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Homebrew build question - tab10672 - 10-24-2015

Hello,
   My daughter is taking an engineering class in college and for a class project the students have to each build something electrically/electronic from scratch so I am going to help her build an early 20's style 2 tube radio.

   My question is, I want to use a crosley model 51 schematic but the 51 coils are loose coupled that move closer/farther to each other for tuning but the only coil assembly I have is a variocoupled coil (at least i think thats what its called) the primary coil mounts at a 45 degree angle and the secondary turns 180 inside it.

   Would this coil work to tune the circuit being that the variable capacitor in the 51 circuit tunes the antenna rather than the primary coil, like in a 1 tube regen.

Thanks, Todd

RE: Homebrew build question - mikethedruid - 10-24-2015

What you have is a variocoupler. It can be used in any of several ways in a regenerative receiver. Here is a simple circuit for the first stage of the 2 tube radio you propose, the second would be a simple amplifier following an interstage audio transformer which would go where the headphones are shown.

[Image: http://40.media.tumblr.com/66fdd422f88a316994ce81540a4e578f/tumblr_nwqrv4mdad1s2snqvo1_1280.jpg]

The two variable condensers are optional in the circuit and provide better, sharper tuning. The condenser across the phones is optional also. I hope this helps.

RE: Homebrew build question - tab10672 - 10-24-2015

Thanks for the info.

Todd

RE: Homebrew build question - mikethedruid - 10-24-2015

By the way, depending on the number and location of the taps on the grid section of the variocoupler, and the size of the variable condenser in it's tank circuit, this circuit can be used to receive shortwave as well as standard broadcast. With the rotating winding used to control regeneration, one can actually go from oscillation level regeneration, to degeneration, which can be useful when dealing with very strong local stations. This means of controlling regeneration also provides very fine control. in slight oscillation it can actually pick up CW and sideband too. Let us know how your project turns out.

RE: Homebrew build question - Radioroslyn - 10-24-2015

> Would this coil work to tune the circuit being that the variable capacitor in the 51 circuit tunes the antenna rather than the primary coil, like in a 1 tube regen.

The '51 schematic is goofy it that it looks like tuning condenser is adjusting the coupling of the antenna. But it's just a series tuned with a switch to change the inductance/frequency of the circuit. Regeneration is achieved by moving the feedback coil closed to the antenna coil.

Typically a variocoupler will have three windings. An antenna winding (smallest), the tuned circuit (largest and in parallel with a 365mmfd condenser will resonate at the BC band)and is connected to the grid of the tube though the grid leak, and the tickler or feedback winding ( size wise it's about 25 percent of the tuned circuit) and is connected to the plate of the tube. Phasing is critical if it doesn't oscillate flip the plate and b+ connections

If she would like to build something on primitive side check out pg 12 here:
http://americanradiohistory.com/Archive-Radio-Craft/1930s/Radio-Craft-1935-03.pdf
The 30 tube could be an 01A or 199 to keep it in the 1920's.

Big fan of regens! Very simple, sensitive, and selective. They just don't like strong signal (overload easily)

Terry

RE: Homebrew build question - tab10672 - 10-24-2015

Terry,

Thanks for the link. I have built a few simple 1 tube regens and over the years I have accumulated a few early 20's parts like commercial grade wiper style coil taps and some neet looking transformers with the bakelite thumb screw terminal blocks, not to mention 2 tipped 01A's so i thought we'd use as many parts as possible (as some early sets did) to make a simple 2 tube radio look as overly complicated as possible.

She has a couple months to complete it so I'll post some pictures when we get it done.

Todd

RE: Homebrew build question - mikethedruid - 10-24-2015

Hi Todd, I found these links also that might be of interest to you.
http://antiqueradios.com/archives/knockout.pdf
http://antiqueradios.com/archives/Radio_Expermenters_Guide.pdf