Sunday, March 27, 2011

A TRANSMITTER POWER SUPPLY

It's surprising how quickly you get into trouble, fiddling with power supply designs. Duncan's PSU2 software does help keep the worst mistakes inside the computer instead of on the test bench, but there's no substitute for the real thing.

Fred Sutter's original "QST-40" power supply is a nice study in empirical design. He used a choke-input supply for stability under varying loads and a fairly low-resistance bleeder resistor to keep the B+ from soaring between Morse elements. The power transformer is one a contemporary ham could have readily salvaged and the rest of the parts were pretty stock -- in 1938.

Today, we have fewer choices. Specs are more stringent, too; there's usually more safety margin for a given current rating and modern measurements of choke values are less optimistic. Hammond transformer has probably got the widest range and availability of any outfit still making tube-suitable iron, so I've worked from their catalog. It's good news and bad news; the plate transformer will run $80 to $100 and the filter choke is another $46 to $50, but you can get them;* the rest of the parts are quite reasonable.

Here's the untested, not-yet-built power supply so far:(Click on it for a large version) There are a couple of options for the transformer and different filter circuits for each one -- either a 279X (850VCT at 170 mA) using a choke-input filter (leave out the 8 uF filter condenser!) or a 273BX (700 VCT, 175 mA), with a pi-section filter. As I have noted on the drawing, you really don't want to try the pi-section filter with the higher-voltage transformer.

5U4 or 5Z3 is your choice; the former is an octal tube, the latter 4-pin but they're otherwise the same. 83V is another possibility.

I also detail two options for the bleeder resistor, which additionally supplies lower B+ voltage to the screen of the 6L6.

The biggest change is in how the B+ is turned on and off; the five and dime no longer sells nice ceramic-based knife switches (more's the pity) and even if they did, it's not a very user-friendly means to that end. So instead, I added separate filament transformers for the 5u4/5Z3 rectifier and 6L6. This results in two good things and one "gotcha:"

1. Now you can turn the B+ on and off by switching the primary voltage; you can even do fancy pilot lights for filament and plate voltage, just like the big rigs!

2. With no heater voltage being drawn, the plate transformer's got a little oomph to spare. (If it has too much, Hammond provides primary taps for 115 and 125V mains).

3. Mind that heater transformer for the rectifier! It's only providing 5V to the tube but it's got the full B+ voltage on the secondary. Hammond hi-pots them at 2kV and it uses "two-bobbin" construction, with primary and secondary on their own plastic spools, but fabricat emptor caveat fabor, or however Cicero would've said it.

The output connector is notional at best; you can still find the old Amphenol plugs and sockets at places like Radio Daze and hamfests. A terminal strip would work as well -- just keep your pets and extremities clear!

I'll close by repeating that this circuit has not been built and tested. While I have done my best to predict how it might behave, I cannot verify its safety or suitability; proceed at your own risk. (On first power-up, I'd want a big old 100W or 60W incandescent in series with the hot side of the incoming power, and I'd use a GFI-switched outlet strip to turn it off and on. I'm not much for excitement.)
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* Angela Instruments, listed under "Sources" in the sidebar, seems to have the best prices at this writing. Antique Electronic Supply, Radio Daze and Parts Express all stock Hammond transformers.

TYPEWRITER DOCTOR

There are fewer typewriter shops today than there were last week. There will probably be fewer tomorrow.

--But the ones that are left are generally run by men (and women) who wouldn't choose anything else. For instance, Cambridge Typewriter in Boston, recently visited by Marko of The Munchkin Wrangler. It looks like a wonderful place! (And the owner has a blog, too).

When you find a typewriter shop, don't keep it a secret; we need to make sure they've got plenty of work, so they'll still be around when our machines need attention.

Friday, March 25, 2011

ABOUT THE QSL TRANSMITTERS

First, a quick note about safety. Fred Sutter, W8QBW, was of retirement age in 1938. His designs suggest a fellow who had been working with radio for many years and like most of his peers, Mr. Sutter was more casual about high voltage than most hams are today; he assumed his readers know the risks.

He got away with it because he was very aware of the dangers. You probably haven't spent your ham career being "tickled" by the occasional contact with B+ batteries; you are unlikely to have ever used a #2 pencil to draw a hissing arc from the tank coil of a transmitter and it is almost certain you have never keyed a spark transmitter operating from wall-socket juice. So you and I don't have quite the same learned reflexes the old-timers had.

One of the less obvious "gotchas" in the QSL-40 is the DC power connection; it's a 5 or 6-pin socket on the back of the transmitter and the output from the power supply is connected to a plug. That's dangerous; there's all manner of juice on those pins. You could still buy that kind of plug and socket new through last year -- it looks as if Cooper Interconnect has discontinued them -- but they work just as well with the plug mounted to the transmitter side and the socket on the power supply.

