily, so we don't guess, we know, and if we don't know, we will tell you we don't know and then put it on our "to learn" list. Even if you have purchased tubes from somewhere else we will offer you assistance.

I received a call from a player that prompted the writing of this preface. He called stating that he had purchased tubes for his amp from a "groovy" supplier only to find that after installation his amp made no sound, so he removed them and reinstalled his old tubes which brought the amp back to life. He then threw away the "groovy" tubes and chalked it up to a "bad batch" of tubes. He called me asking for "good tubes".

The first thing I told him was there really are no "bad batches" of tubes. I ask him to retrieve the tubes in question and explained how to diagnose the problem. It turned out that he had one preamp tube that had fallen down so he was able to get some use out of the very expensive tubes he had tossed away.

We hope the following information will help you to understand a little more about your amp and your tubes. Knowledge is power!

 

#1 How do I change my tubes?

#2 Should tubes glow?

#3 Tube noise

#4 What is biasing?

#5 How will changing from 6L6's to EL34's affect my tone?

#6 What is a phase inverter ( splitter ) and why should I use a balanced one?

#7 Basic amp questions including what is headroom? And what is breakup?  

#8 Are my tubes microphonic?  

#9 What's that rattle noise coming from my combo amp? 

#10 HELP! My amp stopped working!!!

#11 Damn! I broke the guide pin!!

 

#1. Tube Changing

I would like to open this section with a word about amp tech's. As you can well imagine having tens of thousands of customers, we hear about everything having to do with tubes, amps and our customers experiences. We also field hundreds of calls from non-customers seeking support because they cannot get it from their tube vendor, so you might say, we hear it all.

We get the occasional call from a player who has read on a forum somewhere that changing your tubes requires a degree in Nuclear Physics and you will surely die from electrocution if you attempt to change your own tubes... So heeding this false information given by the unknown "amp Guru" who is more than likely afraid to change a light bulb, the player will dial up the closest tech he can find to do this all but impossible job. So at this point let me state that there are a lot of very competent and talented tech's out there that are happy to install and bias your amp for new tubes if necessary, and charge a reasonable fee to do it right. So now the bad news... There are also a lot of tech's out there that will be extremely unhappy that you have purchased tubes online and want them to do the install. The reason for this is the price you paid for tubes online (especially if they came from us) will be right at the same wholesale price the tech pays for tubes, which is why you will find on average that music store and tech tube prices are very expensive! It's called making a living.

The worst case scenario is the unsuspecting player leaves his amp and tubes with a tech, only to receive a phone call telling him that the tubes he supplied were "all bad" and of course the tech can supply him with "good" tubes but good tubes are more expensive. We have even received calls from players who asked the tech for the so called "bad tubes" back, only to be told that they simply threw them away because after all, they were no good... Here is example of an email exchange we get all too often;

From: John [mailto:johnrz01010@comcast.net]
Sent: Saturday, April 25, 2009 6:44 PM
To: eurotubes@cnnw.net
Subject: Help me with my tone!
 

Hi. I hope you are having a good day. I read over your entire site and it is obvious you know tubes and you care about your trade so I am hoping you can help me. I hope I can do business with you in the near future.

Here's the situation.  I have a 1989 Marshall JCM 800 2210.  It was an awesome sounding amp but the tubes were original and needed replacing.  I bought a set of Svetlana EL34s power tubes and took the amp in to have it biased.  As you stated on your web site the amp tech told me the tubes were bad and re tubed the amp with  all Groove tubes.  I got the amp back and it was unplayable.  The sound was harsh and grating.  I put my original pre amp tubes back in and the Svetlanas (naturally he didn't give me back my original power tubes) and got the amp playable but it is still a far cry from what it was.  The amp gets that fuzz you talked about on your site.  I hate that.  The lead notes are mushy and to distorted.

Here is what I am looking for.  I play fast and I need the notes to be tight, clear and singing.  I like my distortion to grind but I don't like fuzz.  I like allot of gain but I like everything to be clean.  I am hoping you know exactly what I am talking about.

Can you provide me a set of tubes that would fit the bill for what I need in my playing? I thank you for any information you can provide me.  It is really frustrating to know what the amp used to sound like and what it is now.  It's actually down right sad.  It's to the point that if I can't get it back to it's former sound I will have to sell it.

 I thank you for your time and look forward to your response.

 John

The moral of the story? If you are afraid to change your tubes and have a trusted tech that has served you well, then simply ask him if he minds doing an install with supplied tubes. What we highly recommend is that you learn to do it yourself. You own the car, so learn to change the oil. There is no black magic voodoo involved and you don't need to swing a dead chicken over your head three times while reciting an incantation. Even if you have an adjustable bias amp it's very easy to do yourself which is why we put up the "how to bias video's". If you do decide to seek out a tech that you have no history with, you may want to read our warranty info at the bottom of the testing and info page here.

Please read on, and prepare to change your own tubes!

I get asked all the time "How do I change my tubes?" Here is an in-depth overview of everything I can think of about changing tubes. First, read your manual! Check to see where your power tubes and pre-amp tubes are located. If you order a retube kit from us, or leave a note in the comments box on the checkout page with your order telling us what amp the tubes are for then we will automatically list on your invoice any of the tubes that are located in special positions. 

