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What are the differences between 12AU7 and 12AX7 ?

fiddlefye

fiddlefye

AK Subscriber
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I'm looking to roll some tube sets through my Opera Consonance PM1 phono pre and looking at various tubes to try, some I have on hand and others I might order. First off though:

What is the difference between the 12AU7 and 12AX7 types (yes, they have lots of other designations)? Is one higher gain, one lower noise? I don't know which is which and just how interchangeable they might be. Help!
 
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From Head Fi...


"I wont dispute what the amps designer claims for his amps, but these tubes are not substitutes for one another. The plate dissipation for these two tubes is noticeably different. A 12ax7 will only dissipate 1 watt max vs 2.75 watts max for the 12au7. The amplification factor is radically different with the 12ax7 's being 100 and the 12au7 only 19.5. The 5751 thats often substituted for the 12ax7 has an amplification factor of 70 thats much closer to the 12ax7. But even this tube will not work in all 12ax7 applications. I would be very careful substituting these tubes for one another, in fact, I would not do this at all myself."

I don't do it either.
 
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The 12AX7 has considerably more gain than the 12AU7. In a pinch one can substitute for the other, but they're really intended for different purposes (leaving one to wonder why so many amp designers used 12AX7s for applications that didn't require their high gain).
 
amplification factor, transconductance, plate resistance, all different. They do have the same basing, and are of approximately the same design internally so you can plug one in for the other, but it may not work so well in circuit. Its not likely to make the smoke come out of anything, but thats about all thats certain.
 
gadget73 said:
amplification factor, transconductance, plate resistance, all different. They do have the same basing, and are of approximately the same design internally so you can plug one in for the other, but it may not work so well in circuit. Its not likely to make the smoke come out of anything, but thats about all thats certain.
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Well spoken and to the point. I don't get into "tube rolling". The engineers were way smarter than me. Want to know what really makes an amplifier sound super? Bring it back up to specs. That includes power supply, coupling, bypass etc.

What will prove the point is to do the double blind taste test. Get some one to swap things around and not tell you what they swapped. Listen to it. Listen to it again. Tell that person that it is OK to say that they swapped something but really didn't when they ask you, "Better One; or Better Two". And that involves many, many swaps, not just one or two. Same source, same speakers etc. Try it!
 
Changing tube types (12AU7 for 12AX7, for example) will likely make a radical change in the way a piece of equipment sounds, and maybe for the better but more likely not. Some types are similar enough that they will still make a big change and be more likely to sound good (5751 for 12AX7, for example). There are lots of sonic flavors to explore with different brands and vintages of the specified number, however, and you can do a lot of tube rolling to find what sounds best to you even by staying within the specified number. For 12AX7s, you'll find that Mullards and Telefunkens sound very different from each other, for example. There are numerous brands and even the same brands changed internal construction over the years so there are probably at least 50 different "flavors" of 12AX7 you can experiment with.

A blind test would be interesting, but I have no doubt that the differences between 12AX7s from different manufacturers is often not subtle.
 
Putting 12AY7 in the place of 12AX7 would probably not be too radical a change and may improve things like noise and microphonics.

mike
 
Andyman said:
From Head Fi...


"I wont dispute what the amps designer claims for his amps, but these tubes are not substitutes for one another. The plate dissipation for these two tubes is noticeably different. A 12ax7 will only dissipate 1 watt max vs 2.75 watts max for the 12au7. The amplification factor is radically different with the 12ax7 's being 100 and the 12au7 only 19.5. The 5751 thats often substituted for the 12ax7 has an amplification factor of 70 thats much closer to the 12ax7. But even this tube will not work in all 12ax7 applications. I would be very careful substituting these tubes for one another, in fact, I would not do this at all myself."

I don't do it either.
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That's what I was wondering first off, which has the higher level of gain. Opera specs the PM1 for 12AX7's, but I actually end up with so much gain (in conjunction with the PV-12) that I can run LOMC carts without the transformer plenty loud if I want (ok, sound a bit funky in other ways). Losing a bit of gain wouldn't lose me any sleep.

The PM1 came to me with Jan Philips 12AX7's. IMO they may be "NOS", but apart from that any of the tubes from them I've had were junk. My CJ PV-12 came with 2 sets of them in 12AU7 when I got it and they were both painfully bright and had weak mid-range (it now has a nice set of RCA clear tops). The pair in the PM1 has the same characteristics, leading me to suspect that they shouldn't be seriously considered for audio applications.

So, trying to find a couple of pairs of 12AX7's that won't break the bank and still sound great. Has anyone tried the New Sensor Mullard or Tung-Sol versions? I have a set of their EL34's and they're excellent. NOS prices on these things have just gone insane!
 
For less gain, you might try 5751s. I've heard good things about some of them, but the RCA Command Series I've tried have had lots of hum. That's in an integrated amp with AC filaments, if you have DC filaments (probably, in a CJ) they might be fine.

For a 12AX7, you should consider the TJ Full Music by Grant Fidelity. They sounded head and shoulders above anything I've ever tried in my guitar gear (and I've tried almost everything!) I don't know firsthand about their sound in a hi-fi rig, but reports are very positive. There's a pass-around on them going. Maybe try to get on the list. I bogarted the tubes for about a month. I had a real hard time parting with them.
 
JonL said:
For less gain, you might try 5751s. I've heard good things about some of them, but the RCA Command Series I've tried have had lots of hum. That's in an integrated amp with AC filaments, if you have DC filaments (probably, in a CJ) they might be fine.

