6N7 Tube QRP Power Amplifier - One Rainy Day DIY Project

Once upon a time I'd decided to join a Big Guns Gang and made 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, 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 can rather speak about the deer's noses, I think. This shiny circuit provides a stabilized 2.5 volts bias and it is very easy to move the triode deeper into the "C" class - you just have to add more LEDs.

Input impedance matching transformer T1 is wound on a little ferrite tore found in the IF stage in some solid state radio - not so important, just try to find the ring (one for the Dark Lord on his dark throne for example) and wind 7 turns of the double twisted wire, then connect the beginning of one winding to the end of another as shown.

Seems that that's all about the scheme, you're ready to Do It Yourself. Oh, yes, L1 is a common RF choke, it should be possible to use a homebrew one, dozen coils of enameled wire on a ferrite rod. All capacitors are mica or ceramics, C3 must be HV type, 500 volts at least. Pi-network data see below.

I've made this amplifier for 30 meters band, Pi-network coil was wound with enameled wire on the pill bottle, 12 spaced turns, ~30mm diameter. If you plan to tune an amplifier to other bands, use following table:

6N7 QRP Amplifier Pi-network Data
Freq, Mc	1.82	3.55	7.055	10.12	14.05	18.08	21.05	24.91	10
Coil, uH	34.5	16.5	8.3	5.8	4	3	2.5	2	1.7
Plate, uuF	231	130	65	45	34	27.5	24.5	22	20.5
Load, uuF	988	673	339	236	189	161	150	144	140

And yet another one note about the glow - I've installed a low voltage tiny neon bulb into the Pi-network's coil, so that I got one extra Glowing (pill) Bottle in the QRP amplifier. This neon lamp shining brightly when output circuit is tuned to resonance. It can be seen at picture below: