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Thread: 12AX7 vs 12AT7 vs 12AU7

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    Default 12AX7 vs 12AT7 vs 12AU7

    What's the difference between the 12AX7, 12AT7, and 12AU7 preamp tubes?
    Can they be used interchangably?

    Thanks

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    Moe's Bluesman Curly's Avatar
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    Default Re: 12AX7 vs 12AT7 vs 12AU7

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Has anybody tried substituting 12AT7's or 12AU7's in a modern preamp?
    How did it sound?

    Thanks

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    honestly, about the only preamp substitution I liked was putting a 12AY7 in V1 of the Bassman RI.

    you have to be aware of the current draw, as well as the gain factor, but I'm no expert on that
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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    would it be safe to put a lower gain tube in V1 in a fender blonde
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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Dumb question time - does "lower gain" mean that the tube would allow more clean headroom in the preamp stage? Or does it mean that the amp will start breaking up earlier?

    Ice Man - I know I tried a 12AT7 in one of the slots of my Blues Jr. but can't remember what it did...

    Actually here's the answer to my question and a lot of other stuff too: Bill Machrone on pre-amp tubes.

    Hope this helps.

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    There are many differences, but the ones that make the biggest difference are the Gain Factor and the Plate Characteristics (including the Grid Voltage Curves).

    The Gain Factors for each are as follows...

    12AX7 = 100
    12AT7 = 60
    12AY7 = 40
    12AU7 = 17

    Keep in mind that this doesn't mean you can put a 12AY7 (40) into a 12AX7 (100) socket just because it has a lower gain. First thing to consider is that the bias circuits for a given socket are designed such that the spec'd tube will operate in the center of the load line - which is based on a given socket's supply voltage and the Plate Load Resistor. If a tube other than spec'd is used, then either the top half of the signal will top out early or the bottom half of the signal will bottom out early. Ya want them to "clip" at the same time to max the level of that stage. The second reason is gain structure. If one stage drives the next with too low of a signal, then some of the dynamic range is being thrown away. In other words, you'll have to turn up the amp to get the same volume and you'll be turning up the noise floor with it.

    In summary, stick with the spec'd tubes for a given amp, or have a qualified designer (not a tech) mod it to use a different tube - even then it's a slippery slope. IMO, the older stuff (Fender, Marshall, etc) stuff is so well designed you'd be hard pressed to find a tube tech, or weekend warrior designer that can actually improve it.

    I hope this helps.

    lovechild

    www.lovechildamps.com

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Why was this zombie thread ever in TGS?

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    It shows up on Google Search, so I figured I'd answer it. Keep calm and carry on :-)

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Moving to Amp Central... finally.

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Quote Originally Posted by Curly View Post
    honestly, about the only preamp substitution I liked was putting a 12AY7 in V1 of the Bassman RI.

    you have to be aware of the current draw, as well as the gain factor, but I'm no expert on that
    That's because that's what they originally came equipped with.



    While they are interchangeable and will function without damaging anything, you may not always like the results, and things may act funny.

    For instance, did you know you could design an amp to use a 12AU7 and it will have an actual usable output?

    EDIT Wow, just realized the date of the OP. LOL
    Nope...

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Quote Originally Posted by lovechild View Post
    It shows up on Google Search, so I figured I'd answer it. Keep calm and carry on :-)
    I doubt the Ice Man has been waiting 10 years for the answer.
    Besides the answer is "none of the above", but rather 5751.......

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    Default Re: 12AX7 vs 12AT7 vs 12AU7


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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Quote Originally Posted by lovechild View Post
    There are many differences, but the ones that make the biggest difference are the Gain Factor and the Plate Characteristics (including the Grid Voltage Curves).

    The Gain Factors for each are as follows...

    12AX7 = 100
    12AT7 = 60
    12AY7 = 40
    12AU7 = 17

    Keep in mind that this doesn't mean you can put a 12AY7 (40) into a 12AX7 (100) socket just because it has a lower gain. First thing to consider is that the bias circuits for a given socket are designed such that the spec'd tube will operate in the center of the load line - which is based on a given socket's supply voltage and the Plate Load Resistor. If a tube other than spec'd is used, then either the top half of the signal will top out early or the bottom half of the signal will bottom out early. Ya want them to "clip" at the same time to max the level of that stage. The second reason is gain structure. If one stage drives the next with too low of a signal, then some of the dynamic range is being thrown away. In other words, you'll have to turn up the amp to get the same volume and you'll be turning up the noise floor with it.

    In summary, stick with the spec'd tubes for a given amp, or have a qualified designer (not a tech) mod it to use a different tube - even then it's a slippery slope. IMO, the older stuff (Fender, Marshall, etc) stuff is so well designed you'd be hard pressed to find a tube tech, or weekend warrior designer that can actually improve it.

    I hope this helps.

    lovechild

    www.lovechildamps.com
    Actually, the reasons you state for not changing the tubes are the very reasons you DO change them. To get those specific results.
    Originally Posted by IanBallard
    Rule of thumb... the more pot you have, the better your tone.

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    Bacteriaolgoist GuitarDoc's Avatar
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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    Quote Originally Posted by IM4Tone View Post
    I doubt the Ice Man has been waiting 10 years for the answer.
    Besides the answer is "none of the above", but rather 5751.......
    For some purposes the 5751 is a great preamp tube. I've been using them in one position or another ever since I've been using tube amps.
    Originally Posted by IanBallard
    Rule of thumb... the more pot you have, the better your tone.

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    Default Re: 12AX7 vs 12AT7 vs 12AU7

    My experience:

    Bassman 59' RI (Circa 1992)

    Swapped 12AY7 (GT) tube into V1 (in place of the stock 12AX7).

    With the 12AY7, the amp had more clean headroom (breakup was not as early as the 12AX7).
    Basically, you could turn the volume knob to 3.5 - 4 with the 12AX7's and then start to breakup nicely.
    With the 12AY7 you were at 4.5 before any breakup started to occur (so higher volume before breakup).

    In theory (please correct me if I am wrong), the advantage of the 12AY7 is: you could gain more clean headroom on the amp -and- if volume wasn't an issue, more of the breakup would come while you were into power tube territory (rather than just preamp tube breakup). So I guess if you were wanting to get to the dirt in a lower volume situation (home, small gigs) then use the standard 12AX7 -but- if you are playing where higher volume is acceptable before you breakup, then the 12AY7 would work well. OR - if you want to get dirt from pedals, then the 12AY7 might be the better way to go. If that makes any sense?

    Hope this helps anyone thinking about the swap.
    Last edited by goodguy; 10-17-2016 at 06:38 PM.

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