DIY power brake/attentuator

[boedonaldson]

Any ideas on how a DIY power attenuator could be made to put between the amplifier and speaker so it can be cranked with small venue/bedroom level volumes. Im thinking like on big mutha of a pot in series with two jack and some kind of heat sink system to get rid of a the heat. Im only a newbie, so i may be way off. I did see something on ebay and it was like one knob to put between your amp and speakers/cab, just like a master volume for the speakers. How could one be made without wearing out the tubes or damaging the amp in anyway. Any ideas? Cheers

[bancika]

The simplest solution is L-Pad, but it will make some tone (treble) loss. But it's simple to make. Two high power resistors and you're done. On my site there's calculator for resistors. -3db will cut volume to half

[vanessa]

I've read that this one here sounds great. Less treble loss than using a L-pad. Do a search over at 18watt.com they love these.

http://www.radioshack.com/product/index.jsp?productId=2102606&cp=2032057.2032187.2032194&allCount=14&fbn=Price%2F%2410.00+-+%2449.99&fbc=1&fr=StorePrice%2FRSK%2F00001000%2F00004999&parentPage=family


And can be built into an enclosure like this

[GibsonGM]

Hi guys,

I'm no engineer, but I do know you gotta be very careful when playing around with a dummy load!  I don't want to be the 'scaredy cat', lol, but someone has to say it Just In Case.  Bancika's L-Pad approach takes into account the output impedance of your power transformer...using something that doesn't match could fry the output transformer (most expensive part of your amp).  But hey, it's your amp!   I'd like to hear more about what people are using regarding DIY attenuators - anyone?

[bancika]

I'm using two attenuators with my little Firefly, I like it at really low volume. One is variable L-Pad and second is fixed attenuator using light bulb (idea from www.valvejunior.com. It works pretty well, bulb attenuator cuts volume to about 1/2 and adds little compression but  there's still a problem if bulb dies. It would leave output transformer without a load, but it's low power amp so I doubt anything will happen for short time.

[Gus]

there MIGHT be an issue with that RS device.  I don't have one and do not know what it is but i would guess it is a autotransformer for solid state amps in homes systems.


all it might be doing is transforming up the Z of the speaker and changing the primary load the output tube(s) work into.



think about this.

[Doug_H]

Be careful using an "in wall volume control" alone as a tube amp attenuator. It is basically an autotransformer and rotary switch. There is nothing in it that "soaks power" or maintains a consistent load for the amp. This means it will change the impedance ratio of the amp to spkr in a way that will substantially increase the load for the amp- a bad thing for tube amps, okay for SS hi-fi systems.

I bought one to experiment with and ended up doing this to make it (more) safe:

http://s14.photobucket.com/albums/a320/Doug_H/Q%20and%20D%20Attenuator/

This is by no means perfect as the load is not consistent- hence the "quick & dirty". It increases a little in low attenuation levels and decreases substantially at high attenuation levels. It serves my purposes well though, as I only needed 6 - 12db of attenuation for a 10W amp. The ideal solution would be to use a 2 pole switch, assigning the other pole to the load which would reflect the proper Z to the primary to compensate as the attenuation level is changed, much like the Marshall PowerBrake works. (I have not been able to find autotransformers with the proper taps for this sort of thing.)

I think the easiest thing would be to just buy a MASS. They are reasonably priced and the newer models divides the attenutation into 2 freq bands which I suspect helps with the "bass loss" issue under heavy attenuation. The MASS motor load on my Q&D attenuator sounds very good, btw...

Doug