Friday, March 25, 2011


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.
* 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.


  1. As the old cartoons in QST used to say, "Switch to Safety!".
    I've been zapped pretty good a couple of times, but never on my own equipment (knock, knock), and I've always tried to instruct the younger hams that even a receiver can have deadly voltages, not the least of which is the 120 coming in.

  2. The "Switch To Safety" slogan and campaign was developed as a direct result of the electrocution death of Ross Hull (with an M.D. in the next room) and the similiar deaths of several other hams.

    --The OTs "got away with it" a lot, but not 100%. Fred Sutter's actually pretty good for the time but he makes some assumptions about his audience that are no longer accurate. Hams don't routinely sling that kind of juice these days.

  3. Since finding the mod iron from an old 173Kc (we worked with cycles then) 1750M rig I've been of the urge to build a variation of the OHS 160 (6aq5 modulates 6aq5( for 75M AM. I have several glow in the dark radios.

    I have good inverted L up and working on 75 so that a start.


  4. I promise to do my utmost not fry myself, anyone else, or even my cat using any design or suggestion I read here first.

    Going through old QST articles, I'm seeing some old designs "updated" for modern time. Some of those tweaks are for legality (keep your regenerative receiver from transmitting), some for safety. I'm leaning towards those plans for my first builds. Maybe even a heterodyne receiver instead of regeneative (but with tubes, of course. If it doesn't glow, I'm not interested).

  5. There are some nice simple superhets in the old Handbooks -- the Jones Super Gainer and ARRL's 1941 2-Tube Superhet are good examples. (Found here.) --His execution of the Jones receiver probably makes for a lot of control interaction; the oscillator and mixer should be better shielded from one another. I'll try to scan a Jones-built version as an example.

  6. If you want a receiver that is safer and also allow for more current RFI avoidance chek out a coopy of the ARRLs handbook from the 50s.

    I've built one of the 5 tube superhets and its a wonderful reciever and fun to listen to for 80 and 75M. there are no shortage of sites on the net for Glowbugs (use as search term).


  7. The three tube simplex super is an easy build and makes for a very useful radio. I've built that one as well.

    Roberta, please feature those links as they have great links to other Glowbugs.


  8. Via any of several links:
    Alles touristen und non-technischen looken peepers! Das machinkontrol is nicht for gefengerpoken und mittengrabben. Oderwise is easy schnappen der springenverk, blowenfus, undpoppencorken mit spitzensparken. Der machine is diggen by experten only. Is nicht fur geverken by das dumpkopfen. Das rubber necken sightseenen keepen das cotton-picken hands in das pockets. So relaxen, und vatchen das blinkenlights.