Here's some tube talk from the point of view of a repair and hotrod guru. I hope you find it useful and entertaining, in the true spirit of the web.


The Driver Tube

Preamp Tubes V.S. Output Tubes
Power Tubes
About Preamp Tubes
Who should you be buying tubes from?
How Do You Find A Tech?
Matched Sets
Who Makes Tubes
Amplifier Classes of Operation

The Driver Tube;
The driver tube is the least understood tube in an amp. If you are putting in matched sets of output tubes (and you really should be) then you are wasting your time and money without a Driver Tube that has been picked for this special job.

Just about every amp with 2 or more output tubes uses a driver tube. Why it is so important is because this tube takes a single signal voltage, and splits it into two. There are actually 2 separate amplifiers in a 12AX7 and in a 12AT7. If each amplifier section in it are working at the same potential - great! But they hardly ever do... so, you need to find one.

As far as I know, no one markets a "driver tube" for musicians, however I do. That's because when I
purchase tubes, I sort them for where in the amp they would perform best. This symmetry in the driver tube allows the 'matched' output tubes to see the same input signal voltage levels, and in turn deliver matching output voltages. When all this and properly biased, the tubes will sound better and last longer. Be sure to read the section on matched sets to fully grasp this vital aspect of your amp. My driver tubes are matched to within 1%. The others claim 5 and 10%. No joke.



Preamp Tubes V.S. Output Tubes;
This is where the tone really happens: in the preamp. Read more about that in "ABOUT PREAMP TUBES". Most amps don't depend on output tube distortion to generate the tone. There is an audible difference in the way preamp tubes distort and the way output tubes distort.

Preamp tubes are capable of much higher levels of distortion (safely), and tend to sound like a finer grit of sandpaper than the course distortion of an output tube. For amps that use EL34's (6CA7), EL84's (6BQ5) like Marshall and Vox; these tubes are likely to impart their own tone and or type of
distortion under normal operating conditions. For almost all Fender circuits you will find the 6L6/5881: this is a workhorse clean tube. When working properly, the output tubes do a good job of NOT imparting their own tone, or distortion. In some Marshall's, you'll find 6550's; these tubes are the big brother to the 6L6/5881. They extend the treble a fair amount, and by comparison can be shrill. They also allow you to hear what's going on in the Marshall preamp circuitry with much more detail. Its a good rig for most Metal genre's, as articulation is a key component to a great sound. Its also a great tube for jazz players. The classic Marshall tone is 3) 12AX7's with 4) EL34's for the Rock Arena, for the blues and proto-rock glean its the 40 to 50 Watt version (as above but only 2) EL34's.


Power Tubes;
Just to clarify, Output Tubes AKA "Finals" although they do a lot of work, are really not "Power" tubes, as they are often referred to. I'm too old to try to change the world, but, Power Tubes are rectifier tubes, and (almost never seen in audio) regulator tubes. In both cases "power tubes" refers to tubes that are part of the power supply. Output Tubes are not part of the power supply, so it doesn't make sense to call them "Power Tubes".

Examples of power tubes are 5AR4/GZ34, 5Y3, 5V4, 5U4, A03.
Examples of output tubes are 6L6, EL34, 6V6, EL84, 6550.


About Preamp Tubes;
First off, nobody's tubes sound any better than anyone else's. Its entirely a matter of taste. From one 12AX7 to another, you will hear differences in tone, dynamic range, and distortion characteristics, if you listen to them at clean, low volume settings. If you want to know what they are really doing behind your wall of distortion, first give them a critical low volume listening. If you lack the discipline to do this, then you will never learn what all the different tube qualities are. Its great that you like to play loud - whatever, I do too.

Tone; Some preamp tubes sound thin, some dark. Some lack midrange, some are rich and fizzy.

Dynamic range; Some are accurate, percussive tubes, others are compressed. One may sound too aggressive and the other too squishy. They both sound good, but one will perk your ears up and make you say "Yea, that's what I've been looking for, that's been the missing ingredient!" Compression is not a bad word. In tubes, the effect is so natural and transparent that you don't hear it as compression. It simply has a different feel, which is what you should be focussed on. It can tame the harshness of one amp, but add the punch needed for another amp.

Distortion characteristics; How quickly a tube is driven into clipping, and whether it then gets smoothly rounded off or generates harmonic overtones determines whether the tube simply compresses or goes into distortion. What that distortion sounds like is affected by the tonal characteristics of the tube, and if it is also audibly compressing. What range of notes gets distorted most easily has a great deal to do with your final product in terms of tone. This is because in guitar amps distortion is overtones, and overtones can be in an area where fundamental notes lay as well.

