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Light Bulb Attenuator


John292

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Hey again, sorry for these mod posts, I got that output jack installed, now I need to know one more thing. I have a 4w 12v light bulb, and I need to know step-by-step how to put it in a small box, I need to know which wires go to the light bulb, and how to install it. I have already read all the the mod threads and it never really says straight up how to do it step-by-step, so if anyone know, please help.

 

Thanks for the help :P

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I'll never say straight up how to do it, because for each application you have to calculate the bulb ratings based on the amp's signal current output, desired degree of attenuation, change in the actual effective output impedance you think you can get away with, make provision for a parallel resistance to save the amp's output transformer when (not if) the bulb blows out, and then you also have options of combining series and parallel bulbs for keeping the total impedance the amp sees in line with what it expects, along with different combinations of volume compression (series bulb) or expansion (parallel bulb.) It's not really a simple topic. Perhaps some day someone with a lot more time on his hands than I will write an article with diagrams of several possible circuits and list some typical values for bulbs for particular power ratings.

 

I will say this: If you keep a power resistor of 5W or greater, valued at about 20x the impedance of the amp's output impedance at the jack you're using, always hooked across the output, it should be sufficient to keep the OT from frying if the output is unloaded. As from forgetting to plug in a speaker, accidentally unplugging while playing, or if a series bulb blows. I've put such resistors on my amps, hooked across the jacks. You'll never hear the amount of power that the resistor draws off, and it's a good safety precaution.

 

:)

"A cheerful heart is good medicine."
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What I'm saying is, there are a lot of ways to do it, and the way you're planning to do it risks blowing the very expensive output transformer in your amp when the bulb burns out. That will be just like pulling the speaker cable out of the jack. I have no idea if the 4W bulb you have is appropriate for the amp you're using it with. I don't have a simple how to. Calculations have to be done for different applications, and I'm not going to do them because I don't need an attenuator right now. You might check on the Valve Junior modding boards like http://valvejunior.com/ (I'm guessing it's a VJ you're working with, you really didn't say) for the particulars of attenuators others have built that are working well.
"A cheerful heart is good medicine."
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