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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.
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:
After many trials and some architectural changes, the final configuration I reached is:
The block diagram shows how the blocks are related and switched between RX and TX. To simplify, the power supply section is omitted. Each block works in both RX and TX, therefore many switches are needed to reverse the signal path from antenna to audio and vice-versa. 3 relays, for a total of 8 switches, are simultaneously driven by the PTT switch on the microphone. Apart of this "reversible" architecture, it's still possible to recognize a quite plain single conversion SSB receiver/transmitter. The big step is done by the tubes: just a 4 stages lineup is capable of about 140dB total gain from antenna to speaker.
(All relays in RX position)
Blocks reversibility requires a certain uniformity of drive and load impedances at their ports. Wideband and narrowband impedance transformers have been used to unify impedances to a relatively small value (300~400ohms, excluding the ports that connect to the antenna). This low impedance works well with the SSB ladder filter and the diode mixers. It also helps to avoid self-oscillations of gain blocks due to parasitic capacitive coupling between relays sections.
Diode mixers are intrinsically reversible, so they don't need to be "reversed", at the price of some conversion losses. Furthermore, they have enough dynamic range to handle the weak signals at reception and the large ones at transmission. Each oscillator output (the VXO and the BFO) is permanently connected to the LO ports of respective mixer and, by design, there will be no shift between RX and TX operating frequency.
The receiver has a simple AGC circuit, that will be better described later. AGC voltage and the S-meter drive current is obtained by rectifying the low-impedance audio at the output of the cathode-follower named AF/AGC buff. During transmission, the same buffer drives the AF side of the balanced modulator, that is a low-impedance port. Similarly, the AF signal section that amplifies the detected audio in RX, works as a microphone amplifier in TX.
Quite uncommon is the configuration that allows to the EL84 pentode to play the dual role of AF PA and RF PA with a limited use of relays. The signal to the grid G1 can be either AF from the audio preamplifier or the RF from the driver, and the plate load is made by two transformers in serie: during TX the output audio transformer is shorted and during RX the RF transformer is a short by itself for the audio signal.
The complete schematic shown here includes the supply section, but omits the low-voltage part needed to drive the relays coils. I've used a voltage doubler from the 6.3VAC of the tube heaters because I chose 12V relays.
(All relays in RX position, click on picture to open in new window)
Special parts list
For a better understanding, I will split its description in the two possible configurations (RX, TX) using two simplified diagrams.
Circuit Analysis in RX mode
You can refer to the complete schematic or to the simplified diagram below, that shows only the signal processing during reception. The circuit is quite straightforward, but I will give some details here that will be useful also for transmitting operation.
LO generation, as described before, is a VXO Clapp Oscillator, working at fundamental series-resonant frequency of CB crystals. The triode chosen, after some trials, is 1/2 of a 6BK7A (V5a), that has given the best output amplitude and VXO span comparing to other triodes from my small stock (12AX7, ECC82). Many, but not all, of the 20 CB crystals I own can span ±3KHz , so don't rely too much on this value. The worst-case to maintain the oscillation is when the variable capacitor is at the minimum value, thus it should be adjusted to a safe value if necessary. I've experimented that the VXO oscillator should be in some way buffered to avoid residual FM and the pulling effect that offsets the frequency between RX and TX. The cause is the variations of the load impedance offered by the mixer LO port. Having no room for a buffer tube, I managed to remove both problems by loosing the coupling of the oscillator output to the mixer. From 3Vpp on the V5a cathode, the amplitude drops to 1.2Vpp, that is still enough drive for the mixer. As an extra care, plate voltage is stabilized at 160V by a serie of two zener diodes.
The 5.2425MHz IF is filtered by the homebuilt, 4 poles ladder filter. 5 poles or more would be better for USB, but I had only 5 crystals in my hand, and one was needed for the BFO, so take it or leave it...
Well, finally we got the audio baseband, starting from a weak SSB signal at the antenna connector. We have used just 3 tubes so far, so we still have 2 in the budget to complete the project with AGC, S-meter and speaker audio.
Generally, the AGC voltage is generated by rectifying the amplified IF signal, but here the sum of RF+IF gain is not enough to get the -10V we need for full AGC. Another option, more suitable for this project, is to rectify the amplified AF. The pentode V3b, with a voltage gain ~150, amplifies the AF from the demodulator to feed simultaneously the volume potentiometer and the grid of the triode V3a, configured as catode follower. The cathode follower doesn't add any gain to the audio signal, but having a relatively low output impedance (~1/1000 of V3b output), not strictly necessary for the high-resistance AGC line, we need it to drive the low resistance coil of an S-meter. The same audio buffer, during transmission, works well to drive the low-impedance port of the balanced modulator, so we would need it anyway.
AF rectification is done in two ways: a Germanium diode for the S-Meter and a tube diode embedded in V2 for the AGC. Originally, the idea was to use the second one for both purposes, but I learned that its high resistance (tens of Kohm) is unsuitable to drive a 200uA FS microammeter. AGC times I found comfortable are about 40ms (attack) and 500ms (decay) with the component shown. It is possible to modify them by playing with R*C values, but not independently.
After the volume control we have the audio PA, operated by the power pentode V4 (EL84). The AF signals at its plate ignores the RF transformer T10 and goes totally to the audio output transformer T11, and then to the loudspeaker. I also put an headphone jack in the front panel of my prototype, not shown in the schematic.
OK, now the receiver is complete, but we have used all 5 tubes. So the question is: can we transmit with the same tubes?
Circuit Analysis in TX mode
It can be useful to refer also to the simplified schematic below, where all blocks are shown as they are connected during transmission. Many considerations and details described in receive mode apply also to transmit, so I don't repeat them.
Starting from the microphone, and going counter-clockwise, there is an RF blocking filter, then the V3b that, as told, amplifies the amplitude by 150 and feeds the cathode follower that drives at low impedance the balanced modulator. With the series trimmer I've been able to adjust modulation level with electret, piezo and dynamic microphones. The AGC detector has been disconnected so that the V1 and V2 will work at full gain. The unwanted LSB is filtered out and the SSB signal amplified by V2 is ready for the upconversion at 14MHz. The same RF bandpass filter used for the antenna signals is reused to reject the unwanted mixing products: yes, there is an impedance mismatch here, but I've verified that adding a 1:9 transformer here doesn't add anything.
As tradition rules, amateur tube equipments are built by point-to-point wiring, with components directly soldered to tube sockets pins and to insulated supports. Mine is no exception:
The chassis is open, with the tubes visible. On the top of the chassis there are mounted the supply transformer, the audio output transformer, one of the relais, one tunable RF transformer and two submodules. The square submodule close to the crystal set is the 1st mixer, while the rectangular one in the center contains the RF filter together with the SSB filter.
Front panel is essential with the few controls needed. On the two sides of the chassis, a touch of style with the wooden finish:
I've been pleased to make some DX with this radio (not many: I prefer to spend my time in my lab), mainly to test its operating capability. It's a QRP, so it's not a breeze: with an 1/2 wave dipole antenna I made contacts from Italy with north Europe, Spain and, my record, with Brazil. So the conclusion is: it works!
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