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The 2007 Radio Workshop

Allen Lein's "Baby Sweeper"
Baby Sweeper

The 2007 Radio Workshop at the Pavek Museum was held on February 18th, the 99th anniversary of Joe Pavek's birth date.  It began with Allen Lein’s demo of his upgraded “Sweeper.” If you have ever attempted to sweep an AM  IF, you’ll appreciate the ease of operation and ingenuity of design that Allen’s device offers. With 19 integrated circuits, it’s a lot of work to build, but performs flawlessly. All of Allen's schematics are linked to the caption below the picture.

Allen’s demo was followed by a brief analysis of plate curves and the information that can be learned from them, including: dynamic plate resistance, amplification factor, mutual conductance, and the proper load values for resistance coupled amplifiers. We even built a few special circuits to demonstrate how a vacuum tube operates under real conditions, including a 12AX7 gain stage with fixed bias and variable plate resistance to show how changing plate loads affect gain.

We then brought out the old Hickok 532 “classroom demonstrator” to show how dynamic mutual conductance was determined with the patented Hickok circuit. We picked this up some years ago from Dr. Frederick Koelsch, a chemistry professor at the U of M. It’s the only one I’ve ever seen. The bottom half is a conventional 532 tube tester, but the top half, inside the lid, has a Bakelite panel with 4 milliammeters and an engraving of the Hickok circuit. The meter on top shows the total plate current through the tube under test. The meter on the left shows the plate current when grid voltage is most negative, while the meter at the right shows plate current with the grid signal voltage at zero.  The meter at the bottom shows the difference between the two states and is directly calibrated in micromhos.  The keys to the performance of the Hickok circuit are accurately calibrated potentiometers, a sensitive and accurate meter, and the absolute predictability of the phase relationships between the secondary voltages of the power transformer.

Hickok 532 Demonstrator
Hickok 532

Then, what we originally planned to be the grand finale’, our old Tektronix 575 Transistor curve tracer converted to test vacuum tubes, was brought out. This whole project started years ago when Paul March of Amateur and Antique Radio Consignment Center in Saint Paul donated the 575 to the Museum. Several years later, Murlan Kaufman from Tektronix stopped in and showed us how to test tubes with it. It’s taken all these years to finally put together the presentation. The nicest thing about the Tektronix 575 is that, with 200 volts available for the collector supply, it’s just right for testing small triodes. And since it has two identical “plug in” ports, it’s perfect for comparing the two halves of a dual triode like a 12AX7 or 6DJ8. Best of all, we were able to do all of this without any modifications to the venerable old Tek. One of our volunteers, Jon Lieberg built a special plug-in adapter. We had to add an external filament transformer, but that was it. The old 575 ran flawlessly..

Tektronix 575
tek 575

This could have been the end of the program, but Alan Douglas’ recent work with curve tracers and the circuit published in his Tube Testers and Classic Electronic Test Gear inspired us to try building one ourselves. It turned out to be a lot more difficult than we expected.

Transfer Curve Tracer

As you can see, it's quite a kluge, but does allow the user to interpret mutual conductance directly from the scale on the scope.  Thanks to volunteers Dick Sigurdson and Glenn Davis for making it happen. Rumor has it that Allen Lein has perfected a new, direct reading transfer curve tracer that will compare two tubes at once. 

Bill Werner with his crystal set driven motor
crystal set motor

Finally, as an unexpected bonus, Bill Werner stopped in to show us a motor whose operating voltage is derived from a crystal set tuned to WCCO.   We connected a signal generator to the antenna terminal and it took off.  It's a beautiful device whose only "flaw" is the damaged top; the result of a lightning strike two years ago.

Thanks to everyone who helped make this the first Radio Workshop in recent memory where everything actually worked!

Steve Raymer      

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