This video, produced by the MIT Science Museum, shows Raytheon workers assembling subminiature tubes having the same envelope (begins 10:33) at their Massachusetts plant. Doubtless, my tubes (date-coded 1964) came from this same assembly line.
My crystal-controlled, 20m transmitter uses a single, triode-connected CK5875. A 57Vdc anode-supply results in an RF output power of 40mW. The receiver uses an identical tube in a 0-V-0 regenerative detector.
The setup was judged to be airworthy last Thursday evening. I began calling CQ the following morning on 14.060MHz.
Unfortunately, an accumulation of freezing rain that morning had de-tuned my antenna such that only 25mW was initially radiated. The icing situation gradually improved towards midday; I wish I could say the same for the band conditions. Still, by 1600z my radiated output power had risen to 35mW. That's when I heard KE4YHY answering my call. Don handed me a 559 report from Alabama; a distance of 1600km. Despite the need for several repeats on my end, we went on to enjoy a 12 minute QSO.The band never did improve in the time remaining before I had to QRT. It's a shame as DL3PB had been listening several hours for my signal (thanks Peter, we'll bag it next week!).
29 Nov. 2010: The band was in rough shape again today. However, my second or third CQ with the CK5875 twins brought a reply from K9IS in Wisconsin (1259km distant). My output power was 40mW. In a follow-up email Steve wrote
"Your 40 mw signal was easy to copy the first time around, but QSB made rough copy after that. I believe you said you were using a 1 tube Xmiter and I never did copy the rcvr. Anyway, it is fun to work someone running real low power. Hope to hear you again soon."
Steve and I worked several times last Spring on 20m. I was running 350mW from a 3A5 twin-triode configured as a "Frank Jones," crystal-controlled, push-pull oscillator. A second 3A5 was fashioned into a regenerodyne receiver. Steve used a variety of rigs with output powers ranging from 5w down to 300mW.
After filling half of a page in my log with DX calls last Spring I reduced my output power from 350 to 57mW. Of course the contacts didn't come as easily, and yet I worked N7EF/QRPp running 1w out in Seattle and W7CNL/QRP in Boise, along with five other stations. And then DH1BBO returned my call one morning with his Elecraft K1. I was especially tickled to hear Olaf report his output RF power as 300mW. I'm not sure which impressed me more; working across to Germany with 57mW or hearing a 300mW German station on a receiver made from a single, 3A5 vacuum tube!
This contact with DH1BBO later inspired me to try my luck on The Bell Ringer with the RF output power throttled down to 10mW. On the first day I (twice!) worked F5NBX for 350,000 miles-per-watt. This was followed by a contact with FM5LD. On the next day I worked IZ0PEC, for 410,790 miles-per-watt.
My eventual aim is to build a QRPp station using subminiature vacuum tubes that's small enough to carry on hiking trips. I intend to power it entirely from a hand-cranked alternator while sending CW with a mouth-key.
In closing, I'd like to recount a couple of wonderful feats of QRPp DX. Some ten years ago, Jim, AL7FS, completed a two-way, 20m, QRPp QSO with Mike, ZL1MH. At that time Jim was burning up the ionosphere with 4.5mW. Mike responded with 100mW.
Around 1997, Yoshi, W6/JJ1SLW, pulled off a QSO with JA6SHL located on the Japanese mainland. Yoshi's 40m Pixie had an RF output power of 20mW!