Crosley 51

Crosley Model 51

I tend to avoid sets with fewer than three tubes because they seldom have sufficient sensitivity and selectivity to be useful receivers in the home. (I enjoy rotating my old radios around the house, using them for baseball games, news, etc.) This 2-tube Crosley 51 (detector plus audio amplifier), while certainly interesting, historic, and very collectible, is of limited practicality by today’s standards.

While it has enough power to drive a high-impedance loudspeaker or earphones, the 51’s front-end RF tuning bombs in trying to separate nearby, strong broadcast stations. I have a 50,000W “blowtorch” located about a mile from the house, and its signal gets smeared all over this set’s tuning band. I have a separate antenna tuner that helps some, but that strong station can jump right into the guts of the radio.

This 1924-vintage radio is a regenerative set, with the audio output of the first tube, the detector, fed back via a tickler coil in the detector plate circuit, increasing amplification. Look at the two photos with the radio out of the case. The feedback is adjusted by the spacing of the two circular coils, which controls their mutual coupling.

At the right side of the chassis, looking from the rear, is Crosley’s interesting “book” condenser for tuning. There are two foil-lined, hinged blocks, similar to leaves of a book; the tuning knob cracks opens and closes the “book,” changing the capacitance for tuning. There is a mica sheet between the two blocks for insulation. Pretty crude, but it’s cost effective.

Perched on top of the book condenser is the glass-cartridge grid-leak resistor. I taped the resistor’s ends to keep it out of the circuit, and I’ve concealed a replacement 2-M resistor under the grid-leak holder.

Now, look at the left side of the radio out of its case. I installed the 3.6-V lithium battery to conveniently supply the C- bias voltage; others may prefer to just supply the C source from the same battery eliminator used for the A and B+ voltages, but I often use a lithium battery to cut down on the number of hookup wires between a battery-powered set and a battery eliminator; if its presence offends some purist in the future, it can be removed easily. These batteries, which are available from Alltronics, come complete with pigtails. Since the grid bias draws no current, the battery life should equal its shelf-life, which for a lithium battery is indefinite.

Also on the left side of the open chassis is the original audio interstage transformer which connects the detector output to the audio amplifier tube.

The controls on the front panel are: tuning (large knob), broadcast band segment selector (small knob, lower left), regeneration (small pull-out knob, upper left), and filament voltage rheostat (small knob, lower right). The filament voltage affects the volume and, to a degree, the regeneration. Tuning the receiver is a multi-step, iterative process that is best learned, not taught.