Heatsink fin area for given temp rise.

As work progresses with the pair of 845 SET power amps,
some serious consideration to getting rid of heat 
in 2 cathode resistors and several others in the circuit, resulting in a
total od 55 watts 
of dissipation.

At first I tried normal white ceramic wire wound R 
all operating at 1/4 or less of their wattage rating and mounted on Al
angles 
but when the prototype was trialed the Al angle small sized heatsinks
must have gone up to nearly 100C...not so good.
On a day here where my shed was 36C inside, the whole amp with R
underneath the chassis 
got fairly warm; too warm to my liking.

So I made a heatsink 185mm high and 210mm wide with a 3mm plate and 2mm
plate with R sandwitched between them 
and silicone adhered and with silicone paste to each plate.
On the back plate facing rearwards, I fitted 12 20mm x 32 mm x 1.6mm
angles running vertical,
and finally I could touch the hottest part of the sink for 5 seconds
without scorching myself,
and my thermometer indicated 47C max for an evening of 23C ambient.
The heatsink parts were all well machine screwed together with high temp
silicone.
The beauty of making one's own heatsink is that you get the size you
want,
and the shape wanted amd you are not addapting a less than perfect asian
made HS.

The useful info I should share with you is the final area per watt
required for dissipation of devices.

I ended up needing 40square cm per watt to get no more than 24C heatsink
T rise.

The total area was measured of the Al. This is surface area of front
back of metal plates holding resistors 
between them, and of both sides od fins. 

Fins are vertical, and have natural unobstructed air flow not dependand
of fans.

The resistors are spread out evenly over the available 210mm x 170mm
plate area,
and values carefully worked out so that the wattage per unit area was
about equal.

I tried a range of fans though, and powered them so they were nearly
silent.
The 12Vdc types are the best, but gees they are fragile!
even when running slow they can still be heard. So although a couple of
slow running 80mm fans 
did cool the heatsink so I got only 15C rise, I decided I couldn't use
any type of fans.

So where one was building a 25 watt class A SS amp, one might have to
dissipate 55Watts,
so you'd need  a total Al fin and plate surface area 55 x 40 = 2,200
sq.cm to get a T rise of about 24C.
This is what I have for my resistor pack on the mono 845.

The placement of mosfets and any hot parts must be so they are spread
out as much as possible or else
you get devices being hotter than you'd want if they are clustered
together
and a heatsink running too hot near the devices and too cold further
away from the devices,
so the devices are stressed and the heatsink would be inefficient.

I don't like hot running resistors in any amps I build, and if you run a
10W wire wound 
resistor which is say 50mm long, with 10mm side size, its T will be
high, maybe 100C++,
and it simply won't last very long, because the welds fatigue or ther
ceramic cracks
or the wire finally oxidizes, hence 2.5W is the max I ever use with any
10W resistance.
Strung netween two tags on a board it still gets hot, and where you have
lots of them,
all that heat has to go somewhere........

The vitreous enamleled ww R and the AL clad R ARE GOOD, and can be
allowed to run hot,
say above 50C but I don't like it at all.
And they are expensive. The AL clads need a heatsink. 

Doing it the way I have and limiting heatsink T to 24C rise 
will mean that elements in the R would probably be at around 30C above
ambient
which is quite acceptable for WW R.

Patrick Turner.