Removal is easy. Simply pull the tubes out. If they are stubborn, pull while rocking them a little bit from side to side but go easy! If you tilt them too much you will break the guide pin. It's a good idea to cup one hand over the top of a power tube when pulling it out so that when it lets loose you don't smack the tube against the cabinet and break it, I'm not afraid to admit I'm talking from experience here!

We get calls all the time from Marshall DSL and TSL players as well as Peavey XXX and JSX players saying that they only have three preamp tubes. We explain that the forth tube is usually hiding under a metal RF cover (these help block radio interference and other noise). These covers simply pull off or turn slightly and pull off. We state on every invoice we send out for DSL and TSL amps that the phase inverter tube is located in V4 closest to the power tubes but we get several calls a week asking "are you sure the phase inverter is closest to the power tubes, shouldn't go in the spot where the tube has a cover on it?" NO!!! We are not lying! The tube with the cover in a DSL or TSL is the V1 tube and not the phase inverter. The V1 tube is the most sensitive tube position in the amp and this is the reason for the cover. Other amps like Mesa Dual Rec's usually have covers on all the preamp tubes.

When installing preamp tubes or EL84's first check the pins to see if they are straight. We straighten all our nine pin tubes when they are tested but you can slightly bend the pins even removing them from the box if you're not careful. A slight bend as in a few degrees is not a problem but if you bend a pin more than about 20 degrees which is right about the angle of this backslash "/" then you should NOT just pull it back into position with your finger! This can and most often will, cause a minute crack in the glass Then when the tube heats up and expands the crack grows and if it goes clear thru the glass then air will leak in, the tube will fail and the silver getter flash will turn white. If you bend a pin beyond about 20 degrees and do not have a pin straightener then use a pair of needle nose pliers to grip the pin very close to the glass and carefully bend it back into position without stressing the glass.

Here is an example below where you can clearly see the pins are bent in different directions and a crack has formed directly thru one of the bent pins. Click on the picture to enlarge.

If the pins are straight then carefully line them up with the receptacles in the socket noting the gap between pin 1 and pin 9 and push them into place.

Power tubes like 6L6’s and EL34’s are easy because they have large pins and a central guide pin. One word of caution! Do not tilt the tubes from side to side too much or you will break the guide pin off!

Any octal base tubes such as 6L6's, 6V6's, EL34's, KT77's, KT88's, 6550's, KT66's and GZ34's to name a few should be gripped by the base of the tube when being installed and removed. Pushing and pulling on the glass can easily cause small cracks in the glass where it is glued to the base and if a small crack is made in the glass then the tube will lose it's vacuum in the first few hours or less of use. When this happens the silver getter flash in the top of the tube will turn white bearing witness to the fact that the vacuum in the tube has leaked out and the tube is no good.

This next tip is the MOST important of all. Whether your amp is old or new you should always clean the tube sockets before installing new tubes and we HIGHLY recommend doing this! Failure to do this can and often will cause you to burn out tubes and here is why. The pins on tubes are all different just like a fingerprint so when you remove an old tube and replace it with another tube the pins will contact the socket collars in a slightly different location. Cleaning the sockets can easily be done by spraying electrical contact cleaner on the pins of one of your old tubes or preferably the new tubes, and working the tube in and out of each socket a few times. Do this for both preamp and power tubes. If you do not have any contact cleaner, then at the very least you should work each new tube in and out of it's socket a few times to help scrape in a decent connection but if you do this then watch the power tubes closely! Just running them in and out without using contact cleaner will NOT insure good contacts!

In an amp that uses a quad of power tubes, 99.9% of the time even though the two inside tubes and the two outside tubes work together as pairs, the two right tubes most often get their B+ high voltage from one tap on the transformer, where the two left tubes get their B+ high voltage from the other tap on the transformer. So if one of the tubes on the left is not making a good connection especially to ground, then it can drastically overheat, red plating the tube and then this will also take out the tube next to it that gets it's B+ from the same transformer tap. Just before writing this I received a call from a player who bought a quad of =C= EL34's (not bad sounding tubes) for his TSL Marshall, he said that he set the bias just as I show in the "how to" bias video we made for these amps and then after playing the amp for five minutes he started loosing power and hearing distortion on the clean channel. He looked at the tubes and saw that the two tubes on the right were fried and the painted logo had turned brown so he called the vender who sold him the tubes but they had no clue about what might be wrong (typical) and told him to take the amp to a tech and that he must have screwed up the bias...

When he called me this morning and described the problem my first question was "did you cleanup the sockets?" The answer was of course no, followed by the statement "my amp is new so why would I need to do that?". I never turn anyone down as far as helping them with tube issues so an explanation ensued. I told him there was nothing wrong with his amp and to clean his sockets, put his old tubes back in, do an initial bias adjustment and then watch the tubes for the first few minuets the amp was on and in the play mode. Then after about ten minuets I told him to do a final bias adjustment if necessary after the tubes were hot. So about twenty minuets later my phone rang again and low and behold, his old tubes were running just fine! His next question was "so what do I do with these burned up tubes? Will they still work?" I proceeded to give him the bad news with an answer of no... Once a tube has red plated its done, it will red plate every time you try it and you can usually tell if a tube has overheated if the paint on the tube has turned gray, brown or black. The first part of the question was a bit tougher to answer because he had not bought the tubes from us. I proceeded to tell him that we send out an instruction sheet with every order about cleaning sockets but we still get the occasional call where a customer will neglect to do this and sure enough he has red plated two tubes. We will usually not warrantee two tubes when this happens.

