Figure 7-2 Doerle receiver inside view in Software

Generator UPC - 13 in Software Figure 7-2 Doerle receiver inside view

Figure 7-3 Original #19 dual triode receiver coupled through a small 15 pfd screwdriver adjusted variable capacitor to the main tuning coil wound on a four pin form A 10 pfd variable capacitor across it gives a fairly good trade-off between slow tuning rate and wide tuning range The signal is fed through a grid leak configuration to one of the grids of the #19 tube From the plate, the signal goes to the tickler wound on the same four pin coil form The top side of the tickler coil connects to a 500 pfd capacitor and a 21 mH radio frequency choke (RF) The choke allows DC plate voltage into the tickler but prevents the radio frequency (RF) alternating current from getting into the rest of the circuit The 500 pfd capacitor takes the RF to ground, thereby allowing RF voltage to build up on the tickler winding Without this 500 pfd capacitor the receiver would probably not regenerate properly Variations in the DC voltage delivered to the detector plate feed through a 1 mfd capacitor to the second grid A 250K, 1 2 watt resistor pulls the grid to DC ground, to provide the necessary zero bias for this class B amplifier tube, yet the resistance is high enough so that the AC avoids the ground and readily flows into the second grid Voltage on the detector plate is changed by a 50K resistor attached to the wiper of a 50K regeneration potentiometer that is connected from the B+ battery to ground Plate current in the detector flows through these two resistors Variations in current creates a varying voltage drop across this pair The varying voltage is the audio signal that is applied to grid of the audio triode The B+ voltage source is fed through headphones to the second plate to complete the audio amplifier part of the circuit The sound that is heard in the headphones comes from the variations in the audio plate current A 006 mfd capacitor on the plate is large enough to take any stray RF to ground, but is not so large to take audio frequency (AF) voltage to ground This capacitor aids receiver stability and audio quality A 20 ohm rheostat in series with one of the filament legs allows adjustment of the voltage A rheostat is a low resistance, high wattage potentiometer In the 1930s, you would have used two No 6 15 volt dry cells (they were about 6 tall and 2 3 4 diameter) as your filament power supply The rheostat would cut the voltage down to two volts needed by the tube s filament As the voltage provided by the batteries dropped with use, you would adjust the rheostat to maintain the proper voltage on the filament In other words, the rheostat allowed you to continue using the filament batteries as their voltage dropped to the point of total failure Since the

filament current for a #19 triode tube is just over 250 mA, so a 20 ohm resistor would allow you to use filament batteries up to 6 volts And quite frequently that 6 volts was the battery borrowed from the family car

The first step in building this or any project is getting the parts In fact, this may be the most difficult step When it comes to building a replica of an old radio like this, you just cannot go out and buy everything you need You may have to adapt and use whatever you have on hand or can scavenge For a project of this type you may want to build the circuit using point-to-point wiring between terminal and components rather than a circuit board to give the radio an old-time look You are probably going to need both a soldering iron and a soldering gun to build this radio You will also want to consult with the resistor color code chart in Table 7-1 and the capacitor code chart in Table 7-2 in order to identify the correct components Building this old radio can be lots of fun, but locating all of these old components can be difficult, and in fact you won t be able to locate the exact same items You ll

get something close You ll have to mix and match whatever you can find Most of these components are available at flea markets if you visit enough of them Many of the components are available new from dealers, such as Antique Radio Supply, see Appendix But often you ll have to pay substantially higher yet fair prices The used #19 tube was purchased from a flea market dealer for $5 Yet, that same tube can be obtained brand new from Antique Electronic Supply for about $6, see Appendix Again, you may have to scrounge, make do, adapt, experiment, and make a few mistakes But the results are well worth it One idea is to build a prototype receiver with whatever components you can find, no matter how new or old, and get it working Then, over the next few years, visit flea markets, antique radio meets, check out dealers catalogs, and accumulate the older, more authentic parts After you accumulate some of those rare, old, beautiful parts from the 20% and 30%, rebuild the receiver And then a few years later rebuild it again And every time you rebuild it, add older and more authentic parts, until you get an exact copy of the #19 Doerle receiver The radio will become more and more of an authentic replica of a receiver from the early days of radio With patience, practice, and plenty of enjoyment, you can build a fine radio Looking at the illustration of the Doerle-1 version, we see in the center a large, slow-motion dial drive