The big RF coil's connections are "live," too; you must keep your fingers clear of them when tuning the transmitter. For that matter, the tuning knob provides the only isolation from HV DC on the shaft of the tuning condenser. He picked the right knob for the job, an Ohmite rheostat knob* made of real Bakelite with a deeply-recessed setscrew, but you probably haven't got one of those handy. So that may need to be designed around. The power supply design is simply unsafe; he used a ceramic-base knife switch to turn the B+ off and on, at a point 600 VAC above ground. He took precautions, but he had experience and parts that cannot be obtained today. I'm working on a "modernized" redesign of the power supply and hope to have a tentative version posted some time this weekend.

There's a little about safety. Now, about history--

There was a whole series of "QSL" rigs, from a 5W transformerless "portable" you could hold in one hand (though not when it was plugged in!) to a 100W push-pull design, with side ventures into a tiny receiver, making "air-wound" RF coils and the use of pilot lights as current indicators.

It had to start somewhere. It didn't start where you'd think or even with the goal you might expect:"Radio" was, in some ways, the West Coast competition to the ARRL's "QST." Formed by the merger of "R/9," a smaller ham magazine with very high production values and the original "Radio," a large-format pulp that evolved from a general-radio magazine to one intended for ham radio operators, it had its own style and tone, a little slicker than that of "QST" and focused on building equipment and operating -- mostly, chasing DX. ("Radio" eventually gave rise to today's "CQ" magazine.)

In his letter to the editor of "Radio," W8QBW is thinking about a 6L6 power oscillator merely as a driver for some big tubes, operating at several hundred Watts.

Instead, once he'd got his prototype working, he put it on the air -- and found himself on a very different path.
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* Many models of rheostats had "live" shafts by design. So the knobs were designed to provide a level of protection the 1930s found adequate. Ohmite's design was ubiquitous and worked well; it's pretty much the default choice for hams and experimenters from the late 20s through WW II.

THE QSL-40

This is an experiment. It is for amateur use only and I may have to take it down, the copyright holder being very much with us (and well worth your time, attention and memberhip).

But for now, a quick look at the project article that launched Fred Sutter's "QSL" transmitters, all using some version of the 6L6 tube:Readers with an eye to history -- or who notice the cut-and-paste -- should be aware the original schematic left out the cathode bypass condenser, replacing it with a direct connection. That meant the telegraph key wouldn't have worked, the transmitter would have been on all the time. QST published a corrected schematic the following month and I pasted that over the erroneous one.

(I have built my own copy of this little transmitter.)

For the record, this transmitter has a lot of exposed high voltage. It is inherently dangerous. Before building and operating such equipment, be certain you understand the risks.

Wednesday, March 23, 2011

THE 6L6 JOB

Not a spy movie but a family of homebrew ham transmitters. The 6L6 is a beam power pentode vacuum tube introduced by RCA in 1936 that kind of caught on -- and caught on big: they're still making them. You'll find them in guitar amplifiers and some esoteric hi-fi amps, too (though purists there are now into triodes and are keeping the WeCo 300B in production for the rest of us).

Hams quickly latched onto the new tube -- it was relatively inexpensive for the power level -- and even though it was marketed as an audio amplifier, the lost no time in seeing if the 6L6 was happy with RF.

It was. Eventually, even RCA had to admit it and someone in the transmitting tube managed to sneak the 807 into production -- essentially, a souped-up, RF-ready 6L6.* But the cheaper receiving tube had won a place in the amateur pantheon and went on the become the heart of thousands of little transmitter designs. (There were plenty of 807 ham rigs built, too).

Fred Sutter's "QSL" series of transmitters -- so named because the little rigs had a footprint no larger than a 4" x 5" postcard (hams exchange QSL cards to confirm two-way contact: talk to enough people, far enough away, and you can get a fancy award!) -- were justly famous; mine is safely (I hope) stashed away Somewhere Around here. Frank Jones (of Radio magazine ham handbook fame) offered his own versions (schematic and a nice example at this link) and I have built a couple of those: The first one is a sort-of breadboard: the DC and heater wiring is all underneath. The empty socket at the left is for the frequency-determining crystal, the first variable condenser is plate tank tuning and the second one is output loading. But its a 1930s design, with relatively low-C (50 pF) and the output circuit is not a pi-network: the "loading" condenser is fed from a center-tap on the tank coil. This means harmonics do get out; at my former location, the first time I fired it up on 80 meters, the ham down the alley was knocking on my hamshack window in minutes, having copied my callsign via his TV receiver. (It did okay on 40 meters). This little rig worked all over the continental U.S. The frames of both tuning condensers are "hot" with plate voltage, by the way, but stubby insulating couplings and nice big knobs reduce the odds of getting bit. (Key jack, lower left, is in the cathode-ground circuit -- touch the cathode side when the key is open and you'll get a nasty tingle).

