12AL8 Regen

 

 

 

BCB Coil, Scramble Wind

5.80 MHz to 7.6 MHz

(6+8 turns, 47pF padder)

 

 

The Rest of the Story

I had never built a regen before so this little adventure began with Norm's 12K5 Regen because I also wanted to try using 'car radio' 12 volt tubes.

Comments about driving a speaker seemed a bit exaggerated but, then, I didn't know what to expect. Regen control also seemed touchy but that might have been the spare pot used.

At any rate, having made my first I decided to try my own hand at it, with the objective to obtain more gain, and that led to partial plagiarization of an old magazine article titled "The 12AT7 Regen."

 

12AT7 Regen

 

My first incarnation used just the 12AL8 with the tetrode section as the audio 'power' amplifier but it should be noted that in 12 volt tetrode's G1 is the "space charge" grid and gain is more akin to a low mu triode than a proper 'screen grid' tetrode. In fact, mu for the 12AL8 tetrode, at a whopping 7.5, is less than the triode section. The 'power' ability, as it were (40mW with 800 Ohm load), is there but the overall gain was not much more than the single 12K5. Regen control, however, was much improved with the triode plate control.

So, with improved regen control making this preferred over the single 12K5, a 12EL6 audio preamp was added in-between the 12AL8 triode regen and tetrode 'power amp' to further boost audio gain, which proved sufficient to drive a reasonably efficient speaker to useful volume on strong local stations. The transformer necessary for speaker operation also allows the use of common 8 ohm headphones for 'harder to get' stations.

 

Construction, the whole project actually, was from spare parts. The SPECO T-7025/5 was left over from previous projects and costs less than 5 bucks. Both the 12AL8 and 12EL6 are around 3 bucks each and the tuning caps are 'plastic japanese radio' types. The second one, for bandspread, uses only the 20 pF section and is under utilized, but I had them. They also conveniently fit in the small CD case, which ended up being a snug fit despite the relatively few parts involved.

The octal plug in coil idea was also plagiarized from "The 12AT7 Regen" with two forms coming from dead octal tubes stripped to the base. Note that I wound my coils to different pin numbers than shown on the 12AT7 Regen article and my shortwave coil tunes a different range.

And for the pudding proof

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BCB Tuning

First station at 740 KHz. Second station at 700 KHz. Tuning is by 20 pF bandspread. Audio pickup by camera mic at 2 feet.

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Shortwave Tuning

First station is Radio Taiwan at 5950 KHz. Second station unknown. Audio pickup by camera mic at 2 feet.

 

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