Re: U3S output fets


The U3A by itself doesn’t have enough drive to make more than 500 mw output from the RD15HVF1 … BUT if you drive the RD15HVF1 with one BS170 on the U3A (at about 150 mw), it should make at least 12 watts…  I home brewed one (before Hans put out his kit) and mine certainly will put out that much power and more depending on the power source voltage.  The nice thing about using the RD15HVF1 is that it puts out a good amount of power above 20 mhz… well into the 200 MHz range… whereas the IRF510 (depending upon the quality of the device in manufacturing) quits at about 20 mhz.   But if you stay below 10 MHz, the IRF510 part is cheaper and will work fine.


You only need to use ONE part in any event!!!!! 



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From: [] On Behalf Of ian liston-smith
Sent: Saturday, February 04, 2017 9:58 AM
Subject: [QRPLabs] U3S output fets


Has anyone had any success getting about 500mW (on 40) on a U3S with a 12V supply from one (or two) RD15HVF1 devices?


I have a U3S running three BS170 fets with a PA voltage of 12V. The bias set so that without drive, total U3S current was about 50mA, and R5 adjusted until it rose to about 53mA.

Maybe my R5 has a bad bit of track, but occasionally the output falls to less than 100 mW, but sometimes rises to over 1 W by itself and the fets (glued to a heat sink) get very hot!

When it's prone to these variations the power fluctuates by about 6dB intermittently, but the drops to the much lower level. I've also observed this on a scope at the fets drains. I've checked for dry joints and swapped the LPFs to no effect.

I suspect it's probably one of the fets misbehaving rather than R5 (although this doesn't really explain the power increase), and it seems heat related. Strangely, on 80m, for the past few days the power has remained fairly stable at about 200 to 250mW - though does vary a little. (Just doing another test on WSPR on 40m - to see what happens.)

Anyway, are one, two or even three RD15HVF1 (used in Hans's 5W PA) worth trying here on the U3S? Or even a couple of VN10 fets?



Ian, G4JQT

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