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12AX7 vs 12AT7 vs 12AU7

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ericj

Member
2008-12-08 10:24 pm
12ax7 is a very popular tube for a gain stage, with it's amplification factor of 100. Low current output ability though. Guitarists love what it does when it goes into overload but that doesn't mean it's not usable in audiophile as well.

12at7 has a bit less gain.

12au7 has even less but can drive more current than a 12ax7. There are even crazy people who have built small guitar amps with a 12ax7 doing gain and tone and a pair of 12au7 in parallel push pull for output, netting just a couple watts -- and a couple watts at full clipping is plenty loud into a really efficient speaker.
 

ashok

Member
2002-06-06 4:43 am
3RS
I remember reading that 12AU7 was supposed to be not as linear, as say 6SN7. I have tried the 12Au7 and 12AT7 in the first stage of a gain stage with CF output. The 12Au7 sounds quite nice. Some NOS tubes sounded brighter ( clearer ?) ( Mullard) than the current production ( EH ). The 12AT7 had higher gain but I thought it didn't sound as nice. What that means is very subjective. I didn't spend much time on it ! For a line amp with no feedback the 12AT7 has too much gain. Obviously couldn't put a 12AX7 there with a gain of 100 !
JJ 12AU7 was nice too.
 

wdecho

Member
2014-10-28 11:45 am
You did not say if this pre was for instruments or audio which will make a lot of difference in what you are after. I built this little preamplifier,

4S Universal Preamplifier for 12A*7 Tubes

where you can roll different tubes. I have not tried the 12AT7 or the 12BH7 but the ones I did try, the one I like best was the RCA clear top 12AU7. After settling on the 12AU7 I did change the circuit somewhat to optimize it specifically for the 12AU7.
 

azazello

Member
2008-08-23 7:28 pm
USA
To the OP: tubes don't have a sound. Circuits have a sound (or don't, if you've designed them to be neutral).
Very interesting!....I didn't know 2A3 RCA and 6P3S Russia have the same sound or haven't a sound...? /maybe sound doesn't depend from inner tube structure....but why one kind tubes,penthode for ex., /or triode/, has different structure?
To make neutral sound for some tubes, You will take different ideas for different tubes, that say that tubes sound different....Never 6P3S will sound like 2A3RCA../Sorry for off.../
 

20to20

Member
2010-06-23 9:25 pm
Very interesting!....I didn't know 2A3 RCA and 6P3S Russia have the same sound or haven't a sound...? /maybe sound doesn't depend from inner tube structure....but why one kind tubes,penthode for ex., /or triode/, has different structure?
To make neutral sound for some tubes, You will take different ideas for different tubes, that say that tubes sound different....Never 6P3S will sound like 2A3RCA../Sorry for off.../

This question of tube sound would stop being asked if those asking realized anything about the internal resistance and capacitance differences of different tubes and how the support circuitry for one tube is not optimal for a different tube. Of course you can expect the sound to be different, although you may not always hear anything has changed if you just plug in a different type. But if it does sound different, what does that mean? Louder? A change in feedback ratio and gain? A change if FR from a different FB ratio? Change in current, voltages and biasing due to all the differences between types? No way to predict if you will hear any difference because it's the amp AND tube that has to work as designed.
 
Last year a friend built a Glassware Akido Tube Buffer to use as a preamp with my VTA ST-120. The standard build specified using 12AU7 tubes. However my friend also gave me a pair of 12AX7 tubes to try with this Tube Buffer.

With the 12AU7's the preamp sounded very good however with the 12AX7's it was as if the music was right out in front. The sound was much more open. I'd say the dynamic range opened up. The tube buffer and amp were used to drive JBL L100T3's. The 12AX7 tubes glowed much brighter and I was told they probably would not last as long....but the difference in sound was incredible.

i think that's the answer you were looking for.
 
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ashok

Member
2002-06-06 4:43 am
3RS
I was sure that I didn't like what I heard due to the excessive gain it had for my application and it had to be padded down etc.
I just rigged it up with a gain stage and CF stage and negative feedback to the Grid of the first stage. It however reduces the input impedance. 12AT7 input and 6922 CF stage. I used a film cap at the output. Earlier it had an electrolytic there.

It sounds nice now. Gain is about 14 dB . Will work on it some more. It has a bit of hum . I'm currently running ac on the heater. Will change that to DC . I hear hiss at the tweeter. Not too bad but I'd like to get that down too.

I used a NOS Mullard 12AU7 earlier. It was good and was audibly slightly brighter (?) than a EH 12AU7. The JJ 12AU7 was more like the NOS Mullard 12AU7. The difference is clearly audible on certain tracks.

I think I'm OT again !
In the circuit cl1238 has given in post 1 there is negative feedback and gain in both stages. So any tube that you plug in , the circuit will still have the same overall gain. But the operating points will vary if the resistors are not changed. That should certainly alter the sound as it will not be optimum for all the replaced tubes. It's just possible that using the 12AX7 at the input will also cause more HF roll off due to it's miller capacitance. Can't just plug in different tubes in without any changes in resistor values. With ideal tubes there should be no difference but these are not ideal tubes . Feedback will cut down some differences and maybe they might sound quite similar after all.
 
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12ax7 is a very popular tube for a gain stage, with it's amplification factor of 100.
in real life it´s about 40ish.



In my amp have ecc83 modded for lowest distortion,sounds fine.
By mistake....i inserted ecc81.
Of course working point is not optimal, but sounds more detail, also bass bottom improved

ecc83 should be driven by supply >600v and >100k Rp.
 
cl1238 said:
I think people did not understand my question.
I suspect you did not understand your question.

What if two 12AX7 or 12AT7 or 12AU7 were used instead - would it make the sound better or louder, more/less bass, high pitch etc...
If a circuit is properly designed for 12AU7 then swapping to 12AT7 or 12AX7 is likely to make the sound much louder and significantly more distorted. It should not affect the bass (although the extra distortion may give the illusion of more bass). What happens to treble will depend partly on Miller capacitance and grid stopper value (if present).