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  • Writer's pictureDan Koellen AI6XG

QRP Tube: Vacuum Tube, Battery, Lightweight, Compact, Portable: Can They Coexist?

Updated: Apr 2

I enjoy SOTA, vintage electronics, and building; so why not incorporate all into one project? This project is a tube transmitter for 40 meter CW than can be powered with a relatively small battery, lightweight and compact enough to fit in a backpack, and interfaces to a portable receiver for transceiver like operation. And throw in a keyer for use with paddles, RF monitoring, semi-QSK operation and use of technology from several different decades.

This post is part of the series about a backpackable vacuum tube based transmitter teamed with a QCX receiver for portable SOTA activation. Other posts are the detailed descriptions of the tube transmitter, high voltage supply evaluation and QCX interfacing, modification and antenna switching.


My first rigs were vacuum tube when I was a novice in the mid to late 1970s. I built vacuum tube transmitters that accompanied my Hallicrafters S-38 receiver for 40 meter CW station. The vacuum tube transmitters were able to load into a very low hanging dipole. Lacking a SWR meter, the dipole was not precisely tuned; a vacuum tube transmitter's tolerance of high SWR was fortunate. Now that SOTA portable operation is my operating focus, a vacuum transmitter may not be a bad idea considering the compromised antennas we sometimes have to run.

My research found a few amateurs still building tube transmitters. But those transmitters are not portable since the high voltage and filament supply required for tube operation are from a power supply with a heavy transformer operating off of mains supply. And the high voltage was generally 350 Vdc or greater. Not something I would want to have in my backpack hiking to a summit.

I found an article from the March 1967 Electronics Illustrated edition, '1 Bottle Xmitter for 40', which had potential. The power supply, though mains powered, was a voltage doubler of a 125 Vac secondary. winding providing about 250 Vdc, maybe less when loaded. This could be in the range of a switching power supply operating from a small battery. The vacuum tube used is a 6AQ5A for which a 12AQ5A could be substituted for operation from a 12 volt battery. The schematic is very simple and made to be assembled with little expense. The circuit depends on inter-element capacitance for output matching and positive feedback for the crystal oscillator, this does not provide reliable operation. This will be addressed later.

Original Schematic from March 1967 Electronics Illustrated

A few other oddities of the design is that the transmitter is keyed if the key is not plugged in, the slug variable output transformer and neon bulb RF indicator. Since all this can be easily addressed, this is a good circuit start design with.

The Concept

I want this to be more than a single tube transmitter but a full transceiver like setup than could be used for SOTA activations. The other elements do not need to be built with vacuum tubes. But, since we are using 1950s and 1960s technology for the transmitter, it would be nice to add in technology from other decades up to and including currently used technology. The elements I wanted to have for this SOTA setup:

  • 40 meter CW receiver

  • Transmit/Receive (T/R) switching

  • Keyer

  • Single small battery

  • Backpackable

  • RF monitor for summit tuning

  • semi-QSK

  • technology representative of multiple decades

I had an unassembled QCX 40 meter CW transceiver kit that could be used for this project since it incorporates a receiver and keyer. Obviously I could just assemble the complete QCX and meet the many of the elements listed above. But that would not be as much fun. Assembling the kit without the transmitter finals will provide a receiver and keyer, recent but not current technology, and potentially logic signals for T/R switching.

A T/R switch will also have to be designed and built to interface to the QCX receiver and the tube transmitter. Either PIN diode switching or relays can be used to switch between transmit and receive as well as protecting the receiver during transmitting.

The RF monitor of the transmitter's output will be an useful tuning and operating aid while setting up and operating the transmitter, especially on a summit. Previously I experimented with various QRP SWR bridges and found that a tandem match is sensitive and repeatable at QRP power levels. This is a likely candidate for the RF monitor.

The high voltage supply will be a challenge. A DC to DC, switching supply is an obvious candidate but they are known to be notoriously noisy, even into HF. And the supply needs to be able to supply several tens of milliamps at over 200 Vdc with very good efficiency to allow operation from a small battery. If I can't find an adequate candidate then a supply will have to be designed.

The overall concept is diagrammed below.

Vacuum Tube SOTA Concept

The Build

The build of this project will be described in separate sections. The assembly of the QCX will follow the standard assembly instructions except for the transmitter final stage. Modifications of the QCX will be discussed in the T/R Switch section. The T/R switch is built as an interface to the QCX with signals from the QCX brought directly to the T/R switch and the QCX LCD readout mounted on the TR switch board. A printed circuit board was designed for the T/R switch that plugs into the QCX LCD socket. The transmitter itself was built with two printed circuit boards and one perf board for the high voltage supply, assembled in a small wood box.

For the build details and more in-depth circuit discussion go to these posts.

Gerber and other files available in GitHub

The Outcome

It Worked!

SOTA summit W6/NC-402 was activated successfully using the vacuum tube transmitter, QCX receiver with T/R switch and interface, an end fed halfwave antenna, and a small battery powering up the transmitter and receiver. Everything easily fit in a backpack and was carried to the summit on a bushwhack hike. Seven 40 meter CW QSOs, including two summit to summit QSOs, were completed. The fun factor was a Spinal Tap 11 out of 10.

The Project's Objectives Were Met:

  • 40 meter CW receiver: the QCX was successfully used as the 40 meter receiver

  • Transmit/Receive (T/R) switching: the T/R switch interfaced directly with the QCX receiver and provided automatic antenna switching between the transmitter and receiver. The T/R switch also grounded the receiver input during transmit.

  • Keyer: the QCX keyer was safely interfaced through an isolated transmitter interface

  • Single small battery: a small 12 Volt 3 Ahr LiFePo battery was sufficient for activation

  • Backpackable: the complete system was carried in a small wooden box that was backpacked to the SOTA summit

  • RF monitor for summit tuning: a visual indication of RF output power was implemented using a tandem match and a LED dot graph

  • Semi-QSK: full QSK was achieved but semi-QSK was used to eliminate frequent and annoying relay switching.

  • Technology representative of multiple decades: several decades of technology were used as estimated below:

    • 1940s: miniature vacuum tubes

    • 1950s: pi and pi-L networks

    • 1960s: tandem match, general purpose NPN Si transistors

    • 1970s: LM555, 78L05

    • 1980s: LM3914

    • 1990s: LED bar graph display, isolated SSR

    • 2000s: LCD character display with no backlight

    • 2010s: QCX design, internal circuits from 1990s and 2000s

    • 2020s: NCH6300HV, KiCAD 6

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