Here is an email exchange that is EXTREMELY common.

From: Trubador tv [mailto:trubador_tv@comcast.com] Sent: Sunday, April 12, 2009 11:56 AM To: eurotubes@cnnw.net
Subject: RE: I bought your tubes

Dear Bob,

Yup, that was the problem. Although I put them in and out with no cleaner before, when I finally got the cleaner and did it, all the gain and sustain came back in full force. dude, ur the best. Now i have "my
baby" back and we're movign to nashville in 2 days to launch this band. peace brother.

marcoe stone/TRUBADOR
________________________________________
From:
eurotubes@cnnw.net To: trubador_tv@comcast.com Subject: RE: I bought your tubes Date: Mon, 30 Mar 2009 17:57:37 -0700
Hi Marcoe:

There is no way we would have time to fix the amp before you need it and the only Marshall amps we have are old vintage amps that we would not loan or rent. I’m not convinced it is an amp issue. Did you
clean the sockets? Even if you did you should do it again.

Thanks

Bob
________________________________________
From: Trubador tv [
mailto:trubador_tv@comcast.com] Sent: Monday, March 30, 2009 2:05 PM To: eurotubes@cnnw.net
Subject: RE: I bought your tubes

Hi, The amp is breaking again. I ordered the tubes in the first place because when I would play it was way quiter and the sustain was kind of gone.

******************************************

Tubes do fail once in a while which is why we warrantee them and if a tube does fail of it's own accord then 99% of the time it will not take out the tube next to it. On rare occasions it will take out it's partner, which in an amp that uses a quad of power tubes would be the other outside tube if an outside tube fails or the other inside tube if an inside tube fails but 99% of the time if the two right tubes or the two left tubes red plate it IS due to a connection issue. I have witnessed this many times in amps where the sockets are a little loose and I've made the bad decision after cleaning them to not re-tension them (covered a couple paragraphs down) and had a tube start to red plate and then grab the top of the tube and push it from side to side and then see the tube calm down and start to function properly. The cause being a poor connection.

One of my customers pointed me to a thread over on the HC Zoo (that's the harmony central forum) where someone had posted about cleaning sockets and several people climbed on the thread stating that this was the most ridiculous thing they had ever heard of! One poster stated that he had four amps and changed the tubes numerous times and never had to clean them, therefore this was a load... Well when you deal with thousands of amps as we have done here over the years you get to know what is a load, and what the truth is, so if you enjoy playing the odds and don't mind a little excess noise or static, or putting your amp at risk by burning out the occasional tube, then by all means please take the novice advise given over at the zoo...

So back to reality, I can't tell you how many calls and emails we get from players who have installed new tubes of all different brands and from all different sources without cleaning up their sockets and and then call us complaining of odd noises like "pops" and "scratchy" sounds and want to know how to "fix it". We also get the occasional call stating "I installed my new tubes and now my amp doesn't work so I must have a bad batch of tubes". In reality there are no "bad batches" of tubes and 99.8% of the time in both these scenarios the player did not cleanup the sockets and after doing so the noises are gone or the amp mysteriously comes back to life.

Just because your amp is new does not mean that your sockets are clean! In fact new amps are just as problematic because sockets are often kept in open boxes gathering dust until they are soldered into place on the board or mounted in your amp where they sometimes will sit around again gathering dust before being loaded up with tubes.

Old tube sockets are often loose which can cause a tube to not function or function intermittently. If this is the case you should re-tension the sockets. I use a dental pick for doing this but you can use a woodworking Awl which comes to a point or a very small jewelers screwdriver to squeeze each of the pin holders in each socket together so the tube fits snugly. For nine pin sockets used with preamp tubes and EL84's re-tensioning the sockets is more difficult so if you find that a tube does not seat tightly in a nine pin socket then very lightly roll the pins between your thumb and index finger holding it by the pins to ever so slightly bend them in which will make them a tight fit in a loose socket. Do not bend them too far!

So to recap all of the info you just read, here are the steps to a successful tube change.

#1. After pulling the old tubes, spray a little contact cleaner on the pins of the new tubes one at a time, then work them in and out of the sockets a few times. If you have no contact cleaner than at a very minimum run the tubes in and out of the sockets several times to help scrape in a good connection.

#2. If you find the sockets are loose then re-tension them as described above.

#3. If your amp has an adjustable bias then do an initial adjustment right after you flip the amp into the play mode. Wait about 10 minutes and do a final bias adjustment.

#4. If the amp is fixed bias then watch the tubes for a few minutes after they are in the play mode to make sure all is well.

Here is a copy of the instructions we send out with every order!

