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Old 21st February 2016, 07:53 AM   #1
cl1238 is offline cl1238  Australia
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Default 12AX7 vs 12AT7 vs 12AU7

Hi, comparison of the 12A?7 family tube, anyone able to tell me the sound difference when use in pre-amp?
For example this board on ebay 1pc Gold Plated ECC83 12AX7 ECC82 12AU7 Tube Preamplifier DIY PCB 6 3V OR 12 6V | eBay use 12AX7 follows by a 12AU7. Is there any difference when use any combination of 12A?7 tubes? eg: 12AX7+12AX7 or 12AT7+12AT7.
I'm thinking of adding this pre amp to front of the tone control circuit by Practical Tone Controls.
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Old 21st February 2016, 08:17 AM   #2
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Look at the datasheets, or do a search.
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Old 21st February 2016, 08:38 AM   #3
ericj is offline ericj  United States
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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.
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Old 21st February 2016, 09:27 AM   #4
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Pin compatibility doesn't mean circuit compatibility!
They are very different animals in need for very different operating conditions to sound well.
Besides: an amp that 'has a sound' isn't neutral therefore not hi-fi...
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Old 21st February 2016, 09:36 AM   #5
cl1238 is offline cl1238  Australia
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One thing I forgot to state is that I am not asking for specification comparison of gain, pin etc...
How does the "SOUND" compare vs each other 12AX7 vs 12AT7 vs 12AU7.
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Old 21st February 2016, 10:48 AM   #6
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Don't you mean, How long is a piece of string?
If you use a 12ax7 and load the anodes with 47k from a 250V supply, it will struggle. Do the same to a 12au7 and it won't mind. It is all down to the biasing and that determines the 'sound'.
A 12au7 makes an excellent driver or phase splitter.
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Old 21st February 2016, 10:49 AM   #7
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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.
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Old 21st February 2016, 12:39 PM   #8
SY is offline SY  United States
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To the OP: tubes don't have a sound. Circuits have a sound (or don't, if you've designed them to be neutral).
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Old 21st February 2016, 12:46 PM   #9
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I think SY has nailed it!
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Old 21st February 2016, 12:47 PM   #10
wdecho is online now wdecho  United States
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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.
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