Difference between 12AU7 and 12AX7?

 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3748
Registered: Feb-07
As you guys know I'm pretty new to this tube thing. My amp has in the input stage a 12AU7 and a 12AX7 for each channel.

What's the difference between these tubes. From my reading it seems the AX7 has higher gain. What's the purpose of using and AU7 and AX7 for each channel? Are these tubes basically interchangeable?

Could I use 4 AX7s or 4 AU7s?
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14717
Registered: May-04
.

As far as pin outs are concerned the 12 series tubes are all interchangeable. The 12AX7 has the highest gain at 100, the 12AT7 is lightly lower and the 12AU7 is the lowest gain of the group. These are rough estimates as each tube will have its own electrical characteristics and just swapping one (new) 12AX7 for another (new) 12AX7 could result in slight higher or lower gain from the circuit.


Obviously, changing the tube type to a lower or higher gain will affect any circuit after that tube.


While it is genericaly fine to interchange the 12 series tubes, you should do so with some degree of caution. Inserting a higher gain tube in place of the lower gain tube could easily overdrive the circuits fed by that tube, that will result in distortion. Using a lower gain tube in place of a 12AX7 could (a slight possibility) deprive the following circuits of sufficient amperage which would flatten the sound.


Lowering the gain with a 12 AT7 or 12 AU7 is the most common procedure and how successful it is depends on the circuitry it feeds. There are other ways to go about reducing gain in most amplifiers.


There are also European numbers equivalent to all American tubes and military use numbers for tubes which would otherwise qualify as a generic 12AX7, AU7 or any other audio tube on the market.


There's no way to tell you why the designer is using a 12AX7 and a 12AU7 other than the two tubes probably serve different fiunctions in the amplifier. One might serve as the phase splitter and the other the pre amp or driver tube. These are "dual triodes" and there are two complete tube sections in each capsule, therefore, one tube can serve both left and right channels depending on the layout of the amp. This is a more common practice with more modern tubes since tubes have risen in cost so dramatically over the last few decades. Vintage amps from the Golden Age of audio will probably have 1/2 of the tube being used to feed one channel since these amps were designed when good tubes cost a few dollars at most.

My advice tends to stay with convention and suggests the designer had various tubes available to them. I would expect there to be a fairly good reason why the designer chose specific tubes for specific applications. To mess with that is in one sense trying to redesign the amplifier.



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Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3749
Registered: Feb-07
Makes sense. Thanks.
 

Gold Member
Username: Soundgame

Toronto, Ontario Canada

Post Number: 1123
Registered: Jun-08
Tube rolling, not for the faint of heart. Not saying you're faint of heart Dave but it does seem common sense that the designer had something in mind, otherwise there would have been additional simplicity and potentially lower cost with going with a consistent tube througout. Inteteresting just the same.
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3750
Registered: Feb-07
Nuck is trying to talk me down from premature tube rolling George. In case you haven't noticed, I have a way of jumping into things with both feet. Nuck has a point, though.... break these in first and get a feel for them instead of willy-nilly swapping in new tubes.
 

Platinum Member
Username: Nuck

Post Number: 14798
Registered: Dec-04
Yup.
A tube, particularly output tubes, change their sound as time goes on.
Hopefully the tubes are matched and the amp is equally biased so they all do the same thing, but this is where studious and careful production comes into play.
 

Silver Member
Username: Hawkbilly

Nova Scotia Canada

Post Number: 886
Registered: Jul-07
What brand of tubes are in it now David ?
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3752
Registered: Feb-07
The 6550 output tubes and the 12AX7 input tubes are all Tung-Sol. The other input tubes (12AT7, not 12AU7) I'm not sure of the make.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14719
Registered: May-04
.

Consider why a manufacturer might choose a specific tube. Beyond the decision to use a 12AX7 here and 6DJ8 there, which are functions of the circuit itself, a brand and a specific version of tube (high/low in the manufacturer's line up) will often be determined by a few not so sonically relevant features.


The first consideration is almost always availability. This has improved over the last 15 years or so as tube manufacturing has remained steady for the demand. None the less, a component manufacturer needs to know their choice of tube will be available for as long as they are building that component and then available in sufficient quantity to allow for spares, say, ten years out as units come in for repair or maintenance servicing. The general attitude of most component manufacturers is to produce a consistent product over time and, should the unit need repair or maintenance, to have the ability to hand the client back what they purchased.

Believe it or not, not all tube component owners engage in tube rolling. And customers get very upset if what they hear coming out of the repair shop is - to their ear - significantly "different" than what they feel they had at the time of purchase. A manufacturer must have the ability to rebuild exactly the same component ten years after the last one rolled off the assembly line. To do that they will need those same tubes that went in on the production line.


