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Re: [Amps] MRFE6VP61K25HR6

To: amps@contesting.com
Subject: Re: [Amps] MRFE6VP61K25HR6
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Thu, 19 Sep 2013 15:59:15 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
David, and all,

> Isn't the MRFE6VP61K25HR6 device rated only 20% duty
> cycle, pulsed, at 1KW?

It can do quite some more than that, but for true 1kW PEP SSB, with speech processing, the thermal issues are difficult to handle.

>    How clean would such an amp be?

An amp using the board mentioned will likely not be very clean at all, as it doesn't seem to use any sort of negative feedback. But this can be corrected. When adding negative feedback, it will clean up a lot, and also it will require higher drive power, making it a better match for typical radios with 100 watt output stages. The ideal is to add so much negative feedback that it requires close to the full available drive power. That affords maximum linearity, and is a much better use of the available drive power, than burning it up in an attenuator!

>    If one wants 1.2KW or better Output, and never have
> to worry about losing a device, is the conservative path a
> pair of these with a splitter-combiner?

Yes, but also adding some protective circuitry. To be very safe, I would include a current sensor in the supply line, a voltage sensor in the supply, a thermal sensor as close as possible to each transistor, and a power/SWR sensor. The outputs of all sensor can be joined, with proper individual tripping points and gains, to feed back to the ALC input of the driving radio, and to shut down the power supply of the amp if ALC action isn't sufficient to keep everything in the green range. This may sound complex, but is actually reasonably simple and very inexpensive to do.

>    Power supplies are readily available and cheap but
> when I was looking it seemed that the necessary switched
> output filters are neither readily available nor much fun
> to build.

Yes. The problem with these filters are the capacitors. They aren't easy to get, because some of them need to be rated for RF currents of at least 10A, the others for at least 5A. I'm also looking for good ones. The best I have seen so far are from England - a forum member hinted me about them. But even those are a bit on the tight side.

Homebrew copper clad mica capacitors are a good option, and perhaps using teflon instead of mica should be tried - but teflon is soft and has a lower dielectric constant. Its advantages are that it may be easier to get, and comes in better controlled thickness. Perhaps somebody has a good idea about other dielectric materials - I would love to hear it! It needs to be available in thin sheets, have good dielectric characteristics, it's desirable that it be tough, and it must survive soldering heat.

Manfred

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