INSTRUCTIONS: PLEASE READ! When replacing tubes you must always clean the tube sockets and if you're replacing power tubes that do not fit tightly in the sockets then re-tensioning is necessary. Failure to do these things can cause tube failure and will void your warranty!  Check out the FAQ page here; http://www.eurotubes.com/eurotubes-FAQ-Frequently-Asked-Questions-Tone-glow-microphonic-biasing.htm

It's easy to clean the tube sockets which can be done by spraying the pins on every new tube with some electrical contact cleaner ( you can even get this at a Radio Shack, they call it TV tuner cleaner ) and then work each tube in and out of each socket a few times to remove dirt and corrosion. If no contact cleaner is available then at the VERY least work each tube in and out of the socket multiple times! Remember this will NOT insure a perfect contact!

If you have been using chinese or sovtek tubes both sometimes have excess solder on the pins which can spread the sockets open and make them loose which can cause a tube to not function or function intermittently. If this is the case you should retention the sockets. We use a dental pick for doing this but you can use ice pick or a very small screwdriver to squeeze each of the pin holders in each socket together so the tube fits snugly.   ****IMPORTANT**** All octal power tubes should be gripped by the base when removing and installing them! Pushing and pulling on the glass can cause small cracks in the glass which can cause the tube to loose vacuum. When this happens the silver getter flash in the top of the tube will turn white.

 

A special note for you Peavey Classic 30 and Delta Blues owners and Mesa DC2 and DC3 owners. PLEASE be careful when removing and replacing those crummy wire tube retainers! If you push the wrong way upon removal or let them "snap" back into place too hard you will break the tips right off of your tubes!!! You will not be able to re-tension your tube sockets easily so we use another method for 9 pin EL84's which is what we call "rolling the pins". We straighten all the pins ( preamp and EL84 ) before the tubes ship and if you find that a tube does not seat tightly in a socket then very lightly roll the pins between your thumb and index finger to ever so slightly bend them in which will make them a tight fit in a loose socket. be VERY careful NOT to bend the pins too far or you can crack the glass!

We get calls and emails everyday from players who have purchased tubes from other "stores" and "depot's" and a couple east coast tube venders for support because they cannot get it from these venders. NEVER FEAR! We take the time to answer your questions and we have real world "hands on" knowledge and experience with thousands of amps. So when that guy on the east coast says "you can't run JJ's in a Mesa" or "you can't use 6V6's in your amp" just give us a call to get the real answers. You can also well over 1000 customer reviews from real players on our site here, http://www.eurotubes.com/eurotubes-customer-reviews.htm

 

#2. Should tubes glow?

Yes! If they are not glowing they are not working! The plate structure should never glow orange or have any spots on it that are orange. The heater or filament which runs up the center of the tube is the only part of the tube that should glow orange.

A blue glow that pulses under load is normal. A strong purple glow that shows up when a tube is not under load at idle means the tube is "gassy" usually due to cathode stripping and is on it’s way out.

We get calls on a regular basis from players saying they need new preamp tubes because they don't glow. The first question we ask is "is the amp working?" 99.9% of the time the answer is yes. So we explain that if the tubes were really not glowing then the amp would not be working. The heater or filament is mostly covered by the plate structure with a little bit of it visible, and the amount of the heater that can be seen will vary so LOTS of players who look at their tubes very closely will think one tube is glowing brighter than another if they see a little bit more or less of the heater or filament.
 
 

#3. Tube Noise

All 12AX7 type tubes are microphonic. Some are just more so than others. You can check your pre-amp tubes by turning the amp on, gaining it up and tapping lightly on each tube with the end of a pencil or a chop stick (my favorite). You will be able to hear the tapping through your speakers, which is normal. It is not normal for a tube to ring like a bell after it’s tapped. If it does ring then it’s what is called microphonic and should be replaced.

In the case of very high gain amps like the Peavey 5150 and Ultra or the Mesa’s and Marshall's you will find that by simply swapping the pre-amp tubes around you can find a combination which will yield the least amount of noise.

We also get lots of calls from players who think they have microphonic tubes because they get lots of feedback in the high gain channel of their amp. One way to check and see if it's your guitar feeding back or microphonic tubes is to simply roll the volume down on the guitar when it starts to feed back and if the feedback stops then it's your guitar, if it keeps going with the guitar off then it's your tubes.

Power tubes on the other hand are rarely microphonic. EL84’s seem to be the most susceptible to being microphonic and having filament rattle. The worst combination of course is a combo amp using EL84’s especially some of the high powered one’s. The speaker vibration can rattle those little tubes to death in short order. All power tubes can be checked for being microphonic just like pre-amp tubes.

We get calls and emails from customers and non-customers all the time stating that all their new tubes are bad so they must have received a "bad batch" of tubes because they are noisy. There really is no such thing as a "bad batch" of tubes but occasionally a tube will fail or become noisy right out of the box even if your tube vender tests them. The most common mistake players make is not cleaning up the sockets when changing tubes. (Refer to "How do I change my tubes" above for info). If you experience noises such as hissing, popping or scratching then 99% of the time it's a contact issue with one of the pins on a tube not making a good contact in a socket.

The second mistake players make in diagnosing a noise problem is to pull all the tubes. Here's the scenario, a player calls stating he bought new brand "X" tubes, installed them only to hear noise so he pulled them all, reinstalled his old tubes and the noises went away so he knows the tubes were bad. This really tells you nothing because you don't know what tube may have been causing the problem or if it was a tube issue or connection issue. The pins on all tubes are just like the fingerprints on your fingers, they are all different and just like your fingerprints the pins on tubes will contact the sockets in a slightly different place.