Next the component manufacturer needs to have a reliable tube that won't go up in smoke during the new owner's intial listening session and hopefully not at all. Tubes that have proven their reliability have a leg up when it comes to use in new equipment. Rugged tubes and tubes that produce excellent sound do a very delicate dance with one another. The musical instrument side of tubes is still what drives tube manufacturing far more than does high end audio. What a working musician wants from their tubes is typically a far cry from what the owner of a 150 watt VTL wants.


Then, as a consideration, you won't find anyone on the low end - and only one I know of at the very top end - supplying NOS tubes as OEM product. The component manufacturer is there to produce the most consistent product they can and NOS tubes not only increase the price significantly they also are not a reliable source should the model catch on and production numbers outpace expectations.


Finally, the sonic values of a tube are considered and its cost, availability and reliability weighed against what someone hears. If the component contains circuits which require very low noise tubes, they will be chosen as such. Not all circuits require low noise tubes and it is often a waste of money to buy tubes sold as "low noise" for most modern line stages. Save your money for low noise phono stage tubes and don't worry about the rest. With the high-ish input voltages of modern digital sources there just isn't a good justification for the aditional cost in a line stage other than the buyer's mental satisfaction.


In the competitve tube market of today review magazines no longer tend to do tube rolling as a general basis for review. Years ago all reviewers had a stash of NOS tubes made from "unobtainium". Expensive tubes in super short supply that were passsed on to the reviewer by some manufacturer wanting a better than average review found their way into any component under review and the "final" assessment was made based on the sound of the component outfitted with these "super tubes". That day has, for the most part, passed and manufacturers need to get their reviews with their stock OEM product. So, while sonic value is lower on the list of considerations than several other qualities, sound is still regarded as being very high on the manufacturer's list of desires when selecting their circuits based around a 12AX7, a 6DJ8 or any other tube used in today's audio components.


I disagree with Nuck about the sound of a tube changing over the first initial days or weeks of listening. I'm somewhat in the minority in that regard. Capacitors and transformers within the circuit obviously settle in and the sound of the component is different at the end of a month of constant use than when it was fresh from the box. IMO tubes are tubes and there is no "burn in" period, they either work or they do not work. You can make up your mind just what is burning in in your amplifier.


I do agree with Nuck that you should not rush into tube rolling unless the tubes in the amplifier are worn out or defective. Listen to the music your component reproduces and get used to the qualities which are important to you just as you should any other component or speaker. You should have your priorities and allow them to settle in to what you are hearing. Most tubed components require more set up care and respond well to such attention to detail when compared to solid state equivalents. Therefore, it makes more sense to invest in excellent isolation feet or a better stand than in expensive NOS tubes that will not preform at their best in a losuy set up. Most tube gear will have higher impedances than many solid state components. You should understand what values are relevant and how to deal with those values should they be present in your component. Specific cables that worked well with a Rotel might not be your best choice with a Conrad Johnson.


Tube rolling, is well worth your time if you can determine which tube will best suit your system. Buying new (or NOS) tubes without sufficient research can prove costly. Just because you've read Svetlana or GE 12AX7's are wonderful tubes is not enough reason to rush into buying those tubes. For a highly ubiquitous tube such as a 12AX7 or 6550 remember these tubes have been in production for decades and buying a tube by brand is no indication you will be buying the tube you read about. This is especially true in NOS tubes where forgeries and fudging information are quite common. I have several sets of Amperex 6DJ8's that all sound different from each other. I gave MW a baggie full of various 12AX7's to experiment with.


Take your time and do your research. Don't buy based on what someone else likes any more than you would any other component. Once you purchase tubes they are typically yours to keep, no exchanges other than for defective tubes. There are, as I mentioned, more than few substitutions for a 12AX7 which make the selection process even more difficult. Determine what the tube is doing in the circuit, a phase splitter's effective will not be as profound in determining the sound you arrive at as would a driver tube.


As always, my advice is to know what you are after in terms of priorities and find the component or the tube that satisfies those priorities.


Spend time reading "Tube Talk".



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Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3753
Registered: Feb-07
Interesting post Jan. I've started going through the "Tube Talk" thread... thanks.

I find my system sounds better after a period of listening than when I first turn it on. I'm not sure if this is a function of the tubes warming up, the circuits and caps warming up, or a combination thereof. But it's noticeable.

I've learned the value of having a set of spare tubes handy. One of my input tubes went up in smoke this week. The dealer sent me a free set of input tubes since it was still under warranty.
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14720
Registered: May-04
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Any tube product and certainly any tube product with large output transformers will sound better after it has warmed up for about 30 minutes. However, should you be considering it, the value of leaving the amplifier powered up constantly must be weighed against extremely shortened tube life and, should anything go wrong while the amp is unattended, say, a tube spits loudly as it burns off some excess gas to the more drastic possibility the tube ceases operation, you are risking potential damage to your amp and speakers.