To hear the differences you need only one of each type, and listen to each one in the socket nearest the input jack. Here the preamp tubes' effect will be the most evident. Learning the way you can refine the tone of your amp with tubes is fun, inexpensive, and worthwhile.

I have sufficient experience to interview you about what style of music, what amp, etc., to be able to advise you on what tubes are most likely to make you "Wow". I don't charge for that if you are ordering tubes from me. But who should you be buying from?
Read "who makes tubes".



Who should you be buying tubes from?
I love the web. I have the time here to say all I need to, and over and over again. And when I say it, I'm not loosing bench time! This is important so listen up.

First off, lots of people do a great job of buying whatever tubes are on the market and screening them for quality. For example; Fender, Groove Tubes, Mesa Boogie, Ruby, and Marshall.
Secondly, the retailers are ready to serve: Guitar Center, your local music store.
Third, mail-order offers low prices: Musicians Friend.
Fourth, the web offers worldwide access to retailers, some web-only.

Now lets get to it. The best place to buy tubes is where there is a tube tester! That's it! When you buy a tube you should have the ability to test it! That's the way its been done since the 1930's. When you buy a tube you expect that is a working tube, without shorts or opens. If you're old enough to remember, those tube testers were in Radio Shack to test the NEW tubes as much as for testing your old tubes!

Tubes go bad in shipping. Filaments open, short to the cathode, vacuum leaks, etc. That's not the fault of whoever made or sold the tube, all tubes have a mechanical aspect, and are prone to mechanical failure.

The next best place to buy tubes is from the place that is servicing your amp.
Are you going to send your new tubes back to foreign country land when you discover they don't work? How much money did you save now? When you buy the tubes from your tech, all your warranty concerns are addressed there; parts and labor. Nobody has to point the finger at anyone else, there are no shipping delays or costs to you when a tube fails. You don't have to make multiple trips between the point of sale and the test machine either.

Yes, that many new tubes are bad. This is the leading reason why people often aren't happy with their new boutique tubes. If you buy tubes online, you can really be asking for it. Where do the tested bad tubes go? For one; unsuspecting individuals trying to save a buck, who may not have access to a tube tester!

Say you buy 3 preamp tubes and 2 output tubes, and 20% of the new tubes in the world are bad. This is realistic, by the way. There's a 100% probability that you'll have to return 1 tube. Then there's a one in five chance you'll be taking that new tube back for replacement, its bad too. Do you want to make three trips or pay three shipping charges? Yes, you will be paying the return freight.

To sum it all up, the absolute best place to buy tubes is from your amp technician who allows you access to one of his tube testers. Your amp will sound better, and you will have saved time and money.

I get that you have integrity, I want to buy my tubes from you.


Tube Failure:
Some tubes are almost impossible to use in a high gain circuit due to Hum or Microphonics. A tube that is humming is likely to have leakage from the AC voltage filament supply to the cathode. This not only introduces hum into the circuit but also throws up to 12 volts of AC on an electrode that's only supposed to have less than 2 volts of DC. This means the tube is junk, even if it works, even if you don't really notice that much hum, unless you want to cause your amp to break too.
A tube with a short from the heater to the cathode is not uncommon. If it is an output tube, then you may get lucky and pop a fuse; hope that you do. Most amps do not have a fuse on the heater voltage. Your audio and transformers can blow instead. If the short is in a preamp tube you will not pop a fuse because the cathode connects to ground via a small resistor (unlike most output tubes, they usually connect to ground directly) which limits the amount of current. Thats really not good because you are still overworking your power transformer. Power transformers are sensitive to "magnetic saturation", an irreversable condition that prevents full output. Its obvious to the amp owner there is a problem, "it just doesent have the nuts it used to", but is very tricky to spot on the test bench. So all you guys that think the tubes that came with the amp 10 years ago are fine; you don't know, you need to get them checked. You are taking a huge risk. You own a vintage amp? Then you should know how important, expensive, and sometimes impossible to replace those transformers are. But getting your tubes checked and possibly getting you amp to sound better than new is fun - do it. All output (power) tubes (finals) will eventually form a heater to cathode short. Output tubes should be checked for merit and h-k leaksevery six months. My store has one for you to use free of charge.

Microphonic tubes have something loose inside them that, when gently tapped or otherwise vibrated (like sound from your speakers) will cause the tube to make its own sympathetic sound. Just past marginal microphonic failure you will hear a grumble when you play a bass note. More microphonic tubes wont let you turn up the treble without tearing out a shriek. Worse still you can't play lead style distortion without your amp squawking at you. There are lots of shades of grey in this. If you are hearing any tones that don't belong then you most likely have a microphonic tube. It may plague you only under certain conditions. But a bad tube is good opportunity to upgrade the sound of your amp.