For a 12AX7, you should consider the TJ Full Music by Grant Fidelity. They sounded head and shoulders above anything I've ever tried in my guitar gear (and I've tried almost everything!) I don't know firsthand about their sound in a hi-fi rig, but reports are very positive. There's a pass-around on them going. Maybe try to get on the list. I bogarted the tubes for about a month. I had a real hard time parting with them.
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That sounds like a good thing. Wouldn't it be great to be able to have something set up where you could try examples of different current tube types and then decided what you like without buying examples of all of them? Maybe we could find some way to arrange something like that on AK. There's a violin shop I deal with which has a room where you can go and try different types of strings on your instrument before you pick out what you'd like to buy. It certainly saves a lot of expense and time.

I've heard of Grant Fidelity. Who makes the tubes for them?
 
fiddlefye said:
I've heard of Grant Fidelity. Who makes the tubes for them?
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I don't recall who makes the tubes. There's info on their site. Grant Fidelity is an AK sponsor (see the banner ads below the thread) and they have their own sub-forum here under Manufacturer's forums. Check around in the threads, you'll find the one about the pass-around.
 
JonL said:
I don't recall who makes the tubes. There's info on their site. Grant Fidelity is an AK sponsor (see the banner ads below the thread) and they have their own sub-forum here under Manufacturer's forums. Check around in the threads, you'll find the one about the pass-around.
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I went to Grant's own website and it turns out the TJ Music is a Chinese manufacturer, but (it seems) only produces their own tubes. In other words they aren't made for them by another company. They look like pretty impressive tubes, not inexpensive compared to some other new tubes though, that's for sure. I'd have to be pretty sure of them to spend that on a tube at the moment. I'll have a look in the forum as well. Thanks!
 
fiddlefye said:
That's what I was wondering first off, which has the higher level of gain. Opera specs the PM1 for 12AX7's, but I actually end up with so much gain (in conjunction with the PV-12) that I can run LOMC carts without the transformer plenty loud if I want (ok, sound a bit funky in other ways). Losing a bit of gain wouldn't lose me any sleep.
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Gain for the PM1 is listed at 43db, which is pretty normal for a 2 x 12AX7 phono stage. Changing to lower gain tubes in a phono stage designed for a specific tube is backwards. You should be looking elsewhere to lower gain, like the preamp.

Jeff
 
vinylkid58 said:
Gain for the PM1 is listed at 43db, which is pretty normal for a 2 x 12AX7 phono stage. Changing to lower gain tubes in a phono stage designed for a specific tube is backwards. You should be looking elsewhere to lower gain, like the preamp.

Jeff
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The gain as it stands isn't a problem, I was just saying that if I lost some it would be an issue. Before I got the PM1 I was using the phono stage in my Yamaha C-4 for a time. The line level output from that is much, much lower than what I get from the PM1. Not that everything relates directly, but it took 12 o'clock on the volume on the PV-12 with the C-4 to get what 8 o'clock with the PM1 gets. It's actually a somewhat hotter output than my CDP.

My main curiosity was whether I can experiment in the PM1 with some of the 12AU7s I have a number of pairs of. Consensus seems to be it isn't a good idea so I'm now hunting for a reasonably-priced replacement for the JAN Philips 12AX7s that came with the pre. This is my second encounter with tubes of that make and I can only conclude that they aren't really suited to audio use as in both applications they produced an inappropriate tonal balance. They might be useful in a system with vast amounts of muddy mid-range, but apart from that....
 
Doing this sub may do something interesting in a guitar amp, but in a hi-fi set, it definitely will work quite worse. Even replacing 12AX7s with 5751s would make the RIAA curve come out wrong in my SCA35, according to a simulation I ran on its phono preamp circuit.

Generally, 12AX7s are used for small signal high gain circuits, and 12AU7s used for lower gain but higher signal level work, and things like cathode followers.
 
wa2ise said:
Doing this sub may do something interesting in a guitar amp, but in a hi-fi set, it definitely will work quite worse. Even replacing 12AX7s with 5751s would make the RIAA curve come out wrong in my SCA35, according to a simulation I ran on its phono preamp circuit.

Generally, 12AX7s are used for small signal high gain circuits, and 12AU7s used for lower gain but higher signal level work, and things like cathode followers.
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I can imagine this is correct as the RIAA curve can be a pretty delicate thing to get right. Part of the issue I have with the set of JAN Philips 12AX7s that came with the pre is that they seem by design to run far enough off the standard for the tube design that they affect that aspect somewhat. This isn't by vast amounts, just enough that with LPs which were marginal in that respect with other phono stages I've had sound "wrong" with this set. I'll find a solution, but any affordable suggestions that will head in the right direction will be appreciated.
 
I know the thread is closing in on 13 years, but I decided to resurrect because my issue is that I have a lot of tubes(Telefunken, mostly) that look to me to be AX7's, compared to others that are marked, but I'm not certain. Some marked AU7's look very much the same. So I've been swapping some of these into my MC30's in the AX7 positions and so far no problems, or major differences that I can discern. I am not putting any unmarked tubes in the AU7 positions, only in the AX7's because my understanding is that if a swap is going to be made better an AU7 for AX7 rather then opposite.

I don't have a tube tester or any other way to make a definitive ID so unless somebody can tell me how to tell the difference between these two similar looking tubes I guess I'll continue to experiment.
 
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