The BACK PACK RADIO

Tom's Designer 1625 Tube Radio front

I've not built a radio in quite some time but a few weeks back the "Bug-To-Build" bit me rather severely. The cure consisted of five 1625 sets. The first started out to be a rendition of Dave Schmarder's All Wave Radio but as I got into it I decided that I really didn't want the short wave part included since I planned to build a short wave set separately. But since I'd never built a long wave receiver that part was definitely left in.

I used 1/4" bent plexiglass for the top and front panels for this set with 1/2" maple laminate for the sides, base and back. It, along with the knobs, were stained and finished with my special flattened lacquer for a soft glow.

Both the antenna tuning variable capacitor and the fine tuning variable capacitor are set back from the front panel 2" and secured to steel bracing built for this set. Both are controlled by PVC rods machined to fit. This premium radio deserved to have all selections available to it that the T725 Bogen Audio matching transformer has to offer so I used a 6-position switch to do just that. The main variable capacitor is one I purchased because of the ceramic mountings although it is smaller in diameter with heavier plates and longer than 365's I've used in the past. It was 405 pf on my meter. I wanted to make certain that I could get to the bottom of the broadcast band with it and to expand possibilities for long wave so I included a 5-position switch to select fixed capacitors in parallel to that variable capacitor with values of 50pf, 150pf, 200pf, 250pf and 300pf. That is maximum over-kill I know but the desire to experiment was strong and, as it turns out, it provides for some surprising and interesting tuning results.

The main tuning capacitor is mated to a vernier drive and the special control knob includes a numbered brass strip so that I can make notes of transmitter locations I find. This is pretty much a first for me since I have always simply wandered around the band blind to any locations found. It also includes the line-out port as shown on the schematic. After boring all the holes for all the hardware I realized that this thing was going to be absolutely bristling with knobs, switches and binding posts. And after wiring all the components it was clear that it was rather crowded in that cabinet and that's when it hit me..I had completely forgotten about the inductance coils! That's what happens when you jump into something you've not done in months and months and the best solution I could come up with was an exterior coil attached to a bracket off the back of the cabinet. And the name BACK PACK was born! I had considered making plug-in coils similar to Dave's radio but there simply was no room for the sockets. The connectors I ended up with are a little clumsy but serve the purpose well. I space wound the medium wave coil with Litz wire on a 4" diameter styrene tube at 241uh and a long wave coil close wound with .031" magnet wire at 1388uh.

This radio has turned out to be the best performing set I have ever built with great sensitivity and selectivity as well. I listen to stations I have never picked up before and some from very far away. I live 60 miles away from any city so flame throwers are typically not a problem here but finding a good selection of AM broadcasters is..but not with this receiver. The long wave was disappointing because there simply are no broadcasts to be heard. All I picked up was the Morse code signal from the airport.

Please e-mail me for price and availability. Order: #T-026


Tom's Designer 1625 Tube Radio angle view