Heat Sink for Dummy Load

[mclagett]

I just purchased a number of Vishay/Dale 50W Aluminum Power Resistors to build a dummy load device that will permit me to feed my amp's speaker output into a Direct In device without frying the transformer in my tube amp.

I'm using a schematic posted on the AikenAmps.com web site which implements a switchable 4/8/16-ohm dummy load device by using a 3-2ay toggle switch to tap into different points in a circuit of the aforementioned resistors. The 4-ohm option taps in at a point that uses 2 8-ohm 50W resistors in parallel (not sure why he does this instead of using a single 4-ohm, but who am I to second guess -- I'm taking this on absolute complete total faith since I am an electronics newbie). The 8-ohm option taps in upstream, adding a 4-ohm resistor in series to give a total 8 ohms of resistance and the 16-ohm option adds yet another 8-ohm resistor in series.

So we're talking about a total of three 8-ohm resistors and one 4-ohm resistor, each rated at 50W. These resistors have to be mounted on some sort of heat sink, but I have absolutely no clue as to how to determine what is an appropriate heat-sink to use. In a footnote to his schematic Rnadall Aiken says "All resistors must be well heat-sinked. If the unit is built in a metal enclosure, the enclosure can usually be used as the heat sink." That would be wonderful, if I could do this, but how do I know how big to make it and what is a suitable material (I'm assuming some kind of aluminum would be good). I could also use a separate heat sink inside the enclosure, but again, how big? If I make it thicker can I make it smaller? Can anyone give me some idea about how to answer these questions.

Thanks very much

[imix500]

What's the output power of the amp? If the amp is say, 100W then you just need a heatsink that can dissapate 100W max. You probably won't really dissapate 100W constantly unless it's something like an RF amp, but it's always good to overbuild, things run cooler that way. ApexJr has some cool tubular type heatsinks for a resonable price and they're pretty efficient.

[imix500]

Oh, btw, he probably uses 2 4ohm resistors instead of 1 8ohm because many amps can provide more wattage at lower impedances, so u have the ability to dissapate 100W with 2 4ohm 50W resistors. You could just use a 100W 4ohm resistor instead, but 2 50W's will probably get rid of the heat faster- more surface area.

[JensRasmussen]

Here is what I use

4 ohm resistors and an option to choose between a total of 4 or 8 ohm load.

\Jens

[JensRasmussen]

hmm pic didn't show up

[Brian Donaldson]

I just dropped them in a large glass of water. They'll never exceed 100 Deg c while there is water there.

[imix500]

True, but then there's corrosion, electrolysis and knocking the glass of water to worry about.

[Brian Donaldson]

But then again, this is setup is used for 2 hours 4 times a year. Then dried off and thrown back in my scrap box. Works for me and didn't take away 6 hours and 30 bucks from my habbit.