Is a strong 12AT7 similar to weak 12AX7??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Axemeister, Mar 7, 2008.

  1. Axemeister

    Axemeister Member

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    Hypothetically, I am just curious if a 12at7 that tests strong is going to have similar characteristics to a 12ax7 that tests weak... Or, do they have inherently different characteristics?
     
  2. rockon1

    rockon1 Member

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    A 12AT7 in a circuit designed for a 12AX7 generally sounds slightly brigher/thinner. Bob
     
  3. Blue Strat

    Blue Strat Member

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    What Bob said. This is because there's a lot different about these tubes other than gain.
     
  4. Blue Strat

    Blue Strat Member

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    How does that Rp figure into the F= 1/(2*PI*R*C) equation when you factor in the plate resistor and coupling capacitor?
     
  5. Wakarusa

    Wakarusa Member

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    Assuming you're looking for the HiPass -3db point at the output (contributions from output C (Co), output R, and load R), then the plate load (Rl) and plate resistance (Rp) are considered in parallel and that in series with the input resistance of the following stage (Ri). So the equation is:

    F= 1/(2*PI*(((Rp*Rl)/(Rp+Rl))+Ri)*Co)
     
  6. Blue Strat

    Blue Strat Member

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    I always loved the "circuit models" where the R combinations seemed counterintuitive. How the hell does a plate load resistor which can only be in series (physically) with the Rp (since only one end of the plate load resistor is connected to the tube) be in parallel with Rp? Is it because the other end, connected to the power supply, considered to be AC ground?
     
  7. Blue Strat

    Blue Strat Member

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    There must be more to it than that. 12AT7s typically sound a lot brighter (thin is often quoted) in 12AX7 circuits. I don't think the quoted frequency response of 12AT7s would be skewed to the upper audio frequencies.
     
  8. VikingAmps

    VikingAmps Member

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    I think the lower plate resistance of a 12AT7 compared to a 12AX7 (about 6X lower) helps to negate the effects of the interelectrode capacitance of the next triode stage. A good experiment would be to try the same swap driving a pentode stage. Don't forget the plate to grid capacitance is an active filter.
     
  9. Wakarusa

    Wakarusa Member

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    However, look to the input side of things where you've got a lopass response from the combination of input resistance, interelectrode capacitance, and miller capacitance (a function of midband gain). Changes in Rp have a strong effect on midband gain and, particularly if you're looking at the 1st stage in an amp, the input series resistance may be something like 34K or 68K. A 12AX7 jacked into your standard Fender preamp with the instrument plugged into jack "2" (68K series input) has a lowpass -3dB point of around 17KHz while the 12AT7 has a -3dB point of around 20KHz. It's a subtle difference though and may not fully explain what you're hearing. For that, I'd look to the base assumption that the 12AX7 and 12AT7 you're looking at have either flat or identical frequency response in and of themselves -- something that my ears tells me simply isn't true, otherwise two different 12AX7 (let's say a Mullard and an EH) would sound the same in the same circuit. Of course, the EH is much brighter.
     
  10. VikingAmps

    VikingAmps Member

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    I wonder if part if not a lot of what we're hearing between different tube types has to do with the spacing and shape of the elements causing different amounts of capacitive reactance. Just because a tube is spec'ed for a certain amount doesn't mean they're all created equal.
     
  11. rockon1

    rockon1 Member

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    OK most of that was over my head however my ears tell me I dont like 12AT7's in place of a 12AX7 usually. I do have an Amperex 12AT7 in one of my amps. It seems to clean up the OD a bit-less muddy........ Not sure if it matters but Im talking in high gain applications. For the most part I seem to lose tone. I say thinner tone perhaps less full OD tone is more appropriate...ok thinner is thinner no matter how you say it! lol! Bob
     
  12. jspax7

    jspax7 Member

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    I've never heard one that does. You might find a strong 5751 that is closer, but in my listening experience, the 5751 makes the clean tone cleaner and brighter, and the overdrive cleaner and more articulate. I personally prefer a 5751 to an AT7 as a substitute for a 12AX7.
     
  13. BLC

    BLC Member

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    Guys, I love technical boards !!! This one seems to be filled with a lot of technical,knowledgable guys. Thanks for allowing me to hang with you !!! You bring a big smile to my face.... really !!
     

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