[Amps] Advice needed for SS amp

[Manfred Mornhinweg]

Bill,

it's great joy to see how much discussion your post has stirred up.

> I'm thinking of water cooling. 

It's probably the only way to go. I suggest that you spend some time 
learning how to calculate heat flow through copper, through copper/water 
boundaries, and so on. Doing that, you will find out facts like these:

- The only practical way to remove heat fast enough from those 
transistors is to have a very short, broad path through copper, reaching 
a sufficiently large copper surface in contact with fast flowing, highly 
turbulent water. That means a tiny copper heatsink with moderately thick 
fins, with a water jacked around, fed by a pump that delivers enough 
pressure to cause lots of turbulence.

- You may need to solder the transistors to the water-cooled copper 
blocks, to avoid the additional thermal resistance of a grease layer, 
but maybe you can get enough thermal conductivity in the rest of the 
system to manage without soldering the transistors down.

- Simply immersing the transistors in water will not afford enough 
cooling, because their surface is too small. The water would boil in 
contact with the transistors, and the steam bubbles would keep most of 
the surface away from the water!

- Do not feed the cooling from the tap. If you do, all it takes is 
pretty cold tap water and a moderately high humidity inside the room, to 
get condensation, and water dripping all over your circuit! Instead use 
a big enough water tank inside the room, that's never much colder than 
the air in the room. Exercise caution if the temperature in the room 
isn't reasonably constant.

> Notice the unusually high power supply voltage. I'm thinking of running a
> full wave bridge rectifier directly off the 240 VAC line, thus eliminating
> the power transformer and giving about 340 VDC no-load. The amp would have
> to be isolated from ground of course,

That's what I proposed too, time ago, and what is used in industry in 
some cases.

> A set of four 10 amp 600 PIV diodes is available on eBay for less than $30

A ready-made bridge rectifier is usually cheaper than four separate diodes!

 > and a 6800 uF 450 VDC capacitor is available for about $50 including
> shipping. There's most of your power supply. Pretty cheap, huh?   :-)

You may need to add at least an inductor, to keep the input power factor 
from becoming too terrible. A 3kW load with a power factor of 0.3 needs 
a circuit capable of supplying 10kVA, and I dout you have that! Maybe 
you need to compromise, use a smaller capacitor and accept some more 
ripple, to get a manageable power factor. It depends on how bad the 
power factor can be, before it gives trouble with your electrical 
installation!

> My main use would be RTTY and CW so linearity is not an issue, but do you
> have any thoughts on what the IMD might be for SSB? 

I have no experience with those transistors, but I have heard rumors 
about the high voltage transistors not being good enough for linear use. 
   Some rumours said it was because of poor linearity, others said it 
was because of hotspotting and device failure. I frankly don't know 
what's true about them, if anything.
Maybe you can linearize them enough by proper use of negative feedback. 
Actually ANY solid state amplifier should use negative feedback! MOSFETs 
typically have far greater power gain than a ham needs. Instead of 
attenuating the drive input and then keeping the full gain, it's MUCH 
better to reduce the gain by using negative feedback. That way the 
excess gain is put to good use by enhancing linearity.

> Since the output impedance is about 50 ohms, I'm wondering if a 1:1
> broadband antenna balun might serve for the output toroid? Just a thought.
> Might not be a good idea. 

Antenna baluns usually don't have galvanic insulation between input and 
output, so you can't use them in an amplifier powered directly off-line. 
  But using the core of such a balun, and rewinding it properly, should 
be possible.

> Like I said, I'm a complete newbie at solid state amps so all comments are
> welcome. This almost seems to be too easy to be true. Go ahead, burst my
> bubble.   :-)

OK. You asked for it! ;-)

I would suggest that you first get some very cheap, small RF MOSFETs, 
could even be switching type MOSFETs that cost one buck each, and go 
ahead building a push pull amplifier with them, that delivers maybe a 
few tens of watts. As driver you can use any 100W HF radio with power 
control and an additional attenuator on the output, so you get down to 
roughly 1 watt of drive. On that small, cheap amplifier you can grind 
your teeth, learn how they work, what problems they give, you can learn 
how to measure IMD, and so on. If you burn up something, the damage will 
be limited to a few bucks, at most.
Once you feel fully confident that you thoroughly understand how it 
works, make a version #2, that basically milks those cheap little 
MOSFETs for the absolute maximum power you can get from them. This will 
allow you to learn about the issues involved with cooling transistors 
that work hard.
Once you got to the point where you can take two MOSFETs rated at, say 
40 watts absolute maximum dissipation each (think of the IRF510, for 
example), and make an amp with them that delivers 40 to 50 watts output, 
with decent IMD, efficiency, and that survives continued transmission at 
full power, you are ready to buy a pair of ARF1500's and build a 1500W 
amplifier with them. At this point you will no longer be a newby, but 
you will know all the basics, so that you will be ready to tackle the 
additional problems that come with high power.

Of course you might want to skip all the previous stages, and jump 
directly into building the high power amp. But that will make you suffer 
all your unavoidable beginner's mishaps on the big amp, each time taking 
out maybe 500 dollars in parts! And it will have you deal with basic 
problems and with high power related problems at the same time.

It can be frustrating to see expensive parts go up in smoke, and not 
even knowing why they did. Instead when it happens with cheap parts, you 
can smile about it and just try again, to try finding out why they blow 
up. It builds a lot of confidence to be able to make a good 50 watt amp 
out of two one-buck transistors, and that's the kind of confidence you 
need to build a big amp. And everything you learn with the small amp can 
be applied to the big one.

Manfred

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