#1 · Aug 13, 2008 21:41 UTC
OK... I realize I'm trodding on subjective territory, here, and I said I'd try to stay away from that. However, as you're well aware, preamps are made up of all kinds of tubes... 12AX7's, 12AT7's, 12AU7's, all of them with varied designators describing their various attributes (dry, warm, clean distortion, smooth sustain). Heck Groove Tubes has about 297.6 different categories of 12AX7 tubes all graded for your listening pleasures.
So, what's the difference? Why, if I put a 12AX7A in a preamp spot and then replace it with a 12AX7C, does the A sound different than the C? Why does a 12AT7, or a 12AU7 change that even more? The answer lies in some basic elements about tubes and the way they operate and respond to the circuits around them. If you look at an amp schematic, which consists of multiple straigh lines, dots, squiggles, parallel lines and a few circles, or half-circles, numbers and letters you'll eventually focus in, like you do with those "Magic Eye" posters and figure out that a tube is surrounded by a life support system of resistors and capacitors all designed to make it "operate properly" - meaning within the constraints that the designer wanted.
Tube geometry (no, this isn't a nightmarish high school math class) defines how a tube will act. Tube geometry is, simply, the spacing between various elements of the tube. How close the grid is to the plate and to the cathode. How large the plate is with respect to the cathode, that sort of thing. If the tube is put together sloppy, well geometries are going to be all over the place. Some tubes will exhibit great charactaristics, others horrid. And from this "slop" was derived a rating system, outlining, in part, how this geometry affects the tube operation. When tubes are assembled, the relative "straightness" and "orientation" of these elements will, in part, determine geometry and final results.
Whenever two conductive surfaces (metals, or the like) are placed near each other, but not touching, they form a capacitor - whether we want them to, or not. The tube, then, as a result of being assembled, creates small capacitors between the plate and the grid, the grid and the cathode and the cathode and the plate. Each of these capactances affects circuit performance (frequency response) in a different way some of which can be neglected. The most dominant of these is the plate to grid capacitance. The greater this value is (the closer the grid and plate are to each other as a result of design and/or assembly) the more "limited" the frequency response of the tube will be and it's the high frequencies that are being cut by this. This is also known as the "Miller Effect".
The next thing that influences the tubes performance is the quality of the metals used inside the thing. The cathode, for instance, is made up of metal that, when heated, shoots electrons away from it. The better the "doping" in the metal the greater number of electrons the cathode can produce. This, then, increases the tubes "amplification factor" which determines how much gain you're going to be able to achieve from the thing.
Finally, the vaccuum envelope... how much air is sucked out of the tube and how much gas is left inside the thing. Too much gas and the tube operates poorly. Too little of it, and it operates poorly. Too much oxygen and it burns up quickly. A slow leak will kill it quick... so, this, too, impacts the tubes overall performance.
12AT7 and 12AY7 tubes exhibit much less gain than their hi-gain (high mu) counterparts. Their interelectrode capacitances are different than the 12AX7's, their grid size, plate size and cathode size, as well as spacing for all 3, are different, therefore the circuits you build around them are, typically, different to reflect, and account for, the differences in tube response/design. That is, if we're designing with these tubes in mind. Most of us will just plop one of these tubes into a spot designed for a 12AX7 and call it "different", whether that different is "better" or "worse", subjectively, for us.
Personally, and this is me, I would use a hi-mu (12AX7) in the very first spot in the preamp. This gives you the best signal to noise ratio (SNR) that you're going to be able to get out of the amp. The higher the gain, in this stage, the more signal you're going to have versus hum/hiss when you're playing your favorite licks at bedroom volume. However, if you're going to use that high gain tube, there, make sure it's a good one. Some of the lower quality tubes exhibit more noise as a result of poorly doped cathodes, or poorly evacuated envelopes, or both. Quiet quality, in this spot, makes it better for everything else. Any noise generated in this stage is going to amplified 10's, or 100's, or 1000's of times by every succeeding stage after that!
