12ax7 vs 12au7?

[harobikes34]

Hi, ive got an old Heathkit IT-12 i made into an amplifier for teh ipods which has a 12ax7 which works well with the external speaker i set it up with. I found a 12au7 in another heathkit unit and i swapped it out and i can hear that the gains less and i can hear more bass. Can anyone tell me the difference between the two and which is more commonly used for listening to music with?

Thanks

[Dominick Costanzo]

Both and many others!
If you like what the 12AU7 does in a circuit that was designed for a 12AX7,
you already have your answer!
It might not be accurate but you're listening

[fossiltooth]

That's a pretty accurate way to say it, I think. The au7 is lower gain than an ax7 for sure. The x's can sound comparatively more middly and aggressive. The amount of gain needed for the initial stage of something like a guitar preamp circuit is gonna be a whole lot higher than what you need for the already healthy signal coming out of an ipod, so I imagine you'd be in good shape swapping those out.

I remember reading something on Dan Torres site where he says a lot of these swap easily in guitar amps, at least. He had something noted like 12AY7 gain = 44, 12AT7 gain = 60, 12AX7 gain = 100. Not sure how the U fit in.

Anyway, someone else probably knows a lot more about this stuff than I do. I'd like to learn more if anyone wants to chime in.

Edit: Here's that link I mentioned: http://bit.ly/mElwov

[Scodiddly]

Basically there's a tradeoff in gain vs. output driving. So the 12AX7 is a high-gain input stage, but you'd see a 12AU7 (or a 12AT7) driving an output stage.

So it's possible that the 12AX7 was bogging down a bit driving the next thing in the circuit, losing some of the bass?

From highest gain to lowest, it's X - Y - T - U

And of course there are some tubes in that series where the two sections have different characteristics.

[analoghacker]

hey,
just reverse "Y" and "T"...

mu of a 12AY7 is lower than a 12AT7... but more importantly the transconductance is much higher on a "T" than on a "U", "X" or "Y". you get more of your mu with a high transconductance tube. however, the 12AT7 is not a particularly linear tube, it's transfer characteristic is pretty curved. it's great for compressing drivers for output stages of guitar amps.

jc

[I'm Painting Again]

and you probably already know but different brands and vintages of each type can sound very different..

[???????]

If you think questions like these will come up more in the future, you owe it to yourself to learn to find and read tube data sheets. Duncan Amps' webpage has a great tube data sheet locator with lots of relevant information.

[Studiodawg]

12AU7 has a rating of 19 vs. 12AX7 with a rating of 100...so this will make 12AU7 "cleaner" and "quieter"...

[emrr]

Ratings assume infinite load impedance, which never happens. Actual gain is dictated by the surrounding components, and will be much lower than the data sheets indicate. Comparison of gain between types is always much closer too. An AU in a typical AX slot will be starved for current, and may not have much less gain at all. It may not be operating at a desirable part of the curve, either. An AX in a typical AU slot will have much less gain due to loading effects. It's a bigger picture, and not so simple, as usual.

[jnTracks]

12AU7 has a rating of 19 vs. 12AX7 with a rating of 100...so this will make 12AU7 "cleaner" and "quieter"...

actually when you swap a 12AX7 for a 12AU7 in the input stage of a guitar amp (which is what we're talking about here) the amp get's more saturated, distorted, compressed, tube-y sooner. that's why people do this. so AU7 is more distorted at the same perceived volume as the AX7, but the volume knob will be up higher.

[Studiodawg]

Exactly...cleaner and quieter at the same volume knob position or more distorted at the same perceived volume.

[themagicmanmdt]

kinda.

not necessarily.

i mean, it *might* work that way.

they are pin compatible tubes that don't need too much changing around, and sometimes sound good plug and play especially with a certain range of values between them -

it's still about having the circuit tuned for each tube and what you want them to do. it's not even about 'optimising' the tube circuit, but rather what the tubes themselves sound like under different conditions, both happy and sad.

i've gotten lucky and done the 'tube swaps' and enjoyed 12AT7's in 12AX7 slots usually. i've never found a 12AU7 sound i've liked yet. it sounds better in the get-go, but it always changes out...

...and usually stock sounds better, unless the circuit is modified when the new tube goes in.

tube rolling is an experiment in luck