Having spare tubes will be the simplest way to perform basic troubleshooting on a tube based component. If you have a complete set of input tubes now, I would recommend you also obtain a set of outputs - at least a single pair. They need not be anything special - I have some old "International" tubes that were white box specials culled from various manufacturers and not at all matched in any way. They are only serviceable IMO as troubleshooting tubes when I'm trying to find out if a tube is at the heart of a problem and I don't care to risk a more expensive tube to find out. Of course, you want to make sure any tube you buy operates properly before you put it away for troubleshooting purposes otherwise, you might be looking at a failed tube when the problem exists elsewhere.

Tubes can go out for no reason other than it was a bad tube. However, if a tube has survived in a circuit for several months to several years and then fails, there's a good chance something else tied to that tube, a bias resistor, coupling capacitor, etc, has failed and this caused the tube to follow suit.


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Silver Member
Username: Ezntn

Greeneville, TN

Post Number: 175
Registered: Apr-09
Any tube product and certainly any tube product with large output transformers will sound better after it has warmed up for about 30 minutes. However, should you be considering it, the value of leaving the amplifier powered up constantly must be weighed against extremely shortened tube life and, should anything go wrong ........

A point I have been pondering myself Jan.
While Cary says warmup time on the SLI 80 is about 3 minutes .. I do find the music has a bit more substance after about 15 or so.
I would think that tubes undergo different stresses and strains during a power up / power down sequence compared to ss components.
Would there be benefit in leaving a tube amp / integrated on versus cycling on and off over finite period of time. When I am home, and in the office, the system is playing. Is there benefit or not to turning the system off for say an hour or so?
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3754
Registered: Feb-07
Does the on/off cycle affect the tubes at all?
 

Gold Member
Username: Mike3

Wylie, Tx USA

Post Number: 2362
Registered: May-06
I leave all my tubes, pre - amp and amp, on 24 X 7.

More things have gone south on me during turn on than from being left on.
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3755
Registered: Feb-07
Hmmm. That's an interesting point Mike. My input tube shuffled off this mortal coil as soon I switched the amp on. zzzzzzzzzz POP!
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 14723
Registered: May-04
.

Thirty minutes should in most cases be sufficient time to get the equipment to near maximum fidelity. The quality might increase slightly after that period but not as significantly as in those first few minutes.

My tube pre amp is at max fidelity in about ten to fifteen minutes and no changes can be perceived from that point forward. My Mac tube amps have lots of heavy iron and copper in their transformers and they will still be improving at a noticeable rate for the first hour or two. The Mac's run very cool at idle and while I add a cooling fan to their non-use cycle they will still have very minor improvements in quality for the first few days after they have been left to completely cool down. Conversely, with all that iron and copper packed into a heavily potted transformer they take a long time to completely cool down. If I were to use then on a daily schedule I could easily allow them to sit powered down for the day and they would be close to their best performance within about 45 minutes. My "normal" operation for the power amps is to allow them to sit powered up virtually 24/7, at idle with a cooling fan running and then switch off the fans about 1/2 hour before I listen to music. They are plugged into an AC conditioner which will shut down should current draw from a defective component dramatically increase. If something cataclysmic should happen when the amps are unatteneded, there's a very good chance they (and the rest of the house) will be protected from further harm.


As I warned David, you need to be very certain of your amp's stability if you intend to leave it unattended for long periods of time. That really is the case for tubes and transistors alike as a defective cap or tube can cause a very loud noise which could easily destroy a driver. You can't predict when a tube or other component might become noisey (I had a filter cap fail while MW and I were listening to music the other day, rather rare but lots of things can happen with audio gear) and it could easily happen when you are not in the room with the amp.


Tube power amps should not run powered up without the load of a loudspeaker on their outputs. If you have your system in an area of the house where a child, pet or unknowing house cleaner could accidentally disconnect a speaker lead, you could end up with a very dead amplifier and a very large repair bill. When I still had a cat he jumped onto the chromed top plate of one amplifier and took out a tube at a moment when I didn't have the tube cage in place.


(That was not what killed the cat though I was tempted ... )


A vacuum tube has a finite life span and anyone using tubes must decide how to expend that life. The two most important features to come along relating to tube life have been the introduction of a soft start cycle and a standby function. Plug in a Dyna ST70 (or almost any vintage tube amp) from the 1950's through '70's and the tubes immediately glow brightly as if they were a light bulb on a wall switch. They are being hit with a solid 120VAC and this will shorten tube life by a considerable margin.

