'Tube rush' is a relatively continuous noise composed of random noise mostly in the middle and high frequencies. If the noise is predominantly high frequencies, we call it hiss.
'Microphonics' is electromechanical noise that is created within the tube when it's disturbed mechanically (by a hand wiggling the tube, by airborne vibrations [called 'music'], etc.). It may resemble the ringing of a bell. The noise is passed along the amplification path and varies greatly in level, from nonexistant to grossly unacceptable. Some amplifier positions/functions are more sensitive than others to microfonics*. For instance, my SET poweramps' 1st gain stages REQUIRE low-microfonics tubes, whereas tubes with unacceptable levels of microfonics in the 1st stage are rather quiet in the 2nd stage.
Generally, gain stages ocurring early in the signal path are more sensitive to tube rush and microfonics than subsequent gain stages.
Often (but not always) the use of some kind of tube dampers such as the excellent Tube Damping Instruments by Herbie's Audio Lab... http://herbiesaudiolab.home.att.net/ ...will reduce microfonics; I use these things even on tubes that have no apparent microfonics. http://s89.photobucket.com/albums/k220/jeffreybehr/ASL%201006-845/?action=view¤t=Fivetubes_700w.jpg
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* One of my campaigns is to be rid of 'ph' when it masquerades as an 'f'.
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'Microphonics' is electromechanical noise that is created within the tube when it's disturbed mechanically (by a hand wiggling the tube, by airborne vibrations [called 'music'], etc.). It may resemble the ringing of a bell. The noise is passed along the amplification path and varies greatly in level, from nonexistant to grossly unacceptable. Some amplifier positions/functions are more sensitive than others to microfonics*. For instance, my SET poweramps' 1st gain stages REQUIRE low-microfonics tubes, whereas tubes with unacceptable levels of microfonics in the 1st stage are rather quiet in the 2nd stage.
Generally, gain stages ocurring early in the signal path are more sensitive to tube rush and microfonics than subsequent gain stages.
Often (but not always) the use of some kind of tube dampers such as the excellent Tube Damping Instruments by Herbie's Audio Lab... http://herbiesaudiolab.home.att.net/ ...will reduce microfonics; I use these things even on tubes that have no apparent microfonics. http://s89.photobucket.com/albums/k220/jeffreybehr/ASL%201006-845/?action=view¤t=Fivetubes_700w.jpg
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* One of my campaigns is to be rid of 'ph' when it masquerades as an 'f'.
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