> I would think that might be overkill, because when you
> bolt the spreader to the heat sink, the two pieces are
> going to conform to each other, in fact, they are going to
> expand and contract as the heat is applied and dissipated
> anyway.
As I recall making temperature measurements for a Heathkit
FET amp it could not have air gap, even at the micro level,
between the plates using the suggested thickness of copper.
The finish had to be pretty good.
One solution is to use a really thick copper spreader. I
think I doubled it. If it bows up a little or if it has
small surface irregularities (like milling lines) there will
still be plenty of area along edges for transfer to the
aluminum. If you thicken up the copper then the only real
critical area is the surface below each FET.
The big problem areas are near where the copper is sheered.
Any bend or burr there can hold the copper off the aluminum
or even warp the whole piece. Better be sure they saw it and
not sheer it, or be prepared to do some machine work.
By the way, the FET's will have a tendency to blow up on
higher bands using Motorola's original layout.... especially
if you let the drain voltage exceed 55 volts. Be sure to
look at the drains right at the FET's with a scope and watch
the peak voltage!!! The boards I played with had a nasty
third harmonic resonance that created some pretty high
voltages.
73 Tom
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|