Homemade "Pentaflex" Single Tube Radio
Hi Friends. This is my single 6A8 tube radio. In a 1934 publication this set was called the "Pentaflex". So that is what I am calling it. The reprint of this book is available from The Xtal Set Society, and is titled "1934 Official Shortwave Radio Manual". This is my first AC operated set. A small power transformer is used to supply the 6 volt AC heater voltage and the 180 volt DC for the plate supply
This radio is a single tube set using an old time radio mixer tube, the 6A8. This tube is not the same as the original Pentaflex radio 6A7 (or a 2.5 volt heater 2A7). The difference is only the socket. I had a nice surface mount octal base socket that was ready for use. The Pentaflex is a regenerative radio with reflex features. Reflexing refers to building a circuit to amplify the RF signals, then after detection running the signal back through the same stage for some audio amplification. That gave you two stages of amplification with one tube.
2007 Update: I received an e-mail asking about a modification to this radio. In looking over the circuit, I don't believe this is a reflex set! I believe the author was confused in thinking that as this type of tube was new. The 6A8 (and 6A7) is actually a two section tube. There is a pentode section, and a triode section. The triode was the oscillator and the pentode was the mixer. Pin 5 is the grid and pin 6 is the plate of the triode section. Early on, this tube was drawn to look like a single tube but it is really two tubes in one envelope.
This finding does not detract from the fact that this is a really nice radio project and I still recommending it. I just wanted to set the record straight as far as the operation. Now back to my original article....
My Pentaflex uses plug-in coils, just as the original had. My first coil tunes from 6 to 10 mhz. The variable capacitor is 140 pf with a 15 pf band spread variable capacitor. I also wound a coil for 9 to 15 mhz and 3 to 6 mhz or so. The coils are all different because that was what I had. Remember we are in a depression here. Use what you can get your hands on easily.
One important difference between my Pentaflex and the original is my use of a 2.5 mh RF choke in the tickler circuit. I noticed that the reception wasn't what I thought it should be. After doing some other experimenting and carefully checking my wiring, I decided to try a choke to see if I could improve the reception. The regeneration is much improved. I recommend that you add this choke to your set.
Another change I made was a safety related change. The original circuit had the B+ voltage on the headphone wire. If there is a short to the metal headphone ear piece case, a shock could result. Therefore I added a Bogen T725 transformer primary where the headphones originally connected. The secondary winding is just right for low impedance (8 ohm) headphones.
Because this radio uses higher voltages, it is recommended that only experienced builders make and use this type of set. If you aren't comfortable with the voltages used in this set, stick with one of the battery sets shown on my pages.
This is turning out to be one of my fun radios. The copper behind the panel has eliminated the hand capacitance problems and the radio is pretty stable. The selectivity isn't very good but not a big problem. I have also been listening to the 80 and 40 meter cw ham bands (ssb too).
I am sure that I have recreated the reception that hams experienced during the early thirties. There was better equipment out there but for many, one tube was the extent of the ham radio luxury. It sure makes me look at radio a lot differently. Now, you plug in the set, use the auto tuner and hit the button and work the dx. It wasn't always that easy. I am sure you will also have fun building one of these sets.
73, Dave - N2DS
Coil Data Frequency Main Winding Tickler Winding Diameter 3 - 6 Mhz 20 turns , 1 inch 16 turns 1-1/2 inch 6 - 10 Mhz 12 turns , 1.5 inch 14 turns 1-3/8 inch 9 - 13 Mhz 7 turns , 1 inch 12 turns 1-1/4 inch Main coil, 20 gauge. Tickler Coil, 30 gauge close wound.
Cool inside views of my radio.
2014 Update I located the original publication of this project in the September 1933 Shortwave Craft Magazine available from American Radio History. I picked out the article with a screen capture and can be downloaded, Pentaflex Page 1, Pentaflex Page 2, Pentaflex Page 3.