We always use to tell those service customers who said their gear worked fine until they powered it up one night that they probably blew out lights when they flicked the switch rather than while they were reading the newspaper. As with a lamp or most other electronic components it is the continuous heating and cooling, heating and cooling, heating and cooling cycle which eventually weaken the internals of any component.


My 1960's Mac amps have what are called "thermistors" in the incoming AC line which maintain a high resistive value when they are "cold" until the voltage slowly begins to warm the component and as it warms the thermistor's resistance drops steadily and slowly until it is virtually non-existent in the line. My tubes remain "off" for the first few seconds and then rise slowly in intensity for the next ten seconds until they reach the point where they will pass a signal. Soft start circuits have become far more complicated and more sophisticated over the last decades but the overall effect is to extend tube life by not allowing 120VAC to slam into a cold tube.


That brings us to the standby circuits which have become common over the last few decades. My 1980's AI pre amp has a standby circuit which keeps the tubes at a trickle voltage and therefore never completely cooled down unless I pull the plug from the wall. The standby circuit also means the tubes reach their maximum potential faster than a tube which must rise from the ashes each time you throw the switch. Standby circuits still eat into the tubes' lifespan though. Minimally and with the advantage of better sound sooner but they are decreasing tube life while they sit at idle. With two sets of (what now sell for) $250+ @ pair tubes in the pre amp, this doesn't always make me sleep any better when I haven't used the system for awhile.


Small signal tubes (12AX7's, 6DJ8's, etc.) have extended lifespan when compared to power output tubes. You could use a tube pre amp for decades and possibly still not have to replace tubes despite the fact virtually any modern pre amp runs in class A operation. Output tubes are likely to be in need of replacement after a few years - at best! - of normal, regular operation. If your amplifier runs hot, it is again the hot/cold cycle which wears tubes out and sets them up for failure. If your amp runs cool - like the vintage Macs or several other modern equivalents - you might get a lifespan of four or five years from a set of power tubes. Running a power amplifier hard will increase the heat and therefore shorten the tube's life by a small amount just as it will shorten the lifespan of any resistor, capacitor or other semi-conductor in the amplifier. Amplifiers must reach a certain operating voltage which determines a certain operating temperature and, therefore, running a tube amp at low volume still eats away at tube life and does not increase the life of the tube just because you aren't pounding out the watts.


You get to decide how to use the lifespan of a tube. The only way to not decrease the useful life of a tube is to not use it at all even in a standby situation.


Keep in mind in any class AB amplifier the tube is constantly being turned "on" and "off" in respect to pasing a signal by the nature of the class of operation. One of the supposed sonic differences between tubes and solid state is the fact transistors totally switch off and on constantly during their duty cycle while tubes take a moment to power down with the signal gradually fading away rather than being completely switched off. Therefore, while the class of operation dictates a constant on/off cycle of signal flow, tubes don't follow the same rules as will transistors.


Be aware, however, that tubes begin to deteriorate from the first moment they placed in an operational circuit. Tubes reduce output voltage while noise and distortion increases over time. While your tubes might live to a ripe old age and still make musical sounds they are simply not the same tube you started with after even a few months of operation.



Biasing arrangements in power amplifiers also play a role in how fast your tubes wear out. Fixed or auto-bias amplifiers tend to extend life due to their unique arrangements of monitoring tube aging. User adjustable biasing tends to shorten life while providing some degree of flexibility to the user. Arguments have gone on for decades over the sonic advantages of various bias configurations. If you'd like to research these opinions, try here; http://www.worldtubeaudio.com/


As the cost of tubes has risen steadily over the last few decades and more and more people become convinced they must have NOS tubes for their gear, attempts to extend tube life have become fairly common. The two things you must be aware of are; 1) tubes have a finite lifespan, and, 2) tube quality begins to shorten from the moment you first use a tube and continues every moment there after.



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Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3758
Registered: Feb-07
This in interesting (for me anyway).

http://cydathria.com/fdm/12AX7_sub.html
 

Gold Member
Username: Soundgame

Toronto, Ontario Canada

Post Number: 1124
Registered: Jun-08
Sounds like yer havin fun dare Dave.
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3763
Registered: Feb-07
Learning lots. When I'm learning, I'm having fun.

Spent an hour or so listening to music today. Really enjoying the gear!
 

Gold Member
Username: Dmitchell

Ottawa, Ontario Canada

Post Number: 3788
Registered: Feb-07
I found this link interesting. It shows that one of the main differences between input tubes (12A*) is the amount of gain they have.

http://thetubestore.com/gainfactor.html

I'm sure there's other diffs, but this made things clearer for me.
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