If you encounter noises and have not cleaned the sockets then clean them and reseat the tubes. We have had amps thru here where the sockets were so corroded and dirty that it took several attempts to get the tubes to quiet down. You can install tubes and have no noise only to find that when everything gets good and hot the noise will come back. This can happen because when parts get hot they expand and move so a marginal connection can become no connection. So if you have some noise then clean the sockets and reseat the tubes, if the noise persists then you can diagnose the problem by replacing the tubes one at a time with an old tube. If you have a tube issue this is the only way to isolate it.
 

#4. What is biasing?

Bias is the negative voltage which is applied to the power tube’s control grid to set the level of idle current.

So that’s the technical definition. Most of the questions I get about bias are:

A. What is it?

B. Do I need to worry about it?

C. Will it affect my tone?

D. How can I adjust it?

Let’s Get To It!

A. You read the technical definition but unless you have some background in tube theory it probably read like an insurance policy so I’ll see if I can do better!

Your power tubes need to idle properly. If they are set too cold (overbiased) they are not producing their part of a full sine wave and are in what is called "crossover distortion. If they are set too hot (under biased) they will distort too soon and the tube life will be shortened. Setting the bias on your amp is like setting the idle on your car. If it’s too high or hot it’s running away with you and if it’s too low or cold it will choke when you step on it.

B. Yes and no. All tube hi-fi amps need to be biased properly. Some have trim pots for each tube and some are cathode biased. Always stick with what the amp maker recommends. Guitar amps are a different story. Amps using EL84/6BQ5 tubes are almost always fixed bias and because of the type of circuit used are usually O.K. On the other hand, any guitar amp that is not cathode biased and uses 6L6, EL34, 6550 or KT88 power tubes NEEDS to be properly biased to sound it’s best no matter what the manufacturer says! Some Mesa Boogie and Peavey amps that use 6L6, EL34 and 6550 power tubes are biased very cold and can benefit greatly form being properly biased and over the past few years we have also come up with different tube sets that will get these amps up to proper bias. You can find these in the Eurotubes online store.

C. In one word YES! Absolutely, Positively, BIG TIME! Well that’s more than one word but I can’t stress enough the difference in warmth of tone and dynamic response that come with proper biasing. Every time I convince a player who owns a fixed bias amp that uses 6L6, EL34 or 6550 tubes to have their amp biased right the outcome is always the same. They can’t believe how good it sounds. This is because tubes that are biased cold or "overbiased" are in what is called crossover distortion and can’t produce a clean, warm tone and are sluggish in response.

D. Saying that an amp is fixed biased only means that the manufacturer used a simple resistor in the final bias stage instead of a variable resistor which is commonly referred to as a trim pot. So, if your amp is "Fixed Biased" and you want the best performance you can and should have the simple resistor replaced with a trim pot in series with a lower value resistor. Any decent technician can do this for about $50.00, including the adjustment. Once this is done then you can be in control of your tone and bias the amp yourself using a bias probe. We have even gone so far as to produce "how to bias videos" for you here! http://www.eurotubes.com/eurotubes-how-to-bias-video-probe-Pro-One.htm

Now let’s talk about the adjustment. There are several different ways to bias a tube amp. The two main methods of biasing an amp are what I’ll call the old school methods. The first is using an oscilloscope to monitor the sine wave output of the amp while the amp is being driven with a signal generator. A fair degree of technical know how and experience is necessary when using this method. The second is the transformer shunt method which involves working very closely with extremely high voltages and I do not recommend using this method.

The newest and by far the easiest method of adjusting bias is by using a bias probe. This is a device that looks like the base of a 6L6 type tube which is installed in one (or two if it’s a dual probe) of your sockets. Your tube then goes in the socket in the top of the probe. Some probes need to be used with a multimeter and some come with built-in milliamp meters.

Some old schoolers don’t think much of the bias probe method but I can tell you from experience that it not only works but it works very well. I’ve compared the two methods extensively and I very much prefer the probe over the scope method, especially for guitar amps. All you need to know to bias your amp using a probe is what the plate voltage is which is measured from pin#3 to ground. Once you know this then the formula is simple. Divide the plate dissipation of your type of power tube into the plate voltage. This will give you the maximum plate current which should never be exceeded.

We will use the plate dissipation for a standard 6L6 for this example which is 22 watts. Example: 22 watts divided by 500 plate volts equals .044 or 44 milliamps of plate current draw at idle. The 44mA should never be exceeded! Then multiply the .044 by .7 or 70% which equals .030 or 30mA. This is where your power tubes will be out of crossover distortion and will perform very well. There is an acceptable window of correct bias and if you want a warmer tone or what some players refer to as a "brown sound" you can use up to 90% of the maximum current formula which would be 40mA rounded off. Your tube life will be a little shorter at this setting but only by about 20%. However, I would NOT recommend this for Chinese tubes or tubes that are sold by the Big guys that have been remarked unless you know for sure what tubes they really are. The JJ Electronic 6L6 is a very stout tube and JJ claims that it will dissipate 30 watts. However this does not mean you have to calculate them at 30 watts to get out of crossover distortion. You can use 70% of 25 watts and they will get a great warm clean tone well out of crossover distortion.

