Does anyone know how this thing works?

http://www.marshallamps.com/images/products/powerbrake/powerbrake.html

Pretty much as described. It's a passive device that mimics the load of a speaker so you can plug a cranked amp into it and output the signal at a lower volume. Other popular devices that do this are the THD Hot Plate and the Weber MASS. Both the Power Brake and Hot Plate use inductors, resistors, and capacitors to accomplish this. The Weber MASS uses an actual speaker core, which is a pretty cool idea. In fact you can buy the parts from Weber and build one yourself if you wish.

None of these attenuators are perfect. They all suffer frequency loss the more you attenuate the signal. The Hot Plate and MASS tend to be  recommended over the Power Break for their ability to compensate for this.

Not real sure which type...
 The 'old ones used resistors to load the output of a tube amp.
 There may be newer versions of this with related circuitry to mimic the variances a speaker loading has on an amp.
 The 'best' [I'm not an output tube amp attenuater fan] method is using a 'dummy' speaker...has a coil, but instead of a cone theres a mounting system for the coil which absorbs the mechanical movement that would be heard as sound from a speaker cone.
 The coil is somehow mounted in a damper [so it can move without going outside it's 'socket'] that lets' it move like a speaker would, but produces no sound...when connected to the amp...loading it for the output tube distortion/compression you get when cranked into a speaker or speaker like load.
 Because the speaker has a moving coil, and kinetic energy from the cone movement influences the loading at frequencies, using a moving coil replicates the transient loading fluctuations more like a  speaker does.
 The old resistance method loads the amp, but the load imparted by fixed resistors is constant, unlike the loading of a moving speaker coil...plus they get very hot...for a small added load they're 'better'...turning a 100w into a 10w...you risk damaging your output section...plus all that energy is converted to heat instead of sound...
 suggestion...use a smaller amp.
 I can't recommend this, but for my own reasons...I used to pull two output tubes from my old Plexi 100w, and not turn the volumes past 1/2...I don't know how safe that was...but with the right steps taken, IIUC it's not that hard...could be wrong...you pull  1/2 of the output tubes out and mod the amp to be a lower watter, then you get full life from your tubes [all their output life used to drive sonic devices instead of dummy loads] and have a 1/2 set of output tubes for spares.

http://www.blueguitar.org/new/schem/marshall/powerbrk.zip  

http://www.blueguitar.org/new/schem/marshall/powerbrk.zip  

Quote from: petemooreI can't recommend this, but for my own reasons...I used to pull two output tubes from my old Plexi 100w, and not turn the volumes past 1/2...I don't know how safe that was...but with the right steps taken, IIUC it's not that hard...could be wrong...you pull  1/2 of the output tubes out and mod the amp to be a lower watter, then you get full life from your tubes [all their output life used to drive sonic devices instead of dummy loads] and have a 1/2 set of output tubes for spares.

Its okay to pull pair of the power tubes as long as you pull one from each push-pull section and you make the appropriate change on the impedence selector. I have a switch on the back of my bigger amps that does just this.

Andrew

Quote from: The Tone God

Quote from: petemooreI can't recommend this, but for my own reasons...I used to pull two output tubes from my old Plexi 100w, and not turn the volumes past 1/2...I don't know how safe that was...but with the right steps taken, IIUC it's not that hard...could be wrong...you pull  1/2 of the output tubes out and mod the amp to be a lower watter, then you get full life from your tubes [all their output life used to drive sonic devices instead of dummy loads] and have a 1/2 set of output tubes for spares.

Its okay to pull pair of the power tubes as long as you pull one from each push-pull section and you make the appropriate change on the impedence selector. I have a switch on the back of my bigger amps that does just this.

Andrew

As much trouble as it is to get an output amp working, it seems a shame to take a large portion of it's output and send it to something that makes it do a large portion less, and introduces new connections {I use Nice Connectors and Cables from output to speakers...not that I don't other places]...this is bad place to have any continuity problems.