"Mass Brakes"...what is in an active Sp Attenuator ?

[petemoore]

  Sorry if I didn't prowrite the names correctly.
  The attenuators Marshall and Weber make which allow a tube amp to sound like it's powering and active load [mass on a spring, like a speaker].
  What is in them ? Is there a coil driving a mass on a spring ?
  IIUC they can provide a lively line signal-out from tube output, how are the small signal attained ?

[defaced]

Depends on the design.  The ones I have seen are some combination of R C and L to get a circuit that can take the power and respond sorta like a speaker. 
Here's a guy who designed/built one and all the fun he had:
http://music-electronics-forum.com/t13773/#post109072
http://music-electronics-forum.com/t20570/

Here's a thread about the MASS Attenuator, schematics included:
http://freestompboxes.org/viewtopic.php?f=2&t=2287

[m-theory]

Almost all modern attenuators follow the basic, resistive load design of the Schotz power soak.  They incorporate large resistors and a large rheostat, and sometimes an inductor.  The MASS adds a speaker motor, to create a dynamic response. 

I've personally only used two, a Hotplate and a MASS, and I can say from my own experience that both will cause significant coloration at any attentation from -8db on.  If you can keep it at -4db, the Hotplate can be reasonably transparent.  The MASS seems to cause a "saturation" effect at virtually all settings, to my ears. 

I have a friend who's had and/or tried everything, in his 30 years of gigging, owning and working music stores, and writing for a leading guitarist magazine.  He says, without any hesitation, that the only attenuator on the market that can truly boast tonally transparent attenuation is the Ultimate Attenuator, which converts the speaker load to line level signal and then amplifies that with a Mosfet amp.