Line voltage is also a consideration especially if your electrical service is old and you run a lot of appliances or have electric heat. During heavy usage hours in the evening you can actually be down by 3 to 4 volts compared to lighter usage daylight hours when your AC can swing the other way and be up by several volts. You may be asking "what the heck does this mean to me?" Here is an example. You bias your amp during the evening when and your voltage happens to be down at 116 volts. You want to push those tubes so you wind up your bias to 80% of dissipation and it sounds great. You fire up the amp during the day over the weekend and your AC voltage happens to be at 122 volts. The amp screams for fifteen minuets and pops a fuse. The difference between 116 volts and 120 volts can mean that your DC plate voltage in your amp can vary by 30 volts! So it's a good idea to know what your line voltage is.

Since I like to run my gear pretty hot I will routinely bias them at 44mA in an amp that has 485 plate volts. The tone is very warm and the tubes stand up very well. I tortured a quad in my Fender twin for a year, set at 50mA at 480 plate volts and they were still going strong until I swapped them out for a quad of KT88’s but that’s another story....

#5. How will changing from 6L6's to EL34's affect my tone?

This is a very frequent question and although there is no perfect answer that fits all amps I can give you a pretty good idea what to expect. I've swapped EL34's and 6L6's around in Marshall's, Fender's, Mesa's and Peavey's as well as many other amps which will accommodate different power tubes and the results are basically the same.

 EL34's in general don't have a low end that is as deep as a 6L6 so EL34's tend to sound hotter in the mids. EL34's breakup a little earlier then 6L6's and the harmonic content of the mids and highs are more prominent. These differences become more noticeable as volumes increase. You have probably heard the term "Marshall crunch" used before. This term is used to describe the sound of EL34 type tubes when they are being driven into distortion. So if you're looking for a great Classic rock tone with lots of complex mids you might want to try some EL34's if your amp allows. The JJ E34L's will crunch up real nice but they do have a deeper tighter low end than a standard EL34. If you like a big low end thump then I would either go with the JJ KT77 which has even a bigger low end than the E34L but still has the harmonic complexity of an EL34, or stick with a good 6L6 unless you have room for the JJ KT66's which have about the same low end as a good 6L6 but have more complex harmonics. Then of course if the mission is huge and your amp allows then the JJ KT88's simply sound BIG all over with a massive low end.

 

#6. What is a phase inverter ( splitter ) and why should I use a balanced one?

A phase inverter or splitter is the tube that keeps your power tubes ( in a push pull A/B type of amp ) running 180 degrees out of phase. This is necessary because in a push pull amp one tube creates the upper half of the sine wave while the other tube creates the lower half. One of the symptoms of an unbalanced phase inverter is that some notes will sustain very well where others will fall off or sound dead. In 99% of all guitar amps the PI tube is the closest preamp tube to the power tubes and 99% of all A/B style guitar amps use a circuit that splits the phase inverter duties utilizing both triodes in this tube.

We get questioned quite a bit on the subject of balanced PI tubes so I’ll elaborate a little. I use to be a real skeptic on this one because most PI circuits are not designed to be perfectly balanced. Ah ha!!! Say what? So it’s snake oil then?  

No, I’ll explain. Most all circuits using long tail phase inverters are designed with a slight amount of asymmetry. The reason for this is because when both sides are dead on, the amp will sound a bit sterile and be missing that bit of 3D swirl that makes a tube amp sound alive. This is also the case with a way out of balance circuit, it will sound sterile and certain frequencies will suffer from phase cancellation.

Logic says (I’m a big fan of logic and common sense) then why balance the current on an imperfect circuit? The short answer is, to get the result the designer wanted, but I’ll go further into this. When I acquired the test equipment to check for balanced triodes I started stuffing balanced tubes in everything I have to see if there was a real difference or if it was snake oil. The results? In some amps it was nothing short of amazing and in others it was noticeable but not jaw dropping.

A good friend of mine who has a Marshall 6100 30th anniversary, (one of those amps I just love to hate, I call it the little Blue bastard), had fought getting a good tone with it for years so he brought it back in a few months after we started using balanced PI tubes and when we changed the PI tube for a balanced tube we both were astounded at the change. The amp simply went from so-so at best to, holy crap, is that the same amp!!

So, morale of the story? It’s just like the old argument that a rectifier tube does not change the tone of an amp because it’s not directly in the tone circuit. This of course is completely false and anyone who has ever done an A/B and compared a chinese, a sovtek, a JJ and different NOS rectifier tubes will tell you there is an amazing difference in dynamics and tone. A good balanced PI tube does make a difference and it’s not snake oil. All you need is a pair of ears to hear the difference.

You will still find a lot of skepticism and bad press from the "nay Sayers" on the benefits of using balanced PI tubes. The only question I would ask them is "have you personally had experience in listening to the difference between balanced and unbalanced PI tubes in different amps?" Oh, and request an honest answer when posing the question.

 

#7. Basic amp questions including what is headroom? And what is breakup?  