I haven't touched, really, on the differences in EL34 versus 6V6, or EL84 vs. 6L6. There are some relatively complicated other issues, surrounding those, that include interactions between the power tubes and the output transformer, the power tubes and the speakers (reflected impedances) and other things. Let's get through preamps first... then delve into power amps.
Let me know if there are any questions you might have...
Enjoy...
Dar
So, what's the difference? Why, if I put a 12AX7A in a preamp spot and then replace it with a 12AX7C, does the A sound different than the C? Why does a 12AT7, or a 12AU7 change that even more? The answer lies in some basic elements about tubes and the way they operate and respond to the circuits around them. If you look at an amp schematic, which consists of multiple straigh lines, dots, squiggles, parallel lines and a few circles, or half-circles, numbers and letters you'll eventually focus in, like you do with those "Magic Eye" posters and figure out that a tube is surrounded by a life support system of resistors and capacitors all designed to make it "operate properly" - meaning within the constraints that the designer wanted.
Tube geometry (no, this isn't a nightmarish high school math class) defines how a tube will act. Tube geometry is, simply, the spacing between various elements of the tube. How close the grid is to the plate and to the cathode. How large the plate is with respect to the cathode, that sort of thing. If the tube is put together sloppy, well geometries are going to be all over the place. Some tubes will exhibit great charactaristics, others horrid. And from this "slop" was derived a rating system, outlining, in part, how this geometry affects the tube operation. When tubes are assembled, the relative "straightness" and "orientation" of these elements will, in part, determine geometry and final results.
Whenever two conductive surfaces (metals, or the like) are placed near each other, but not touching, they form a capacitor - whether we want them to, or not. The tube, then, as a result of being assembled, creates small capacitors between the plate and the grid, the grid and the cathode and the cathode and the plate. Each of these capactances affects circuit performance (frequency response) in a different way some of which can be neglected. The most dominant of these is the plate to grid capacitance. The greater this value is (the closer the grid and plate are to each other as a result of design and/or assembly) the more "limited" the frequency response of the tube will be and it's the high frequencies that are being cut by this. This is also known as the "Miller Effect".
The next thing that influences the tubes performance is the quality of the metals used inside the thing. The cathode, for instance, is made up of metal that, when heated, shoots electrons away from it. The better the "doping" in the metal the greater number of electrons the cathode can produce. This, then, increases the tubes "amplification factor" which determines how much gain you're going to be able to achieve from the thing.
Finally, the vaccuum envelope... how much air is sucked out of the tube and how much gas is left inside the thing. Too much gas and the tube operates poorly. Too little of it, and it operates poorly. Too much oxygen and it burns up quickly. A slow leak will kill it quick... so, this, too, impacts the tubes overall performance.
12AT7 and 12AY7 tubes exhibit much less gain than their hi-gain (high mu) counterparts. Their interelectrode capacitances are different than the 12AX7's, their grid size, plate size and cathode size, as well as spacing for all 3, are different, therefore the circuits you build around them are, typically, different to reflect, and account for, the differences in tube response/design. That is, if we're designing with these tubes in mind. Most of us will just plop one of these tubes into a spot designed for a 12AX7 and call it "different", whether that different is "better" or "worse", subjectively, for us.
Personally, and this is me, I would use a hi-mu (12AX7) in the very first spot in the preamp. This gives you the best signal to noise ratio (SNR) that you're going to be able to get out of the amp. The higher the gain, in this stage, the more signal you're going to have versus hum/hiss when you're playing your favorite licks at bedroom volume. However, if you're going to use that high gain tube, there, make sure it's a good one. Some of the lower quality tubes exhibit more noise as a result of poorly doped cathodes, or poorly evacuated envelopes, or both. Quiet quality, in this spot, makes it better for everything else. Any noise generated in this stage is going to amplified 10's, or 100's, or 1000's of times by every succeeding stage after that!
I haven't touched, really, on the differences in EL34 versus 6V6, or EL84 vs. 6L6. There are some relatively complicated other issues, surrounding those, that include interactions between the power tubes and the output transformer, the power tubes and the speakers (reflected impedances) and other things. Let's get through preamps first... then delve into power amps.
Let me know if there are any questions you might have...
Enjoy...
Dar