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Vacuum Tubes - Page 3
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Thread: Vacuum Tubes

  1. #21

    Re: Vacuum Tubes


    From my experience with guitars, I have heard that tube amps sound better but I've never heard each back to back. They're also more expensive. I wonder why tube amps are better anyway.
    Well, depending on the particular class/driving aspect of the
    amplification circuit, there are two main reasons that tubes tend
    to sound "better" than transistors. All sounds put off harmonics
    (higher frequency modulations based off the "original" tone
    frequency) and tubes put off harmonics based on the 5th of the
    original tone compared to transistors that put off harmonic based
    off of the 3rd of the original tone. The 5th of the "root" tone is the
    warmer tone (it exists in most all scales based off of the root
    tone) while the 3rd (different tone between major & minor scales)
    is more desonant than the 5th (the 3rd is the next warmest tone).
    Overtones (or harmonics) based on the 5th will not be quite as
    "edgy" to listen too, especially in minor key music.

    The next reason is tubes sound a little warmer is because of the
    way they "overdrive" a signal (distortion, like typically used in a
    guitar amp). A transistor has a rather sharp signal clip when
    overdriven, while a tube tends to "squish" the signal rather than
    just cut it off at a certain point. This is quite noticable in a
    slightly overdriven class B push-pull amp, some of these would
    be stock Fender amps made before ~1978 and Marshall amps
    made in roughly the same eras (particularly the '59 plexiglass
    heads).

    In my recording experience (though not really technically
    explained), I've found that on drastic volume differences the
    tube amps tend to have a "swell" effect up to the louder
    volume rather than the instantanious jump that transistor amps
    makes when the same signal is given.

    This being said, transistor amps have several benefits over
    tube type like quicker/cleaner response, less "modification"
    to the original signal (in class A type amps), and they can to
    certain degree's simulate tube characteristics (with Mosfet
    transistors).

    It really boils down to a matter of personal preference and
    specific application.
    ~Guitarlynn

  2. #22

    Re: Vacuum Tubes

    Good explanation Guitarlynn. It's scary that you know all this stuff. You must be an acoustics engineer or something. I don't understand about that harmonics stuff though. Why must amplifiers add sounds to the original? That's probably the only thing I didn't understand. Why can't they simply amplify without altering? Granted a slight loss in fidelity and slight noise are expected (though usually not enough for the human ear to tell) but adding tones? What's up with that?

  3. #23
    Mentor cga's Avatar
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    Re: Vacuum Tubes

    Speaking from the playback end of things...

    In an absolute sense, an amplifier should not alter the input signal. Most modern solid state amplifers, in fact, have almost no additive effect on the signal going through them. Tube amplifiers almost without fail tend to alter the signal. They tend to soften the sound, for example.

    The main reason for this has to do with the fact that most tubes have very high impedence as opposed to transistors. The highish impendence, interacting with the speaker's internal impedence via the wire, result in a filter effect, thus altering the sound. This, in addition to the harmonics issue and the fact that most tube amps (good ones at least ;D) roll off the high frequency just a bit. This all combines to creat the "classic tube sound" that I like.

    No, it is not as acurate a signal as a modern "sand amp". But to my ears, it is much nicer. A good, low power tube amp driving some nice, high efficiency speakers just has a certain magic in the midrange that I simply do not get from transistors. My Denon transitor amp and Mirage M-590 speakers certainly have deeper bass, more dynamic range and can play louder. Heck, it is a great system.

    But I still find myself drawn back to the sweet glow of tubes and their lush sound.

  4. #24

    Re: Vacuum Tubes

    Oh I understand the softening stuff. It's basically a loss of fidelity. It blurs wavelengths together, like how the Gimp blurs pixels together when you soften. Like a capacitor with extremely low capacitance. My curiosity is about the harmonics stuff. I don't see how it could be added without intention in the design.

  5. #25
    Mentor cga's Avatar
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    Re: Vacuum Tubes


    Oh I understand the softening stuff. It's basically a loss of fidelity. It blurs wavelengths together, like how the Gimp blurs pixels together when you soften. Like a capacitor with extremely low capacitance. My curiosity is about the harmonics stuff. I don't see how it could be added without intention in the design.
    No, actually all amplifier circuits have some harmonic components by nature. All electromagnetic signals generate harmonics. Back in the navy, we often used high range harmonic signatures of electrical generators on "other guys" boats to ID them.


  6. #26

    Re: Vacuum Tubes

    Why at a different frequency though? The only way I can think of that electromagnetic fields would cause added sounds would be like an echo, but echoes are the same frequency.

  7. #27

    Re: Vacuum Tubes

    very good addition Cga!!! You're right indeed!
    The transistors are much more efficient than the "jamming" amplification
    of tubes. The harmonics (or rather "overtones" to be a little more precise)
    are simply created by current flow through the circuit akin to "resonance"
    to what we would think of audiably. Super clean, non-driven Class A
    modern power amps are not as noticable to changing the signal, but there
    is no softening or warm-tone to them either. Take a tube amp sometime
    and turn it up while the speaker(s) are disconnected, you'll find you can
    hear the audio through the tubes vibrating themselves. More or less
    (un-precisely) the tube has a low-level side and a high-level side. The
    signal is jammed through the middle and forced into the high side for
    "amplification". This effect is the "classic" tube sound with an outdated
    tone control circuit that furthers this tone.

    I spent 2 years engineering tube amplifiers while playing in bands
    (hence "guitar-lynn&quot. I understand them audio wise and electrically
    quite well.

    Nothing against transistor circuits though, they all have a place that
    are preferred.

    ~Guitarlynn

  8. #28

    Re: Vacuum Tubes


    Why at a different frequency though? The only way I can think of that electromagnetic fields would cause added sounds would be like an echo, but echoes are the same frequency.
    Every analog electronic circuit is nonlinear, despite the effort engineers put into making it linear. (nonlinear circuit: the input to output characteristics is not a straight line). Put a single sine tone at the input and you get this sine tone at the output plus a sine tone at all multiplies of the input signal frequency - this is harmonic distortion. (Math: Time domain: Out(t) = In(t)*Transfer_function(t), '*' denotes convolution, Frequency domain: Out(jw) = In(jw)Transfer_function(jw)). In real life there is more than one signal on the input, therefore the conditions in which the circuit works for one signal are altered by the second signal (and vice versa). Because of nonlinearity the effects of intermodulation occur - both signals are mutually amplitude modulated, which means (read about Fourier transforms and modulation) we get tones at every freq_a - freq_b and freq_a + freq_b. As signals are not single tone on the input, on the output we get unwanted noise through all the frequency spectrum. This is intermodulation distortion.

    Why tubes are "better" ?
    - their parameters vary less with temperature
    - have bigger input resistance
    - lower self noise level
    - (guessing) help achieve better linearity

    Why they are not ?
    - need higher voltage to work
    - have shorter lifetime
    - are bigger

    hth

  9. #29

    Re: Vacuum Tubes

    Ok, I somewhat get it now. I guess I would have to study more electronics to really understand it at the lowest level. While I like electronics, I know little about how it works.

  10. #30
    Mentor cga's Avatar
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    Re: Vacuum Tubes


    Ok, I somewhat get it now. I guess I would have to study more electronics to really understand it at the lowest level. While I like electronics, I know little about how it works.
    One cool thing about vacuum tube stuff is that all the components are big. You can actually hold them in your hand and really see what is going on. When you look a tube circuit drawing and then look at the circuit, it is easy to see each component and follow the drawing, unlike much of solid state stuff that is microscopic and integrated.

    Tubes are a great way to learn electronics.


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