Another 6L6 rig, I built in a candy tin. Shown here sitting atop my RME-45 receiver to show how small it is (and to show off the receiver; I was copying W1AW and trying to keep up with some QRS guys earlier tonight, with a 6' hank of wire for an antenna since there was a storm outside: RME built good receivers). There should be a coil plugged in the socket at the right -- my box of transmitting coils is also Around Here Somewhere. This one does have a newer-type pi-net output, though a little more C on the output side wouldn't hurt. The crystal socket is out of sight behind the tube. And here's the mysterious inner workings -- a couple of RF chokes (plus the tinned-busbar VHF choke just left of center -- the breadboard rig has one, too), a voltage divider for the screen, grid resistor, feedback condenser, cathode resistor and a whole lot of bypass condensers. The BNC connector was for a VFO but I never wired it up. there are a couple of little braces soldered in place, too -- the sheet metal candy tins are made of isn't as sturdy as a real chassis. Oh, don't be fooled by the meter scale (0 to 5 Amps); that's one of a batch of meters surplussed from a 1950s RCA commercial transmitter, all 1 Ma/1 V/1kOhm. Very easy to shunt for the range desired, in this case 0-100 mA).
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*All-glass, with a low-loss base; 6L6s were made in both metal and glass-envelope variants. Much later, the 807 begat the TV receiver sweep tube 6BG6, little more than an 807 with an octal base. The 6L6 was scaled down (6V6), scaled up (6550) and made in industrial versions, too.

Tuesday, March 15, 2011

A REMARKABLE GIFT

I suppose you could argue it's not a gift, as it replaces most of the content in the Spring number of The Indiana Historical Radio Society Bulletin. It was nevertheless wholly unexpected and quite delightful:It's a crystal set, sent out to commemorate the 40th Anniversary of this fine organization.

Like (I suspect) most members, I took mine to the radio room immediately and hooked it up. It works just fine; one tunes it by fanning out the hank of wire into a section of a torus; a full half-donut hits near the upper end of the broadcast band and a few degrees in from there finds WNDE (1260 kc/s), clear as a bell and plenty loud.

This nicely-framed xtal set is a variation on the "postcard crystal radio" of days gone by, a little bit of the past brought forward.

Sunday, March 13, 2011

CLEAN SIGNALS FOR 1929

It was the (international!) law, but it would have been a good idea even if it hadn't been required: beginning in 1929, radio amateurs had to build their transmitter to meet nominal requirements for stability and spectral purity.

The rules weren't especially stringent; existing designs could be adapted and a beginner's transmitter wouldn't cost any more, though it might take more time to adjust. The benefit was that sharp, stable signals get through where wobbly, "squeggy" ones would prove too hard to read.

...It happens that for radiotelegraphy, a "1929 type" transmitter still makes signals stable enough and clean enough for amateur use. Even if the other ham is running a software-based radio on a plug-in computer card, he (or she) can tune it in.

And you can build one! Canadian ham VE7SL shows how, complete with original schematic, recent photos, modern schematic and ways to couple it into your coax-fed antenna, and then ends with a photographic round-up of 1929-style breadboard rigs by a dozen-odd builders. (And if you'd rather build Ross Hull's wonderful 1929 Hartley, why, he'll show you that one, too!)

I'm going to have dust mine off. H'mm, I need to find me a type '45 or '10 transmitting tube.*
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* The ' in place of a first digit replaces the manufacture's ID number -- "2" for RCA, for example.

Saturday, March 5, 2011

THE FIRST AIRCRAFT CARRIERS

They not be quite what you're picturing; nowadays, "aircraft carrier" summons up an image of a floating airfield nearly the size of a Georgia county.*

But you have to start somewhere; the British started before the 20th Century was two decades old, with the Thorneycroft Seaplane Lighter: a ship. That carried aircraft. An aircraft, in fact: singular. And it was on its own for the taking off and landing.

It might sound a little mad but the concept worked so well for the Royal Navy that they built 46 of them. There was one "survivor" known, but it was stuck in the mud and corroded away.

And then a maritime journalist spotted one in the Thames -- stuck, rusting, but remarkably intact!