Other failure modes are:

No orange glow in the tube (the heater filament is blown).
The big grey things (plates) in the tube are glowing almost red,
there are glassy crinkling sounds,
you could fry an egg on my tubes (You really need a repair, and new output tubes).
My amp sounds muddy, its not as loud as it was, and my bass is on 10 and it still doesn't have enough bass (you need new tubes).
The tube has smokey blue-grey color (the vacuum is lost)
The glass has white powder floating around in it (the insulation on the heater is falling away, and there will soon be catastrphic failure). DO NOT USE, do not even put that in the tube tester!

Now wait a minute, you said you just put new tubes in right? And it sounded great for a while, but now yadda yadda yadda. Here's what happened. You heard from a friend or teacher that you need to put in
Brand X tubes to get a good sound, because you were not completely happy with the sound you were getting. Then you went to the store and got those puppies. Somehow you knew that there is a high failure rate in new tubes, and bypassed all mail-order possibilities. Now, you either made the mistake of putting them in yourself, or the technician you took it to biased your amp one of three of the four correct ways that won't work to keep your amp out of the shop. You didn't let some retail clerk put them in, right? Again, the technician you took it to biased your amp one four correct ways, but not the one method that will work to keep your amp out of the shop. This happens all the time. And by the time you figure out your amp REALLY sounds bad, the tubes are off warranty by one day.

I said there are 4 correct ways to bias an amp. One of them is better, so your amp is more likely to stay out of the shop. Unknowing or unscrupulous repair shops will make you a steady tube customer. Who's to blame? Maybe the tubes they are using, maybe the tech, maybe you had something really go south in your amp. Either way, if your back for a third set of output tubes in a year! Go find a tech who knows what the good tubes are, who really makes them, and how to keep your amp out of the shop. Follow his recommendations, and pay a little extra for (probably his) good tubes because you will save money in the not too distant future.

Let your amp cool for 3 minutes, that means completely off, before you attempt to move it. This will help keep your tubes from shorting out. They are the same as light bulbs: a vacuum, a heater. And if you unscrew a light bulb while it's on it usually blows because of the violent shock taking place while the hot metal filament is transitioning from it's hot almost molten like state to it's cold, very hard and brittle state. You banged it when it was changing from hot and soft to cold and brittle. Same as a tube.



How Do You Find A Tech?

Call the amp manufacturer, and get the name of a factory authorized service center. If there isn't one, ask your friends if the shop they use does good work. Use the Yellow Pages - Believe it or Not - This is the heading: Musical Instruments - Repairing.

A busy shop will not have the best turnaround times. A good shop will always be busy. In my area shops doing quality repairs have 4 to 8 weeks turn.

My shop is open 4 days a week. I charge a premium of twice the labor for emergencies. I also will go to the customer, but for a minimum charge. I bring everything under the Sun and fix it on the spot.



Matched Sets:
There are variations in any manufactured product, this is especially true in tubes. What matching does is pair up tubes that are equal in potential. Typically, we're talking about Output Tubes, although there is a need for matching in driver tubes. By using matched sets, you get an equal amount of output voltage swing and current driving capability from each side of the push-pull audio transformer.
To try to visualize this: picture each of your hands as an output tube. Put your hands together like you are praying, only with a sheet of paper in between your hands; that's the speaker. Now push from
side to side. Notice that the speaker moves to either side of center. Now what happens if one of your arms is made to push harder? Your weak tube gets beat up, and the strong one has to work harder, try it. Lots of people match output tubes and do a very good job of it. Its expensive to do, in labor and electricity, and its worth it for you because in the not so long run its cheaper.

Matching tubes can also entail BURN-IN time. This addresses "infant mortality". Infant mortality describes the effect of most parts that will fail prematurely will do so in the first 24 to 48 hours of use. What burn-in is, is simply putting a device (tube) under conditions that resemble an actual circuit, and let it sit there and run. Then you test it to see if still works. If it does, two things happened:
1) The future reliability of that same tube went way up.
2) The BIAS POINT of the tube drifted. Again, you get a more reliable tube, because it wont need to be readjusted two days after you put the new tube in! Read more about bias.


Fender now owns Groove tubes. At this time, Groove Tubes actually does make a few tubes. They make a KT66 and one particular 12AX7, Both use gold in the construction of the control grid. They have purchased the original tooling to produce US made 6L6s'. That's all they make at this time. The rest of their tube line is, as it always has been, a representation of all the worthwhile tubes made, mostly currently. They, like Fender, Marshall, Mesa Boogie, Ruby, TNT, do a very good job of screening out the bad tubes and putting their own name on them. To the best of my knowledge, Groove Tubes is the only one on that list that manufactures any tubes, and that represents only a small portion of what they offer. Fender is now shipping "Fender Groove Tubes'" in their amps. All the ones I've seen still carry a third name, the name of the actual manufacturer: Sovtek.

