Design Con 2015

A super-het radio runs 5 years on a C-cell, plus a pentode radio

-October 11, 2011

Ron Quan, analog aficionado and inventor of the dreaded MacroVision VHS content scrambling system, saw my blog about a radio that uses Russian rod tubes. I saw Ron Saturday at the last eFlea of the season and we got to talking about the rod-tube radio. In the course of telling me how much he liked the blog, Ron mentioned that he did a super heterodyne radio that would run for 5 years continually on one C-cell. I asked him to send a picture, which he did yesterday. As a bonus Ron also included a picture of a pentode tube radio he designed. The pentode is cool since it is regenerative, the low-power radio is neat since he used a crystal headphone to save power. Ron notes:

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  • On the low powered radio, a crystal earphone was used. I figured driving a 32 ohm headphone drained too much power. Typical transconductance of the IF amplifiers was about 900 micro mhos, but the equivalent tank resonance resistance was about 50 K ohms. This gave about a gain of 30 per IF stage including loading effects of the subsequent stage. The hardest part was taking apart a 10 mm IF coil (without a built-in capacity or) and unwinding it and then rewinding it again for the desired inductance and tapping ratio for the oscillator circuit. Anyway, the whole radio including its audio amplifier, oscillator, mixer, and two IF stages had a total current drain of about 140 micro amps. The sensitivity and selectivity was quite good, able to pull in KFMB (San Diego) to KOMO (Seattle) off the ferrite antenna coil. I figure my next version may drain less than 100 microamps.

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  • The regenerative radio was built with a pentode using 27 volts for the plate and screen voltages. Although the filament was meant for 2 volts, the two D cells (3 volts) provided longer running time, which also powered the three transistor audio amplifier for 32 ohm headphones. Unlike most regenerative radios, this one had no external antenna, and the built-in ferrite loopstick antenna coil received KOMO 1000 KHz from Seattle. Also because the regeneration control enhanced the Q of the tank circuit, selectivity was comparable to a superhet. Not bad for just one LC tank circuit.

Way to go Ron. If you folks like these projects leave a comment and maybe I can convince Ron to dig out or reverse-engineer the schematic. A lot of savvy guys like Ron don’t realize the rest of us need a detailed schematic, even if it is drawn on a napkin, to figure out what is going on. To my Ham buddies like Phil Sittner and Alan Martin, Ron probably gave enough information in the blurbs above to build the darn things.

[Update, Nov 29, 2011] Ron Quan was nice enough to send a schematic and a few comments about the regenerative radio:

  • Here is the schematic for the regenerative radio that was designed and built in 2007. I decided on a 1G5 tube, which is normally used for an audio output tube. My initial test showed that this radio, located in Silicon Valley, did quite well receiving KFMB (San Diego), KKOH (Reno, Nevada), and a couple of Sacramento stations. Later on, I picked up KOMO 1000 in Seattle. After getting the tuner section working, I had to decide on the audio amplifier. So I thought of “bridging” the older tube technology with germanium (2N217 and 2N1304) and silicon transistors (2N5089) for providing the audio.

Click to enlarge:

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