It is presently being restored and will be displayed at the Fleet Air Arm Musem: an aircraft carrier not much larger than a modest modern houseboat; but an aircraft carrier all the same.
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* Originally required to have no border farther than a half-day's horseback ride from the county seat. Not a bad idea, especially in light of modern-day fuel prices.

Friday, March 4, 2011

USEFUL LINKS

Clock parts -- for instance, the easily-breakable glass over the dial -- gears, hands and a staggering lot more: Timesavers. I may be able to put my vintage hamshack clock back in service!

Battery eliminators for very old radios: delightful versions housed in cases styled like classic radio batteries can be found at Antique Radios, Inc. Or you can get them with a plain case, if you'd rather. Since the older battery sets usually have you juggling a high-current 6V "A" battery (lead-acid, originally, and who wouldn't want that in the parlor?), several "B" batteries of up to 180 Volts (if you'd gone all out and bought a big loudspeaker) and "C" batteries for tube bias in the 4.5 to 45 Volt range, you can see that a nice battery eliminator would be a thing of beauty. Back in the days when wall-socket juice was the most modern of modern conveniences, a little company called Raytheon turned exactly that product into a lasting legacy; they're still building high-end electronic widgetry today.

The source or inspiration of these links would be FB ham operator W1UJR. If you like classic ham gear, you'll want to see what's in his hamshack.

Thursday, March 3, 2011

IT'S THE ULTIMATE

At first glance, you might well ask, "Ultimate what?"
[click for a much larger version]
The "73" Ultimate Telegraph Transmitter, is what; more familiarly, a telegraph key. This company built a dizzying array of versions and sizes, from fancy all-chrome versions with a high degree of fit and finish to lower-priced amateur editions with exactly enough hand finishing to function. The majority of them were squarish (like mine) and came with a nice metal cover (missing from mine). They used at least two different operating mechanisms, with the one shown above being a little more common. They are very small -- 2-1/8" by 3-3/8" is typical, though at least one was much smaller. And it is the amateur version, solid but a little rough.

Mine has two levers; not all of these keys do. The right-hand one is moved to the left and makes a simple contact closure. The left-hand, moved to the left, does the exact same thing. So why's the other one there? I don't know.

The fun begins when you move the left-hand lever to the right. That's where the semiautomatic action starts to make the "dits" (usually written as "dots") for you. Like most other bug keys, it uses a weighted pendulum and a flat spring or reed to bounce back and forth; but the way this key shoves the pendulum into motion is madly complex, a square ascending spiral involving three levers, two pivots and a roller! I've tried to trace it:
[click for a larger version]
Start by finding the pendulum; it runs left to right across the center of the key. Got it? That's where we'll end up. Now, move your attention to the black fingerpiece at the very bottom; the pink arrow shows what direction you'd move it. From there on up and around, the fuzzy yellow line follows the mechanism and the pink arrows show direction of movement, The lever attached to the fingerpiece moves a second lever (which it pivots away from if you moved the fingerpiece the other way but they move together in this direction); the second lever is pivoted and at its far upper end, it pushes a roller to the left. The roller is in one end of another lever shaped like a squared-off "C" and the bottom of it holds the reed and pendulum. The C-shaped lever is pivoted at its bottom left corner and thus, when the upper end is moved, the bottom end moves away from you, bumps into a stop screw and -- at last! -- the pendulum begins to vibrate. There's a little electrical contact on the pendulum arm, on its own flat, U-shaped spring, which is what actually makes the dit. Whew! When you let go, a little spring next to the stop screw pushes everything back to the at rest position.
And if you weren't confused enough, here's a back view that shows the around-two-corners lever a little more clearly.

I first saw one of these keys in 1996 at a railroad museum in French Lick, Indiana; it was in a display case and I was unable to get a clear enough look at it to figure out how it worked. I've wanted one ever since.

Photo links are from this page at Tom Perera's online museum, where a key-fancier can easily spend hours. I was able to find a few more examples via a websearch as well.

Tuesday, March 1, 2011

MCELROY "JUNIOR," AGAIN

I have written of them in the past and a reader recently pointed out another one: the McElroy "Junior," a semi-automatic telegraph key built for the ham or telegrapher of limited means.

The base and frame were made of stamped and welded sheet metal and they were the least-expensive bug ever made. They're also vanishingly rare, which leads to an interesting quirk: while you can find a nice Mac "T-bar" bug for about $500 and his high-end models for up to a couple hundred more, the humble little "Junior" sells for as much as -- see for yourself:Yes, that's $1026.01. I did not bid on it; bidding rises early on these and keeps on going up.

I do note, very typical of McElroy, that the inner construction appears to be quite robust and seems to include some kind of additional pivot support; mechanically it is a better key than first glance might suggest.