Hi Martin,
> For those who wants the pictures of the EB-104 construction I prepared a
> PDF guide from the pictures grabbed out from the faulty site. Download
> here:
>
> http://www.ok1rr.com/download.php?view.167
Thanks for that! I downloaded it, and had a thorough look at it. Well, I
had thought that there would be some good information about some modern
legal limit SS amp design, and then discovered that this is "only" the
assembly procedure for a design that must be roughly 20 years old, and
has been available on the web almost "forever"! Well, that's my fault,
I should have read closer.
Still, a few comments about the job shown in those photos:
It would be better to FIRST bolt the transistors to the heatsink, with
thermal compound and all, and THEN solder them. This technique results
in minimal chance for undue mechanical stress on the connections. When
doing it in the opposite order, as shown in these photos, inevitably the
transistors will end up with more mechanical stress on them, which might
lead to failures in extreme cases.
Judging from the photos, the solder didn't flow very well under the
transistor tabs. I normally tin them lightly before installing them.
This results in much better soldering, closer to the transistor body,
which is important specially in the source tabs, to minimize the
inductance. True, with 50V MOSFETs running at HF this is not too
critical. But with 12V transistors running VHF and higher, it can make
the difference between an amplifier and an attenuator!
The soldering of the power resistors also looks really poor! I suggest
to improve this, before having the resistors fall off when unsoldering
themselves from their own heat! The rule used by old-time radio
technicians, who often mounted components a bit like that, was: First
you wrap the leads two turns around the support (or the other resistor's
leads, in this case). THEN you solder. Tin-lead solder is a very soft
material that easily creeps. It is not mechanically strong enough to
support a large component like these resistors over a long time, without
some additional help like the wraps. And that's specially true when the
component gets hot in use! I have had my share of power resistors
falling off, when I mounted them in this way! Also, these resistors
might dissipate a good portion of the heat through the leads, if they
are properly mounted (short leads to something large, like the board).
This additional dissipation capability is often useful.
Finally, a comment about the use of thermal compound, and this comment
is just my opinion, not a guaranteed truth: It seems to me that when
spreading out the thermal compound to a thin layer, and then mounting
the part, it's just too easy to get areas which aren't properly filled
by the compound, just as happened here. One problem is getting the layer
even enough (almost impossible), another is avoiding the inclusion of
air bubbles, which might never come out. For that reason, I think that
my method of applying the thermal compound is better: I apply a rather
large blob of it in the middle of the surface, and small clean beads of
it around the screw holes. Then I mount the part, and torque it down
properly, so that the compound squeezes out of the joint as much as
possible. If necessary, I warm up the assembly to get the compound to a
lower viscosity, and then re-torque the bolts. This method never leaves
any air bubbles or unfilled areas between the part and the heatsink. Its
drawback might be that the compound layer ends up thicker, but I don't
think that really happens, judging from the low thermal resistance I
obtain from these joints. And it should not be done if the compound is
too stiff, because the flange of the transistor might be deformed. But
then, such stiff compound should not be used anyway, because it will
never form a thin layer! The Arctic Silver compound surely is fluid
enough to use this method.
Well... I'm still on the lookout post for some legal limit SS PA design
that has advantages over the age-old and well-established ones, which
typically use 50V MOSFETs of ancient Motorola lineage, in class AB. The
advantages could be in obtaining high efficiency by running class E or
class F with envelope restoration, or at least in doing away with the
power supply by running directly off the line, and using low cost
transistors. If nobody comes up with something like this soon enough, I
guess I will! We need to innovate!
But right now I'm playing with PICs rather than RF MOSFETs. There's less
to blow up with them. When I get fed up with those little cute digital
toys, I might return to the big and serious world of PA development! :-)
Manfred.
========================
Visit my hobby homepage!
http://ludens.cl
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