"Five by Twenty"
A 5 tubes SSB QRP Transceiver for 20m Ham band
Can you build a complete SSB QRP transceiver with just 5 vacuum tubes? I actually did it, and it works without any transistor or IC.
A prize-winner project at MAdS 2006 by IW9ARO.
Many projects of homebuilt ham transceivers obey to some intriguing rules, that often are quite challenging in miniaturization, or refusing to use any Integrated Circuit, or being powered by a lemon. This project has a similar inspiration: only tubes and diodes (then no transistors or ICs at all). Looks easy? Let's try to do it with just 5 tubes, operating in true SSB, 5W transmission, and superhet RX decently driving a loudspeaker. This is not a theory project, but a real QRP transceiver to make real QSOs, and in fact it stands in my shack together with other radios I built or bought.
Nevertheless, it is not an easy design. And even if I'm trying here to give all informations to anyone willing to make a copy of it, skills, experience and instrumentation are needed to get it working. Bottom line, I hope you will like the ideas and solutions I used, and perhaps some of them will be re-used in your own tubes-radio design. Don't forget that the high voltages around vacuum tubes CAN KILL, so take extreme care when working with them.
Because of its original concept and the good execution, this project has won the first prize of the homebuilt equipment competition at MADS 2006 edition in Italy.
PROJECT CONCEPT AND TARGETS
To get the maximum possible from 5 tubes and a bunch of diodes, you cannot separate RX from TX. Each tube must play a role in either mode, similarly to what is normally done with the SSB filter in all commercial SSB transceiver. A wise choice of multi-function tubes (I.E. triode+pentode) has been exploited, still remaining in the range of easy-to-find tubes. Passive (diode) mixers are intrinsically reversible, making the two conversions from BaseBand to RF and vice-versa very straightforward.
Another important choice is the LO generation. A really usable SSB transceiver must stay on frequency, and a stable VFO is a tough design by its own with tubes. So I made my life more easy with a 9MHz VXO, using CB crystals oscillating at the fundamental frequency. A "coarse tune" rotary selector chooses one out of 12 CB crystals (can be more ore less), and a variable capacitor makes the fine tuning within ±3KHz.
From LO oscillator choice, it comes that the IF frequency is a non-standard (~5MHz), so the SSB IF filter has to be designed, built and aligned. With stock 5.2428MHz crystals I built a quite good 2KHz BW ladder filter, whose characteristics are shown later on these pages.
TX output power is an important parameter that affects design choices. With a simple design and one EL84, 10~15WPEP is an affordable target that I actually missed, mostly because of HT limitations. But I'm happy with my 5W pure QRP, expecting DX contacts as good as my FT817 with an 1/2wave dipole does.
So, in summary, what I request to my 5 tubes is:
- True USB
- Covering the SSB portion of 20m ham band
- VXO with crystal selector and fine tuning +/-3KHz
- Superhet RX, sensitivity <0.2uV
- Driving a loudspeaker, >0.5W
- TX PEP power: >5W
- No semiconductor components, with exception (sorry...) of diodes