We get the question all the time, “what is headroom?” Headroom is often referred to as the volume your amp will achieve before distortion sets in. A more accurate description should include the explanation of dynamics and that most all music by it's nature is very dynamic with peaks and valleys. A couple of exceptions would be hard core screamer rock and punk that simply comes on full tilt, smashes you up against the wall and then finally drops you at the end of the song. Another would be rap which consists of a 60 to 100Hz pounding synthetic kick drum that never varies in volume and some guy trying to rhyme because he can't sing....

Now a good example of punk with great dynamics is the new Green Day "American Idiot" CD. While I don't really agree with their politics I think this is one of the best recordings I've ever heard and the song writing is top notch. It has a permanent home in my CD changer. I would give a good example of rap but there isn't one....

I digress, so back to dynamics and head room. A great example of this is evident on any Robben Ford or Ronnie Earl CD. These guys are Blues and Jazz based and you can hear passages that go from very subtle to really digging in.

A typical 50 watt guitar amp when cranked up to about half way on in the clean channel will be right at the point of breaking up when you dig in on a note or a chord. So imagine that you are recording a passage that is very dynamic and starts out softly and then builds in volume and that you want the tone to stay clean. Even with the amp half way up you are only using about 10 watts of power to play a light passage but when you dig into a note and the amp is called on to reproduce it then it will be using most all of it’s power to do so and this is referred to as headroom. If your amp did not have the power to reproduce the clean tone you wanted at the peak of the passage you would be out of headroom!

We’re often asked “what is breakup?” Breakup is simply distortion. When the typical 50 watt guitar amp is about half way up you will start to get preamp tube, power tube and speaker distortion ( which is what I call the sweet spot ). One of the most asked questions is “how do I get more power tube breakup?” The answer? TURN IT UP! The other alternative is an attenuator between your amps output and speakers which allows you to crank the amp up higher and then choke down the output before it gets to the speakers causing a lower overall volume.


 
#8. Are my tubes microphonic? 

When you tap on a tube with the amp turned up you will hear the mechanical signal thru the speakers or the tube is dead. The definition of microphonic is that a tube when given a signal at a specific frequency will oscillate and carry on that tone by it's self like a ringing bell.

 

One way to check and see if it's your guitar feeding back or microphonic tubes is to simply roll the volume down on the guitar when it starts to feed back and if the feedback stops then it's your guitar, if it keeps going with the guitar off then it's your tubes.


 
#9. What's that rattle noise coming from my combo amp?

Great question! Combo amps like Peavey Classic 30's, Fender Blues Juniors and Fender Hotrod Deluxes (these are the worst models) are nothing more than torture chambers for tubes because every time you play a note the back wave from the speaker pounds on the tubes. This vibration and continual pounding will shake the guts loose in just about any tube in short order. This is a universal problem with all brands of tubes, and the JJ's are no exception. I have LOTS of NOS tubes including RCA's, Mullards and Telefunkens which are among my favorites that rattle when they get anywhere close to a combo amp.

 

We get calls all the time from players who have just purchased brand new amps right off the showroom floor asking for new tubes because their tubes are bad. When we ask what the problem is they say have microphonic tubes and that is sounds like broken glass or a jar full of BB's when they play lower notes. We then have the unfortunate task of explaining that this is not microphonic tubes, it is mechanical noise and you have just purchased a torture chamber for tubes...

 

These noises are heard easily in a quiet environment or trying to record with it rather than playing at a gig where these noises are rarely heard. This is why combo amps are rarely used to record with or extension cabs are used for recording. In most cases just about the time your power tubes are starting to loosing their high end (about six months) they will also start to rattle and if you play at high volumes the rattle can start much earlier.

 

We test for tube rattle but a tube can be fine one minute and develop a rattle the next and this is something that we nor any other tube vender will warranty. Tubes were never designed to be placed inside of a speaker cabinet...

 

We get questions about tube rattle all the time from players who hold tubes up to their ears and knock on them to check for rattles and I always tell them one thing, STOP IT! Stop beating on your tubes or we will have to turn you into the ACVT! (Association for the Cruelty to Vacuum Tubes)  It's not good for them! You cannot tell if a tube will rattle by holding up to your ear and knocking on it and this is why.

 

When a power tube is cold the insides of the tube are cold and contracted and in a very different position than when the tube is up to operation temperature. When a tube heats up the internal plate structure including the cathode, anode, filament and other internal components all expand at a greater rate than the glass that houses them. This is why the only thing touching the glass are the mica spacers which have a very small contact area because they literally move against the glass as the internal components expand into position.

 

So as you knock on a tube when it is cold it will not tell you anything about what it will do when it’s hot. A cold tube can sound perfectly tight and when it gets up to temp it can rattle horribly in a combo amp, and of course the inverse is also true where a tube that appears to rattle cold will be perfectly tight when it’s expanded into position and up to operating temp.

 

Tube rattle in combo amps, or torture chambers as I call them, is the biggest problem having to do with tubes that there is…

 

After experimenting with tube dampers in torture chambers (combo amps) for quite a while we finally decided to build them right. We had our own mold made for these and our “EuroDampers” are made from a softer than standard durometer, special high temp silicone. These are BIG dampers and measure 1.45 in diameter by .375 tall and are usable on all 9 pin preamp tubes and EL84’s. You can see high res pictures of these on our tube damper page here;  http://www.eurotubes.com/eurotubes-EuroDamper-Tube-Damper-Rings.htm

 

 #10. Help!! My amp stopped working!!!

