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Adding mass to a heatsink !

glenn jarrett

pfm Member
I have an Cairn 4808A amplifier which i repaired last year
the only problem was i couldn't get the bias high enough for the rated output of the amp
as the first 10w of the 30w rating operates in class A then A/B up to 30w
what i want to know is if i add 10mm aluminium flat bar to the face of the heatsink and use longer fixings for the output transistors etc
would this give me more dissipation or just hold on to the heat ?

i know this is a factory built amp and the heatsink should be fine but i think its more like a bit of exaggeration from the manufacturers figures !

See Arkless comments below

For 10W class A you need 0.8A bias which is a measured 176mV across the 0.22R wirewounds.

Careful though... many companies exaggerate class A power and those heatsinks look a tad on the small side to be in a an amp capable of 10W class A but which then goes on to give 30W in A/B

If you set it to 10W class A then keep an eye on heatsink temp and on bias drift. It will quite likely need readjusting somewhat as it heats up. If it gets above around 65-70C then back off a bit to get to 60C


My reply
set up 2 meters on mv and measure the voltage across the emitter resistors on both channels
with amp just powered no speakers or input connected
the first measurements before adjustment was 7 .0 mV left and 2.2mV right the heatsink luke warm
after adjusting both pots up to 140mA as soon as you set it it starts to drop counting down as the heatsink temp rises
after some time it settles to 63mV and 62mV with a heatsink temp of 56 oC

Arkless reply

So nowhere near 10WPC in class A then.... in fact 1.3WPC in class A for 56C heatsink temp with 63mV across the source resistors....


20190729_135336 by glenn jarrett, on Flickr
 
Adding mass to the heatsink without adding to the main heat dissipation (Fin) area will simply lengthen the time taken to heat up to a given temperature with the same input power.

I agree with Jez, those heatsinks look small for a 10 watt Class A amplifier.
 
I had a similar issue on a small Sugden A25 integrated and I drilled a series of 1/8 on the top of the casing in line with the output transistors to help the heat to get out and it worked fine.
I also advised the owner this little amplifier wasn’t made for disco party !
 
When an amplifier produce's say 10 Watts in class A and then goes on to produce far greater in class A/B it will dissipate far more heat and need much bigger heatsinks than one which produces 10W class A and then it clips ie 10W is yer lot.
 


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