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#278881 - 04/08/10 09:27 AM
Re: Bought the Peavey 212, its on its way, thinking of a mod though...
[Re: fat_boy]
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stickyfiddle
Spam Commander (Forum Council Member)
Old Timer
Registered: 22/04/04
Posts: 5874
Loc: London, UK
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You can't just put a pot on your speaker cable! You'll melt the pot and potentially blow your amp up!
Master volumes are generally put between the preamp and poweramp section.
The output transformer (the bit that sends the power amp signal to the speaker) needs to have a minimum load, or 'impedance' on it at all times- this amount depends on your amp, but is usually 8 or 16ohms. As standard, your speaker will match that impedance.
If you want to attenuate that signal from output transformer to speaker you need a proper attenuator- Weber Mini Mass, Marshall Powerbreak or THD Hotplate are the obvious options. They make the amp see the same load at all times, so it doesn't get hurt, while reducing the effective output, and thus volume to the speaker!
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PSN: stickyfiddle
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#278971 - 07/08/10 10:44 AM
Re: Bought the Peavey 212, its on its way, thinking of a mod though...
[Re: fat_boy]
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Scrooge
Old Timer
Registered: 30/11/02
Posts: 1259
Loc: Aberdeen
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using very long speaker cable lengths reduces the power output of the speaker, by the resistance of the wire. They didnt talk about any damage to the amp though.
The longer a particular wire is, the more losses due to heat there are in the transfer of energy. You will also observe higher losses by using a thinner gauge of wire, but this is where the danger comes into play. The thinner the wire is, the less ability it has to handle electrical power and the more it will heat up. The danger here comes into play when the copper starts to melt!
You would either need a dangerously thin and still lengthy, or an impractially long piece of wire to achieve a desirable reduction in electrical energy for your purposes.
Of course the potentiometer would need to handle 100+watts, and need a max resistance something like 3 times the speaker impedance to cut the current back to a quarter so a 48 ohm pot would do.
Theoretical question: do you think a quarter will be enough? Thats a 6dB cut.
The guitarist in my band uses a marshall power brake with his 50w Marshall non-master volume head, its a good solution and he gets a great sound from it, but it is still loud and he runs the power brake round about the 20dB cut setting.
OK, took a look at the products you mentioned and indeed they are just big resistors, although they adjust for tone quality as the volume reduces.
The products mentioned are a lot more than just big resistors.
If you take a look at the schematic or even a photo of inside the powerbrake you can see theres a lot more going on there than just a big resistor. Even the cooling systems have a lot of thought put into their design.
The weber product in fact works on a completely different principle, not a resistor at all.
There are even schematics to make them.
I'm fairly practical with my electronics, but I shy away from anything mains related because of the danger and liability involved. Given this is a device designed to soak away more than 100w (RMS!) of power I wouldn't be keen to rely on my own craftsmanship. Some of the schematics online are pretty crude L-pad based designs I wouldn't want to use with a valve amplifier.
I hope that your research has proven interesting, I certainly enjoyed reviewing some of this info myself because I had the same questions you have, back when I was shopping for amps in my guitar playing days 
If I wanted to save some money with a homemade project, I would probably start by getting a second hand speaker and trying to build something along the lines of the weber mass system. You could possibly even build it into a speaker cabinet. Other ideas include THD yellow jackets for example. I also wanted to try making a guitar speaker cab based on an isobarik design, which should theoretically shave 3dB off.
Best of luck!
Steve.
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I've always prefered natural sounds to the synthetic...and valves grow on trees, dammit! 
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