6N7 Tube QRP Power Amplifier

Description: A powerful vacuum tube QRP amplifier designed for an 800mW QRPP homebrew telegraph vacuum tube transceiver named "3T." The decision to build this amplifier followed a period of using less than a watt of QRPP power for nearly a year. After repeated attempts to make contact on a specific frequency without success, the creator decided to proceed with the project. The amplifier employs a grounded-grid design, which is straightforward, matches well with coaxial impedance, provides adequate amplification, and typically does not require neutralization. After experimenting with various triodes, the 6N7S (6H7C) valve, a Russian variant of the 6N7 vacuum tube, was selected. This vacuum tube features a pair of triodes in parallel, resulting in over six times power amplification and effective input impedance matching through a 1:4 autotransformer. The maximum plate power dissipation for these triodes is 12 watts, allowing for over 25 watts of output power without overheating, making it suitable for QRP transceivers like the Yaesu FT-817, Elecraft K2, and Icom IC-703. Additionally, the amplifier incorporates an automatic bias circuit stabilized by two red LEDs, providing a stabilized 2.5 volts bias. The input impedance matching transformer T1 is constructed using a ferrite toroid from an IF stage of a solid-state radio, with seven turns of double twisted wire. The RF choke L1 can be homemade using enameled wire on a ferrite rod. All capacitors used are mica or ceramic, with C3 specified as a high-voltage type rated for at least 500 volts. The amplifier is tuned for the 30-meter band, with the Pi-network coil wound with enameled wire on a pill bottle, consisting of 12 spaced turns with a diameter of approximately 30mm. A table for tuning the amplifier to other bands is provided.

The vacuum tube amplifier described incorporates a grounded-grid configuration, which is advantageous for achieving high levels of amplification while maintaining simplicity in design. This configuration is particularly effective for QRP (low-power) applications, where efficiency and reliability are paramount. The choice of the 6N7S valve allows for significant power amplification due to the parallel arrangement of the triodes, which enhances signal strength without introducing excessive heat that could compromise the components.

The automatic bias circuit utilizing red LEDs not only serves a functional purpose but also adds a visual aesthetic to the amplifier, reminiscent of holiday lights. This feature allows for easy adjustments to the biasing of the triodes, facilitating operation in Class C mode, which is ideal for QRP applications where power efficiency is critical.

The input impedance matching transformer, T1, plays a crucial role in ensuring that the amplifier can interface effectively with various transceivers. The construction of this transformer using a ferrite toroid enhances its performance by minimizing losses and improving the overall efficiency of the amplifier. The RF choke, L1, is an essential component for filtering unwanted frequencies, ensuring that the output signal remains clean and within the desired frequency range.

The choice of high-quality capacitors, particularly for C3, is vital in maintaining the integrity of the circuit under high voltage conditions. The use of a Pi-network for tuning further allows for flexibility in adjusting the amplifier for optimal performance across different frequency bands. The specific design details, such as the number of turns in the Pi-network coil and its physical dimensions, contribute to the overall effectiveness of the amplifier in achieving the desired output.

This amplifier design is particularly well-suited for amateur radio enthusiasts looking to enhance their QRP operations while enjoying the unique characteristics of vacuum tube technology. The synergy between the components and the thoughtful design choices culminate in a versatile and powerful amplifier capable of delivering impressive performance in a compact form factor.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: