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Re: [Amps] question about SPE 1.3K FA amp

Subject: Re: [Amps] question about SPE 1.3K FA amp
From: Manfred Mornhinweg <>
Date: Tue, 10 Nov 2015 18:41:43 +0000
List-post: <">>
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.


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