2 ohm DIY Attenuator

[Rockwell666]

Hi Folks, A friend of mine needs an attenuator for his 61' Fender Concert with a 2 ohm output trans. 2ohm attenuators are pricey and I figured I could wire up a cheapy with some old parts I have laying around.
I have a few questions about Power handling of L-pads/Rheostats. Would a 75W Rheostat be enough power handling? I know a 100W Rheostat would work but I can only find a few online bidding sites selling NOS units that don't have a shaft to attach a knob. mouser/digikey only have 50w units for 100$cad. what if I found a 100w dual ganged 4 ohm rheostat, couldn't we just wire the gangs in parallel to achieve the 2ohm load I need? I plan to make a basic one with a brite switch and bypass switch so my buddy can enjoy his concert to its full potential.

[xtian]

Would a 75W Rheostat be enough power handling?

I would guess yes, unless he plays punk music flat out. Fender concert puts out 40 watts clean?

[Rockwell666]

ya they say 40W clean but at 500v on the plates its prob doing closer to 50W. I feel the 75W rheostat with alittle caution and maybe a fan for cooling would be fine. I just wanted to see if anyone had any experience. the 2ohm attens are crazy expensive

[roberto]

You can also use the standard 8 or 16 Ohm attenuator you and/or your friend already have/has and then add an impedance adaptor between the amp and the speaker.

[martin manning]

How much attenuation is needed? Does it have to be adjustable? Play around with this to see how much power is dissipated by the resistors in an L-pad: http://www.tedweber.com/webervst/lpad.htm

[tubeswell]

You can also use the standard 8 or 16 Ohm attenuator you and/or your friend already have/has and then add an impedance adaptor between the amp and the speaker.

Fenders 'EC-series' of amps uses simple 25W cement resistor and switch combinations between the OT Secondary and the Speaker for power attenuators.

The EC tremolux (with a 25W 8R speaker) has a 8k:8R OT with 2 resistors and a DPST switch for full power or 1/4 power attenuation. The '1/4-power' option puts 8R in series with the 8R speaker as well as 16R in parallel with these - to keep 8R overall.

Similarly, the EC twinolux (with 2 x 8R speakers in parallel) has a 4k:4R OT with 3 resistors and a douple-pole 3-way switch for full power or 2 attenuation options.

For a higher power amp with a OT with 2R secondary, (e.g. a BFTR) a you could do either type of attenuator using the same resistors with more resistors added in parallel to get increased power dissipation and decreased resistance.

[roberto]

I have to say that pure resistance attenuators always sound artificial to me.

IMHO this is still unbeaten: