need help re: attenuator

[jrhoden]

im about to build an attenuator

alot of the schematics ive seen online are for 50watt and under...using a 50 watt l-pad...can i use the same plans and just switch out the 50 watt  l-pad for a 100 watt l-pad use with a 100 watt amp? or do i need to change the caps as well ?

[Dai H.]

be careful with "X"watt L-pads because some are rated in "music power" and won't handle 50, 100 watts, etc. Maybe the rating makes sense for the usually intended application (which I think is to be used in line with a tweeter in a hi-fi loudspeaker) but I think you could blow up the L-pad(and consequently damage the amplifier if the load goes open) if it's hooked up to a woofer(gtr. speaker) and overpowered. Someone on another forum advised to derate them drastically, and as far as I can see this is sound advice. Also, try comparing the physical size of the "50watt", "100watt"etc.  L-Pad to a wirewound rheostat of the same rating(an L-Pad usually has TWO rheostats in one housing or sometimes two ganged). That should tell you something (about the power handling). One idea is to put a fixed L-Pad in front of the variable so the fixed handles the bulk of the power, and the variable the rest. Limits the range, but if you are trying to significantly decrease volume it might be fine as practically used.

[MikeH]

Quote from Teds Weber

Amplifiers are generally designed to operate clean, and most manufacturers specify the output power as clean power. When you run the amp full blast, the output devices actually become more efficient because distortion is not considered. Under those conditions, the amp may be capable of putting out as much as 50% more power than its rated clean power. Be aware of that when choosing an attenuator, because the idea of using an attenuator is to run the amp full blast into power tube distortion and then controlling the level going to the speaker. If you are going to dime a 50 watt amp, you should choose a 100 watt attenuator.

[jrhoden]

ok Mike..

so what about using a 60/100 head?....use a 200w  l-pad or two 100w l-pads?

[cheezit]

Agreed on overrating the resistor by at least 2x or so.  And watch the heat!  I did a 50W version that my Fender Tweed Deluxe makes super hot.  And a 100W version (2 50W resistors in parallel) that my 50W Hiwatt will burn out if I let it.  So heat disappation is critical---all that energy has to go somewhere.

[jrhoden]

yeah i thought about the heat...

theres gonna be a fan or fans lol

so should i go with two 100watt?