Marshall has dropped Sovtek from certain model amps, and is currently shipping Svetlana tubes in
their amps (pronounced Sevitlana). Prior to that they had used Tesla tubes for a long time, and are currently made also. EI was apparently on the wrong end of Western politics, and the manufacturing plant in Yugoslavia fell down, went boom. Its production of tubes rises and falls to this day. The highly abused Chinese Sino 12AX7 was the tube everyone loved to hate, while at the same time widely used for the production of new amps. The tooling is said to have been put outside, and has gone to rust. I guess another form of friendly fire. So much for the most reliable 12AX7A I ever used.

There is more than one tube manufacturer in China! Lets not lump everyone together unfairly. Just like there is more than one Russian tube company, there are few enough choices in tubes to start getting a National Bias (is this a joking matter?). For anyone who cares, there is a market for American made tubes, as proven by the gentleman who purchased the tooling and licensing of the name from Westinghouse - who is making the 300B in the good old USA. The import 300B's sell for up to $300, so it's a fair guess the American made Westinghouse will cost double. By the way, the 300B is it for true hi-fi. If you want to buy a guitar amp that uses this 42 watt triode please tell me.




This is for geeks like me. Its misunderstood, there's too much misinformation out there, and my best customers are the ones know something, so here goes. I'll keep it simple, and cover some stuff you wont find elsewhere!

Preamp tubes are in a self adjusting circuit, so they never need biasing. We are referring now to output tubes.

Bias is an adjustment that determines how hot a tube runs. It also determines what the duty cycle is of a tube in a push-pull circuit. The amplifier design determines what class of operation the tube will run
in, and so in turn determines what the correct bias point is. Regardless of class, there is a range of proper adjustment!

You need to know what you are doing to make this adjustment or you can seriously damage your
amp, sometimes irreversibly.

Three 'Classes'

Class A: The tube or tubes in the amp each have a 100% duty cycle. That means that the tube is always doing its job of pushing and pulling the speaker. Any amp with only one (output) tube has to be Class A. If there is more than one tube, it can be wired in parallel or in push-pull. Push pull amps can be any class. The parallel configuration is 'single ended', Class A. Most guitar amps 15 Watts and under are Class A. These amps tend to sound very natural, and sustain well.

Class AB: Here amps use tubes in multiples of two. They are arranged 'push-pull'. The duty cycle is 100% at low volumes, and at some point the tubes pass the baton, allowing for operation of each tube more than 50% of the time, and less than 100% of the time. By switching out a tube and allowing it to cool, it is possible to have an output that exceeds the wattage rating of all the tubes added up, i.e.; two 30 Watt EL34's in a 75 Watt amp. Most guitar amps are class AB.

Class B: Also arranged in multiples of two and in push-pull, the duty cycle for each tube is just over 50%, and is never 100% if a signal is applied. These amps use very little current without a signal applied (idle current), and so the tubes may feel relatively cold if on and not being played.
These amps also employ extremely high (dangerous) voltages, like 700 Volts in a Musicman HD-120. Musicman are the only guitar amps that I'm aware of that use Class B operation. Class B amps tend to sound aggressive.

Since the Audio transformer design dictates Class of operation, how much voltage V.S. current at idle is also a function of design. Class A runs a lot of current at idle, Class B barely a trickle. Watts equal Amps times Volts, so depending on the design, you may be setting the bias either too hot or too cold when you set the bias to say, minus 52 Volts.

Tubes also vary, and picking an arbitrary bias voltage can only be useful if all your tubes run at the same potential. Its not the same thing but if it helps, think of it in terms of efficiency. Under any given set of conditions different tubes will perform more or less work. Tubes within a manufactured lot vary. From one model of tube to another of the same type, there is variation. There is an even greater variation from one manufacturer to the next.

To build a 6L6 you need to meet the specifications for tube type (beam power, pin basing), for the maximum Wattage rating (ability to deal with heat), maximum plate voltage, maximum control grid to cathode voltage, some inter-electrode capacitance stuff. The point being that there is a lot that's not said about the design. They vary by design and by the fact that they are manufactured (manufacturing tolerances).

I'll bet this is more concise and complete than anything you've seen. I hope you enjoyed it. Feel free to print it for you own personal use but do not duplicate it, as it is copyright protected material.

If you want more information than this, I'll provide it for a fee.


revised 2014

[12AX7A] [Donate!]

All Content Protected Under USA Copyright Law
Copyright 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014 Gregory Raynard

[Amp and Guitar Repair HOME]

Website developed and managed by 44kMiSec