Radiophile.com - Tiny Tim model no. unknown

Tiny Tim model no. unknown, ca. 1939

This truly tiny (7-1/2 inches / 19 cm wide) radio uses the same cabinet and the same chassis as its sibling, the G-E GD-520. Both radios were made by the Continental Radio & Television Co., maker of Admiral radios, and use C.R.&T.C. chassis no. 5J. Information about the G-E version is readily available, but there's nothing in any of my references about this Tiny Tim model. Since it's essentially the same radio, I'm assuming that the date of manufacture is approximately the same too.

The Radiophile.com method for eliminating those troublesome resistor line cords
When I received this set, its original resistor "curtain burner" line cord was very frayed and dangerous looking, so it really had to be replaced. When faced with this problem, I usually go to great lengths to secure an original type replacement resistor line cord. Well, resistor line cords are getting quite hard to find, so this time I thought I'd try a different approach to get the radio working nicely. I converted the set to non-resistor-line cord operation by using tubes slightly more modern that those available when the set was designed. Although not an "original" chassis restoration, this type of modification is an easy way to remove the resistor-line-cord hazards from sets that you might want to play a lot.

The tube complement for this chassis is 6A8GT mixer/oscillator, 6K7GT IF, 6Q7GT detector, 25L6GT AF output, 25Z6GT rectifier. This is an early version of the [From a 1940 Sonora brochure]

All American Five tube set, and it turns out to be quite easy to substitute later tubes that perform the same functions. The filament voltages of the original tubes add up to approximately 68 volts. All of the tubes have 300-mA filaments, so the filament string dissipates 20 watts total. The resistor line cord must drop the difference in voltage between the line voltage and the voltage needed by the filament string. This is 52 volts, and at 300 mA, the line cord would need to have a resistance of approx. 165 ohms (which agrees with the value that's marked on the original schematic). Also, the cord would need to dissipate approx. 16 watts. That's why resistor line cords get warm in operation.

Well, that was "state of the art," for cheap midget sets anyway, in 1939. But here in the future, we can look back to 1940, when a whole new series of tubes was introduced, with higher filament voltages and a lower (150 mA) filament current. Let's revamp the 1939 design to use the 1940 technology.

Function	Original tube	New tube
Mixer/oscillator	6A8GT	12A8GT
IF amplifier	6K7GT	12K7GT
Detector	6Q7GT	12Q7GT
Audio output	25L6GT	50L6GT
Rectifier	25Z6GT	35Z5GT

The radio works great with its new tube lineup. The filament voltages of the new tube set add up to approx. 121 volts, so no resistor line cord is needed since no extra voltage needs to be dropped. Also, the new filament string draws 150 mA, so the power dissipated by the tubes is now only 18 watts—a slight reduction from before for cooler operating temperatures inside the cabinet. All the tubes except the 35Z5 rectifier are direct plug-in replacements for the original types. In the case of the 35Z5, the socket pinout is different, so it must be re-wired, but it's easy to use any All American Five schematic that uses the 35Z5 as a guide, and simply duplicate the circuit, pilot light and all.

This method is adaptable to many other resistor-line-cord sets as well. For example, if one wanted to do a set like my Silvertone 6197, one could use the following substitutions.

Function	Original tube	New tube
RF amplifier	6K7	12K7
Detector	6F5	12F5
Audio output	25L6	50L6GT
Rectifier	25Z6	50Y7GT

The filament voltages of the new 150-mA tube lineup add up to roughly 124 volts, a perfect match to today's line voltages, and again eliminating the need for the resistor line cord completely. As in the Tiny Tim case above, all substitutions are plug-in replacements except the rectifier, which requires socket rewiring.