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I built 5 Strelow style solid state substitutes for 01A tubes and tested them in a Pearson - ERLA PS-5 TRF
Introduction:
Gary Strelow in an article in the February 1996 edition of the Antique Radio Classified Magazine describes under "Restoration Topics" "A Tube Substitute that glows". More specifically he proposed and built a substitute of a 01A type triode, which glows like a 01A, has volume control by a rheostat between the filament pins like a 01A - but does not look like a 01A. Thanks to the miniaturization of electronic components it is possible to hide the substitute in the base of a dud 01A, the glass flask of which has been temporarily removed and later glued back. The picture gallery below shows the steps to built 5 of those substitutes, and their integration into a 1925 Pearson - ERLA PS-5 5-tube Tuned Radio Frequency (TRF) receiver.
Implementation:
The substitute consists of a 6V micro-bulb (no base), replacing the filament string of the 01A, a Field Effect Transistor (FET), a photoresistor, a bias resistor and a coupling capacitor, as shown in (copyrighted) picture 9. Volume control in 1920's TRFs often is done by regulating the filament voltage. The TRF's unchanged circuitry causes the microbulb to brighten monotonically, when volume is increased, like the original filament wire. Optical coupling to a photoresistor, which sits in the source leg of a FET, causes its resistance to decrease and the FET's amplification to increase. Typical drain voltages are 18Volt (instead of typically 90 Volt plate voltage for the original 01A).
I bought a lot of 12 "good" Cunningham CX-301A on eBay, of which 5 were duds, 2 had subcritical gain and 5 were usable. I experimented with two duds, breaking their vacuum when trying to gain the space inside the glass flask. The silver liner immediately developed bubbles and looked bad and I discarded the two. I then used the remaining 5 bad tubes for the project.
Tests:
I tested the tube substitutes with a 1925 Pearson - ERLA PS-5 TRF. This is a rare radio, for which I did not find a schematics, but had to do it myself, as shown in (copyrighted) picture 7. Incidentally it was new for me that Pearson was not only an ERLA brand name (all components of the set are stamped ERLA), but itself a Chicago Corporation (see pict.5). I tested the radio first with regular 01A tubes, powered by 3 battery eliminators. The radio with the substitutes worked immediately, and with considerably more volume.
Power economy:
The 18 Volt supply should deliver 1-10mA drain voltages for 5 devices, that is max. 50mA.
A 9 Volt battery has about 500 mAh, so lasts at least 50 hours, rechargeable 250 mAh, so lasts at least 25 hours. A Radio Shack RC/car 9.6V/1600mAh Ni-MH battery pack sells on eBay for 8+6$. The 6 Volt supply has to deliver 36mA for 5 minibulbs, i.e. 180mA. A 1.5 Volt D battery has 18000 mAh, so lasts 100 hours, rechargeable has 6000 mAh, so lasts 33 hours.
Investors wanted:
To commercialize these substitutes, the main problem is to find a company, which does the (simple single-layer) round printed circuits, without asking up-front 1000$ for prototyping. The 5 components needed cost less than 1$ in quantities of 100. Please send me an e-mail if you have a good idea.
Gallery: (watch descriptive legends under pictures, when mousing over thumbnails) |
