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Thread: general differences between 12a*7 and 5751 tubes?

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    Fudgeitallologist PVFan's Avatar
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    Default general differences between 12a*7 and 5751 tubes?

    just curious as my new amp has a 5751 front. What are your thoughts?

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    LoveMachineologist jeremy's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    similar to a 12ax7 but with less gain. usually gives you a brighter, cleaner sound, little less compressed when running the volume up high on a nmv amp

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    Default Re: general differences between 12a*7 and 5751 tubes?

    The 5751 is a military spec tube, with a gain of 70, vs the 12AX7's gain of 100. Thus it is going to be closer to a 12AT7 (gain of 60) in terms of gain than a 12AX7. However, because it is a military spec tube, it is supposed to be more rugged than a 12AX7 or 12AT7

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    Default Re: general differences between 12a*7 and 5751 tubes?

    Can't add much more to what's been said already, very accurately put the above posters.
    Switching it for a 12AX7 would increase the gain and decrease the head room a little too.
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    Super Toneologist marvar's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    as a side note: genuine 5751 tubes are balanced, as per military spec.
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    Imperator of Indignation idsnowdog's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    What I have noticed is they overdrive differently. The 12AX7 smooths out and darkens, while the 5751 clips like a power amp tube and is bright and open. If you're looking to add a bright harmonically rich edge(spank & sparkle) to an amp the 5751 makes a good compliment. I use a 12AX7 in the first stage and a 5751 in the second stage of my ADA MP-1, since I can drive each stage seperately. Together they give a lush harmonically rich distortion.

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    Default Re: general differences between 12a*7 and 5751 tubes?

    That reminds me, I've been meaning to try a 5751 in my 18 Watter.
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    Fudgeitallologist PVFan's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    100% interchangeable ?

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    Imperator of Indignation idsnowdog's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    Quote Originally Posted by PVFan View Post
    100% interchangeable ?
    As far as I know, yes. A Fender Champ with a 12AY7 in the first stage and a 5751 in the second is instant SRV/Knopfler.

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    Dean Hardtail Fanologist Jeff_H's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    5751 - This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7 also has a gain factor of 70, but since the plate resistance is different, the 5751 is closer to the 12AX7. The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonic response. 5751's also have matched triode units. Late versions produced by Philips/ECG don't have the extra support and don't perform as well as the older 5751's.

    http://www.vacuumtubes.com/12ax7.html

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    Minion of One Andrew Lamprecht's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    Quote Originally Posted by PVFan View Post
    100% interchangeable ?
    Yeah, you can change them. Here's some info for you.

    http://thetubestore.com/gainfactor.html





    "The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 type has a gain factor of 100, while a 5751 (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain. Not only will this make the amp quieter, but it can also alter the sound by making the power section work harder when you turn it up. Many guitar amp users (particularly harmonica players) like to reduce the gain to get a different sound or calm the amp down to prevent feedback."
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    Dean Hardtail Fanologist Jeff_H's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    "Mike I got your name from Myles's on the Dr Z forum. He's the one that recommended the NOS RCA 5751 for the V2 in my Carmen Ghia. Wow, I have never changed one tube that made such a big positive change in any of my Amps. I bought my Ghia new about two months ago and am now changing out the stock tubes. Thanks, Russ"

    https://www.kcanostubes.com/content/...751-blackplate

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    Default Re: general differences between 12a*7 and 5751 tubes?

    Quote Originally Posted by Koreth View Post
    The 5751 is a military spec tube, with a gain of 70, vs the 12AX7's gain of 100. Thus it is going to be closer to a 12AT7 (gain of 60) in terms of gain than a 12AX7. However, because it is a military spec tube, it is supposed to be more rugged than a 12AX7 or 12AT7
    . Except in my opinion 12AT7's suck in gain stages. Admittedly similar in gain but 5751's sound much better in my opinion.

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    LoveMachineologist jeremy's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    9 year bump for that eh?

    i dont know if 12at7's suck in gain stages but they do react differently than the 5751 even if they have similar gain. in some amps i like a 12at7 better than the 5751 but these tend to be older style non-mv amps. although i did like the 12at7 in an old matchless sc30 a ton for whatever reason

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    Slutbucker Pimpologist ArtieToo's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    But a good bump. I've got a couple of NOS mil-spec 5751's that I was thinking of trying in my little Tubemeister 5. Good info.

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    Default Re: general differences between 12a*7 and 5751 tubes?

    I just put an old 1961 GE 5751 in my Mesa Blue Angel's reverb position...the tail is longer, and less 'boingy' which is wonderful.
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    Default Re: general differences between 12a*7 and 5751 tubes?

    Quote Originally Posted by Mincer View Post
    I just put an old 1961 GE 5751 in my Mesa Blue Angel's reverb position...the tail is longer, and less 'boingy' which is wonderful.
    Thats interesting, I must admit that I am guilty of putting whatever tube I have on hand in the reverb driver slot cause its not a gain stage and doesnt matter. But I will have to play with this idea a bit.
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    firstlessonologist guitfiddle's Avatar
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    Default Re: general differences between 12a*7 and 5751 tubes?

    I have an old RCA 5751 in V1 of my OR15. It tamed the fizz a bit when the gain is run high, and the power section gets a bit more od when I turn it up. It still has more gain than I could use, but it definitely sounds better.
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