Here is my take on the Interflex radio that is on the glowbugs website. I got first signal at 4:00am Zulu 4 October 2002. I got to listen to the local collage girls volleyball team beat their arch rival from the north. It took a little doing since I somehow got the plate and grid mismatched. I mean it says "G" and "P" right on the ceramic socket, how hard can it be? And you have to have a ground. I have two other crystal radios and they don't need the ground, but then they have the antenna connected directly to the coil. I have learned that I need the antenna link in my town because I have AM 1440 KBRK's transmitting tower 12 block away from my QTH and if I connect the antenna directly to the coil I get nothing but KBRK.

The black cylinder in the upper right is just some 2" PVC with a "D" cell alkaline battery in it. The box in the upper center is a 60v power supply. Counter clockwise from the upper left hand corner is the coil, 50 wraps of green #20 hook up wire on a 5" long piece of 2" PVC with taps. Plus another 10 wraps of the same wire for the antenna link. Next are the antenna fahnestock clips. Then is the 365pF variable cap. Then the not too pretty but very workable cat-whisker crystal detector. Then the phone fahnestocks, and the variable resister and finally in the upper right hand corner is the 30 amplifier tube. Behind the 30 are the battery input fahnestocks.

A word about the variable resister. Try as I might I just could not (read, did not have the patience for) wrap that damn resistive wire around the cardboard former. So I pulled out the bearing that I had inside the wood case and inserted a 25 ohm 3 watt pot that I got at Radio Shack. That $1.50 saved my sanity and the project.

I got the phones at a local antique shop for $16. I don't know how old they are but the ear pieces are bakelite and they are labeled American Bell, Allied Radio Corp. Chicago, Ill. they work very well.

The Dial

I took the dial as it appeared in the article and cleaned it up a bit and then just printed it on to a CD label and put it on a CD then glued a Radio Shack knob to it. Probably the easiest part of the project and it looks the best.

The Crystal

Definitely not pretty but it looks semi-ok and it does work. You can't see it but there is a galena crystal in there. I got it and the cat whisker from Antique Electronic Supply. the rest of the parts I got from my local hardware store in the lamp section of those little draws that they have back by the nuts and bolts. The base is just a 1 inch copper pipe cap. A friend of mine told me that it might be difficult to get it to work, he was right, it is tricky. I can usually get a signal though but it takes some hunting to get a good clear signal. I tried a 1N34A to see what it would be like and that worked great. I'm thinking about doing another cheat and hiding a diode under the base and bypassing the galena.

The 60 Volt Supply

I cannot say enough about this circuit. You can download the circuit from Linear Tech. Just do a search for the LT1082 and you will find it on page 10 of the datasheet. It will give you 90v at 120ma from a 15volt supply. I replaced the 78.8k ohm resister with a 100k mini-pot so I could vary the voltage out, I run mine at 60v. To save money and space I used a 12.6v 300ma Radio Shack transformer that I had laying around and use an LM317 to get my 15v. The voltage out of the transformer runs high enough to run the regulator to 15v with out a glitch. I've built dozens of power supplies over the years and this seems pretty normal. I've never done a study of it but I have seen transformers rated for 12.6v run as high as 21v rectified and filtered (17.8v should be right). With this setup I was able to get 60v at about 35ma before it tripped out. Of course with a larger transformer I could get more milliamps but this is fine for me. I only had one 47uF 100v cap on hand and so I used it instead of the recommended 100uF cap. My advice is to use the largest cap you can get or even parallel some to get as much capacitance on the output as you can afford or have room for. I get some 60Hz hum which one would not get with a real battery of course. I can get rid of the hum by putting my finger on the headphone output fahnestock but this also attenuates the sound level.

The Holder

I got the box for $4 from the Hobby Lobby. I have since stained and lacquered it. It looks good and it has that old time feel to it. Some day I'd like to find a decent label that I can print out and glue on.

All in all I really like this radio. I ran the phone output into a voice coil that I had in my junk box and I can actually get a decent volume out of a loud speaker. When I'm working in my shop now I have it on pretty much all the time. I have a 1N34A diode jumpered into the circuit now since it is almost impossible to keep the signal at full level for long periods of time when I use the galena. And I built a 1.5v supply as well, it goes through the D cells pretty quick. I've also got a 400uF 100v cap that I need to add to the 60v supply to get rid of that 60Hz hum (I hope).

An interesting observation. When I use the galena I run the filament pot wide open to get the loudest signal but when I use the 1N34A I only run about 2/3 of the amperage that I run for the galena. Any more than that and the sound actually gets softer. Also I ran the 60v supply up to 90v and then down to 40v and there seemed to be no difference in the sound level.

Comments, Questions? Send to rhaub@itctel.com