[Helge Fyske (LA6NCA)] may not be an actual spy — then again, he may be; if he’s good at it, we wouldn’t know — but he has built a couple of neat vacuum tube spy radios in the past. And there’s no better test for such equipment than to haul it out into the field and try to make some contacts. But how do you power such things away from the bench?
To answer that question, skip ahead to the 3:18 mark of the video below, where [Helge] shows off his whole retro rig, including the compact 250-volt power supply he built for his two-tube 80-m Altoids tin spy transceiver. In the shack, [Helge] powers it with a bench power supply of his own design to provide the high anode voltage needed for the tubes, as well as 12 volts for their heaters. Portable operations require a more compact solution, preferably one that can be run off a battery small enough to pack in.
By building his power supply in a tin, [Helge] keeps to his compact build philosophy. But the circuit is all solid state, which is an interesting departure for him. The switch-mode supply uses a 4047 astable multivibrator chip as a 50-kHz oscillator, which switches back and forth between a pair of MOSFETs to drive a transformer. This steps up the 12-volt input to 280 volts AC, which is then rectified, filtered, and regulated to 250 volts DC.
To round out his spy rig, [Helge] also designed a tiny Morse key, which appears to be 3D printed and fits in its own tin, and a compact dipole antenna. Despite picking what appears to be a challenging location — the bottom of a steep-sided fjord — [Helge] was easily able to make contacts over a distance of 400 km. His noise floor was remarkably low, a testament to the solid design of his power supply. Including the sealed lead acid battery, the whole kit is compact and efficient, and it’s a nice example of what vacuum tubes and solid state can accomplish together.
Not to be a pooper, but looking at that motorcycle battery and those big long wire leads, there is absolutely nothing tiny about that setup.
It’s the 12 to 250V DC supply that’s small, not the battery. He could just as well power it from a little LiPo, but it’s not the battery that’s interesting here.
That’s a 12v lead-gel battery actually, very common in the amateur hobby. Way less dangerous than our typical car/automotive battery. It doesn’t outgas, (it’s sealed) and needs no maintenence (in the traditional sense). Because if this, they can be stored in closed rooms, too. Doing same with, say, a car battery could result in an explosion at some point. Because, if too much gas is being freed, it becomes a detonating gas. That’s why CBers hads stored their car batteries in safe places to drive their linears. The smart guys among them, I mean.
Reminds me of the SOE biscuit tin radio of WW2.
Building projects like this is really a labour of love. I wonder if it can be extend to DSB with a few more valves? G7VFY
Sure. Double Side Modulation (DSB) is just AM without a carrier.
If the carrier doesn’t need to be removed entirely (surpressed), but just lowered, then it’s not that difficult.
If you are using a Royer Oscillator then what about a capacitor across the two collector connections
Something about using a standard 3.5mm headphone jack and audio cable for 250V makes me very uncomfortable.
Right. They obviously never had heard about electro-mechanical relays.
Nostalgia or not, in these days, keying should always be done remotely, with galvanic insulation.
*Real* amateur-radio transceivers like the Yaesu FT-101 did use hybrid designs in a good way. Tubes+FETs, relays-switching transistors. A quality power supply (the high voltage for the tubes was *NOT* taken from the mains, but always provided by internal PSU). Modular Design..
Anyway, let’s don’t expect to much from these old timers. They mean good, but are kind of stuck in the past. Akin to Amiga users in the vintage computer hobby, maybe. They don’t even realize that young people may watch their videos, not knowing about the dangers of high-voltage.
Those old timers simply continue to do things the way they used to do. They’re perhaps not even aware of any safety concerns. I mean, many of these old tube radios weren’t even retro-fitted by enthusiasts with power cords that have protective ground.
The “old-timers” are what have enabled the younger generation to still have the hobby. No one that is not aware of ALL aspects of electricity should be anywhere neat a HV powersupply. Don’t they test anymore at the FCC?
N0OME
Another “flaw”, maybe. In the tube radio era, radio hobbyists had used “anode batteries” (90v DC).
Exactly because the mains were so dangerous! Too much current available, and AC at 50/60 Hz! Yikes!
That’s why radio tinkerers went a different way (for receiver radios).
They substituted the heater battery (6,3v) by a little transfomer that plugged into the mains.
Doing so resulted in a long battery lifetime. The anode battery was mainly used for providing voltage, only a little bit of current flew.
It was the heather battery that ran out quickly, so it made sense to substitute it by a transformer.
Back in the early 20th century, they were sold in toy stores, in the form of little transformers for electric model trains.
High voltage B-batteries are now hard to get. But it’s easy to make your own by plugging ten 9v PP3-size “transistor radio” batteries together in series. 1 watt at 250 volts is only 4ma, so the load is no problem and they will last a long time.
The other vintage ways to get high voltage were a vibrator, or a dynamotor (merged motor-generator). Both could be 80-90% efficient if designed right.
For battery operation, radios normally used tubes with directly heated cathodes. The filament power was an order of magnitude less than indirectly-heated cathodes.
I really hope that thos design here has a fuse (quick) and a current-limiter installed. And that a protective ground connection for the metal chassis is used. Otherwise, one side might be “hot”. Such a design shouldn’t be used anymore since the 1960s or so.
There is no “ground” for a battery powered device sitting on a picnic table. You could literally touch *anything* in the circuit with your bare feet in a puddle without getting a shock.
On the keying: While the voltage may be high, the current is low. Helge clearly knows what he’s doing! I doubt there’s anything he could touch to get a serious shock (without reaching inside the tins while the unit is operating).