[Amps] question about SPE 1.3K FA amp

[Manfred Mornhinweg]

> Probably something similar to the NXP BLF188XR

According to what's stated on some websites, they used an MRFE6VP61K25H.

 From the IMD data given in previous posts, I suspect that the amplifier is 
designed to reach clipping level at about 1000W or even less. The 1300W rating 
would then imply severe saturation, with large IMD, unless they use some bias 
modulation scheme to actively iron out the distortion.

Running a single of these devices at 1300W output necessitates saturated 
operation, otherwise the dissipation would be too high for long term device 
survival under practical conditions with a real-world heatsink.  If this is 
implemented with active linearization, for example by bias modulation, it's 
fine. Without that, the IMD must be terrible, and this would really be a 
"linear" amplifier only up to 1000W or slightly less. Even then, the thermal 
stress might be higher than one would like.

On eham there are 8 reviews of this amp, and all of them give it the highest 
possible rating! That does say something: The amplifier certainly has its users 
quite happy. But one should also mention that all these users have owned the 
amplifier for zero to 3 months, so the reviews can't tell anything about long 
term survival of the FET.

At least one user mentions high fan noise, though. According to photos on the 
web, this amp has fans for the heatsink, fans for the power supply, and even a 
dedicated local fan for the RF output transformer!

 > Perhaps the day of SS amps is here.

Definitely. With tubes for legal limit amps costing 1000 dollars and rising, 
while FETs for the same power class cost 200 dollars and dropping, factory-made 
amps really have to go solid state. Tube amps will soon be just a niche for 
homebrewers who have a well stocked junk box, and nostalgics much in the same 
class as those people who love tube audio.

I can indeed see SS amps much like that SPE becoming a kind of standard: Single 
gemini LDMOSFET, built-in switching power supply with power factor correction, 
relay-switched low pass filter bank, optional built-in automatic antenna tuner. 
If this is done with active linearization, allowing the amp to reach saturation 
without producing excessive IMD, then the efficiency is also good enough to have 
relatively low dissipation, which would allow an essentially unlimited life 
span, along with lower power drain and low heating. Otherwise, larger devices 
are showing up on the market, so soon there should be a single device allowing 
relaxed operation in class AB at 1500W, for those old fashioned people who like 
that class!

I really wonder whether the SPE uses active linearization. The photos I found 
aren't clear enough to find out. If I had to run a single of those FETs at the 
highest possible power, with acceptable linearity, I would use active 
linearization, varying the operating class from A at very low power levels, over 
AB and B right into class C at mid power, and back toward saturated class B at 
peak power.

And the price for such amps needs to come down, of course. I wouldn't spend 5000 
dollars to get 11dB higher power! But many hams will, obviously.

Manfred

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