We get panic calls and emails all the time from players who have amps that have simply stopped working. So here is a guide to help you try an trouble shoot some common problems. I'll divide this into two sections starting with power tube issues first and then preamp tube issues.

Power tube issues.

#1. If your amp blows a fuse and you have not changed your power tubes in over a year then stop. Do not pass go. Do not replace the fuse and play the amp again unless you had a power surge caused by a power outage, a blown breaker, or you just decided to play your guitar while bathing and survived, or you own a 5150 (more later). If you did have a power surge or a blown breaker and popped a fuse then replace it and try the amp again. If you blow a second fuse STOP! You probably have a power tube going short and if you keep replacing fuses or "God forbid" use a 10 amp fuse to cure the problem you risk causing serious damage to the amp. If your 5150 blows one of the conveniently located internal fuses on the PC board then pull the chassis out of the box and replace the fuse, usually all will be well. These amps like to blow these fuses just to screw with you...

#2. If you have just installed new tubes and you have blown a fuse then carefully remove the power tubes to make sure you have not miss-indexed one. There is a guide pin on the bottom of all octal tubes including 6L6's, 6V6's, EL34's, E34L's, KT77's and KT88's, these power tubes have a timing key on them and this has to lineup with the notch in the socket. If you turn an amp on with a tube miss-indexed you can damage the tube and the amp! You may think this is impossible and get a good laugh about it but believe me, we get lots of calls from players and customers who have done this! If the tubes were indexed properly and you did not clean the sockets or the sockets are loose and do not grip the pins on the tubes tightly then clean the sockets and re-tension the sockets if necessary. (Go to the top of this page to read about this under the heading of "How do I change my tubes?") I cannot emphasize enough that having good connections between your tubes and sockets is of paramount importance!

#3. So lets now assume that you have the tubes installed properly and there are no connection issues but the amp still blows a fuse. Remove the power tubes, replace the fuse and power the amp up. If the amp blows a fuse with no power tubes installed then it's not a tube issue and the amp needs to see a good tech. If you can power the amp up with no tubes installed and the fuse does not blow then power down and install one power tube and power up again. Repeat this process adding the second tube to the equation. If the fuse blows then remove the tube you just added and you will be holding the trouble maker in your hand. You can use this trouble shooting procedure for amps with any number of power tubes.

#4. If your amp uses a rectifier tube and you simply lose power or blow a fuse then this tube can cause both of these things.

#5. If your tubes are more than 6 to 8 months old and have seen regular use do not simply replace one tube, replace them as a set. If your tubes are new then contact your supplier about the failure. If you bought them on evilbay do not pass go...

 Preamp tube issues.

#1. If you have a single channel amp and you suddenly loose power or your signal drops down substantially and your power tubes look fine then it's probably a preamp tube causing the problem. The first thing to do is to clean the sockets and reseat the tubes. If a tube was making a marginal connection to begin with then as the components heat up and parts move a bit due to expansion a marginal connection can become no connection. I can't even count how many times we have instructed players who have called with problems to clean their sockets and reseat their tubes only to be amazed that their amp came back to life. Connections people! You know what they say, if you don't have connections you don't go anywhere!

#2. If you have cleaned the sockets, reseated the tubes and still have nothing, then replace the preamp tubes one by one using an old tube that was known to be functioning. This will allow you to find a malfunctioning tube. If you simply replace all the preamp tubes then you will never find out which one is causing the problem.

#3. If you have a multiple channel amp and all channels go down then it's probably either the V1 preamp tube which in most amps is the preamp tube closest to the input jack or the phase inverter tube which in most amps is either the preamp tube that is farthest away from the input jack or closest to the power tubes. So trouble shoot these tubes first and you will likely find the source of the problem.

#4. If you have one or two channels go down on a multiple channel amp but your clean channel still functions then look to the tubes located between your V1 tube and the phase inverter tube. In most modern multi channel amps the signal is cascaded thru several tubes compounding the gain.

 #11. Damn! I broke the guide pin!!

OK, it happens to the best of us. If you tip or lean a tube a little too far you will hear a faint "snap" and then when you pull the tube out the central guide pin stays in the socket or drops into the amp. #!$!! Another scenario which is what we call a "noob incident" (we were all "noobs" at one time) when the "I GOT NEW TUBES!" adrenalin is pumping and you stuff the tubes in the sockets before even noticing that silly guide pin. Miss-indexing a power tube can cause some catastrophic results when the power is applied but some are lucky enough to miss-index a tube in the one or two positions that will cause no harm. Then when the amp does nothing and the tubes are removed the jammed in guide breaks off.

If you do break a guide pin then the tube is still usable as long as the glass is not physically broken. If this happens the silver getter flash in the top of the tube will turn white and it's game over. If the vacuum is still in tact then all you need to do is draw a line with a sharpie or white-out or even nail polish that runs up the side of the base of the tube so you can properly index or line up the tube with the timing notch in the socket. The first picture below shows a couple of "abused" tubes and the second shows a socket and the timing notch and the third with one of the abused tubes installed.

So carefully mark the tube to show where the timing key once pointed and then mark the chassis on your amp indicating where the timing key in the socket is pointing and carefully install the tube.

 

 

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