6N7 Tube QRP Power Amplifier

Posted on Feb 5, 2014

A Super-Duper Powerful Vacuum Tube QRP amplifier for my 800mW QRPP homebrew telegraph vacuum tube transceiver `3T` (I promise to write a separate article or two about this three tube transceiver project later). It was not an easy decision to me because for that legendary time I`d almost a year used the QRPP power of less than a watt, but there was a rainy spring day,

6N7 Tube QRP Power Amplifier
Click here to download the full size of the above Circuit.

I`ve called CQ again and again without any takers and, at very last, somebody took over my only rock bound frequency. "That`s all" - I said - "Enough!". and turned on my Soldering Iron. Let me describe the scheme. Grounded-Grid design has been chosen because it is simple, easy to matching to coaxial impedance, provides the necessary level of amplification and typically requires no neutralization. After several experiments with triodes and double triodes I`d chose a 6N7S (6H7C) valve - Russian glass shaped version of well known 6N7 vacuum tube. I know that there was a 6N7G and 6N7GT American glass tubes, but as I heard these ones was not as common as metal 6N7. Anyway, pair of tied in parallel triodes inside this vacuum tube in the grounded-grids amplifier circuit offers more than six times power amplification and accurate input impedance matching via the 1:4 autotransformer. Maximal plate power dissipation of these twins is 12 watts, so you can got more than 25 watts output power without overheating - it should be interesting if you have Yaesu FT-817, Elecraft K2, Icom IC-703 or any other QRP transceiver, if you need some, but not too much extra power and, of course, if you love the magical glow of valves. By the way, about a glow - in this amplifier I managed to use an automatic bias circuit stabilized by two LEDs. As said one of my friends: "I love that Christmas Tree biasing!", but in my case these LEDs are red, so we...

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