Crystal / Tube #63 — The Multi-Radio
Update! I entered this radio configured as a hobby class set (single coil set with a wave trap) for the 2006 Crystal Set Receiving Contest.
Fasten your seat belts, friends and neighbors. This will be a wild ride. I am proud to introduce you to my new Multi-Radio. I didn't know if I should list this in my tube or crystal section, so you are seeing it in both.
I have built modular sets before. They had a separate antenna tuning unit that could be moved from one radio to another. I am taking this a step further by using interchangeable circuit modules. In seconds I can change my radio from a crystal set to a one tube radio. Not only can I select the detector type, but also if the radio is to have one or two tuned circuits. I may build a transistor module too. I got the idea for this set from my breadboard project. The mechanics of that project worked so well that I did the same with this project. Each chassis base, made from 1/8 (3mm) thick Garolite® is fastened to the oak base with two thumbscrews and two T-Nuts under the base. By removing the screws and disconnecting the circuit from the variable capacitor, a new board can be placed in that spot in seconds. I have become a real quick change artist.
Why did I make this radio? Perhaps my shrink, Doctor Peebles could answer this better than I, but I will attempt an answer. It has to do with cost. The two most expensive parts in my dx crystal sets are the big litz coils and the premium quality variable capacitors. By making a board with one LC circuit, I am able to reuse these parts over just by making a new receiver module. Several of you have commented that Schmarder guy is pretty tight with his money. This proves it. If you would like this kind of economy, several great radios with only one litz expense, please continue reading.
The detector board is 17x8 inches or 43x20 cm. The variable capacitor is one of those 350 pf "holy grail" type capacitors. It combines all what is best in a capacitor, ceramic insulators, silver plated plates and a great wiper arm. These are hard to come by, so keep an eye out for them. The coil is made from 660/46 litz wire. The form has a 2 inch hub (50mm) with a 7 inch outside diameter (18cm). There are 48 turns on this coil, producing an inductance of around 240 micro henries. The form is made from polystyrene. If this litz is outside your budget, 165/46 litz will work ok too. Put the rest of the money in a better variable capacitor. By the way, a vernier drive is essential for the easy operation of this radio. Tuning 120 channels over 180 degrees rotation is tedious.
I would like to describe the tube module first. This is a single 1625 tube receiver with only 12 volts on the plate. How does it work with so low voltage? I feel it is the high current heater that boils the electrons off the cathode in such an intense manner that any plate voltage will attract them. For those of you that don't want to take chances of electric shock with high voltages, this is the project for you. I drew two diagrams. One is how I wired my dx set, and the other is a "lite" version that will still work great. The circuits are below. My comments only relate to the full schematic.
The 4 diodes is an audio peak clipper. In my past dx radios I had trouble with noise pops causing near deafness. The diodes limit the peaks. There is also a volume control. Sometimes I turn the regeneration control way up to narrow the receiver response. That also makes the radio much too loud. It is good to be able to control the volume independently.
This set uses the famous Bogen T725 transformer as a match between the tube and earphones. A three position switch selects the tap. You can use your favorite taps, or just forget the switch and connect to one directly.
The tickler is made from a small spider coil. There are 6 turns of 40/44 litz wire wound on the form. This coil is about an inch (24mm) from the main coil. The regeneration control is set 1/2 to 2/3 up on the pot, which is the optimum range. If you have too many turns on this coil, the audio will be lower at the point of oscillation. The regen pot operates very smoothly too. Proper phasing between the main tuned circuit and tickler is required. If your set doesn't go into oscillation when the regen control is turned up, please turn the tickler coil around.
The tube module is powered by a 12 volt @ 500 ma wall wart power supply. Since the heater draws 450 ma, and the rest of the radio only a few milliamps, this is a perfect supply. If you need one, I have a bunch of them. Since under near full load there is some hum ripple present, I added a filter network to smooth the dc. That filter network consists of a 560 ohm resistor and a 1000 uf capacitor. The resistor isolates the part of dc to be filtered only to the plate and screen supply.
This story is kind of long, isn't it? Now it is time to describe the crystal radio module for my multi-radio. Again, I used ideas from my past radios to make this set. My crystal detector uses a true Hobbydyne circuit. It uses a differential capacitor, like Jim used. Differential capacitors are hard to find, so go ahead and use a single trimmer from the coil to the diode. A small variable capacitor on one of the legs of the differential capacitor is for tracking adjustment. If adjusted properly, as you turn the differential capacitor, you won't have to retune the main tuning control. This doesn't work perfect for me, but it is an improvement.
My crystal module uses a single FO-215 germanium diode. As always, a 1N34A can be used. The FO-215 diode provides higher performance in that with the good impedance match it provides between the large litz coil and output transformer, the selectivity is improved.
This module has a built in Bogen T725 transformer. But since this radio has large litz, a great variable capacitor and Hobbydyne circuit, the output impedance is larger than the input of the Bogen. So I have provided a quarter inch jack with a cut off switch so that an external transformer can be connected. Some examples of what I can connect are shown on my transformers page. To finish up the module, I used a 1 pole, 3 position rotary switch to select the transformer tap that is best for the headphones that I am using.
The Antenna Tuning Unit also has a nice dual section variable capacitor. This one has a 400 and 200 pf sections. A dual 365 will work, but I like this one better because of the lighter antenna coupling that the 200 pf section offers. The crystal set and tube modules can be attached to this board for a single tuned circuit. The detector board can still be used as a wave trap.
When this board is used only as an antenna tuner, the capacitance change caused by the module being removed is enough to mess up the calibration. I put a small trimmer capacitor on a separate Garolite® strip that can be connected to compensate for the difference in the capacitance. Also, since this capacitor combo only allowed tuning to 540 khz (antenna dependant), I added a 33 pf fixed capacitor to act as a padder to get me down to 530 khz.
The capacitor shaft has to be isolated from the vernier. This is accomplished with a ceramic shaft coupler. I added a wire from the ground connection to the vernier drive. This is because the hand capacitance detuned the circuit. Now there is very little shift when I remove my fingers from the tuning knob.
One of the finishing touches on this set is the dial. Having a dial calibrated in kilohertz makes it fun and easy to know where you are. After I found the radio to be as good as it could be, I began the calibration of the dial. I used the tube module so I could get a zero beat note with the stations. I start at 1700 and work my way down. Many times I can count ten beats for the next 100 khz down. Of course, this is in the evening. A signal generator or digital readout MW radio helps.
I also calibrated the dial in wavelength (meters). I wanted to get used to this wavelength measurement technique and what better way than to have a dial with both calibrations. The dial material is white styrene which pretty well hides the tag edges at a short distance away. These dial markings are now available from me. I can also provide the styrene you can cut in a circle. To use the labels, you need a knife and pair of small side cutters. The labels are cut. At the edge closest to the dial, I cut close to the marking at an angle, so the label edges don't stick over the dial edge. Your hand has to be somewhat steady. As you go down the dial, if the distances crowd the labels, just leave off some of the labels. Here is my labels offer page.
How does this radio work? It works great! The tube section works as well as it's higher voltage cousins. The crystal radio has that sharp tuning appeal. I love the smell of litz being tinned in the morning! It smells like great DX in the evening!
Good DX! Dave - N2DS