My earlier receiver was also plagued by microphonics. While a vibration damping mount for the CK5875 detector tube would have greatly reduced the problem, I decided to side-step the issue entirely by switching to a remarkable, Soviet-made, subminiature pentode.
The 1Ж24Б (1J24B, 1SH24B) is a marvel of ingenuity dating from the 1950's. Examining the close-up photos on this German site, one might be tempted to exclaim, "Where are the grids?" In fact, three grids are present, only, they aren't helically-wound, neither are they ladder or screen structures; they're collinear! This device is the product of some clever electron optics (see post #7) involving tiny sheet beams.
Another virtue of this tube is its extraordinary emission efficiency; a definite plus given my eventual goal of building a minimalist's vacuum tube-based portable QRPp station.
Details of the 1Ж24Б are covered in this 1962 Soviet vacuum tube manual. An article on the topic of these tubes appeared in the July 1960 edition of the journal, "Радио." Please click here to download this magazine issue. The article begins on page 34 (page 40 of the scanned document).
Aside from the vague speculation they may have been been used in the MIG-25 (Foxbat), it's clear these tubes were used for portable military equipment of the Warsaw Pact nations; an example of which may be found here. They were used for similar applications, and during the same time-period as the likes of my Raytheon subminiature tube was used in NATO equipment.
I rather like the idea of melding the CK5875 and 1Ж24Б - these former Cold War adversaries - into a single radio.
The 40mW transmitter amounts to a simple, crystal-controlled, Miller oscillator followed by a high-pass, L-network for impedance matching. For maximum efficiency L1 needs to be a Hi-Q inductor.
The regenerodyne receiver IF tunes from 2.877 to 2.807MHz in order to provide 20m coverage from 14.000 to 14.070MHz. L2 and C4 constitute a 16.877MHz trap filter. Adjust L2 for maximum 16.877MHz energy at the filament. VC2 cancels the residual trap reactance at 14MHz. On paper it improves the sensitivity by 4dB; it's not strictly necessary. While tuning across the band VC3 needs to be re-peaked every 10kHz or so. VC4 is the main-tuning variable capacitor. VR1 is the regeneration control.
The CK5875 filament draws 100mW at 1.2Vdc and the 1J24b's each draw 15mA. Altogether, the receiver only consumes 217mW of input power. Not a peep of SWBCI has been heard on this receiver in several weeks of operation.
Despite poor band conditions, last week Solidarity provided me with several pleasant QSOs. On Thursday, K9IS replied to my CQ with his Elecraft K1 from Wisconsin. Steve was running 3w to a dipole. He was 579 on the regenerodyne and he gave my 40mW signal a 339 report.
An hour later N4KGL/4 answered my call. Greg was operating his little, Wilderness SST @ 3w into a Hamstick dipole that he mounted atop a 20' mast. Greg was on his lunch hour and operating from the parking lot adjacent to his office. Details of his setup may be found here, here and here. Greg wrote later
"I had good copy at the beginning for the essentials and then fading in and out....When I turned on the rig I was hearing nothing in the SST's 8 khz receive window. So you were the first signal that I heard."
Working a guy standing in a parking lot on his lunch hour (a stone's throw from the Gulf of Mexico) with 40mW and some vintage pencil tubes stuck in a plastic protoboard here in snowy Vermont is just plain cool!
A few minutes after I signed I heard K9IS calling me again. He'd heard Greg's and my QSO. In fact, he'd continued to copy my CQs on a variety of rigs ever since our contact. He said that my signal had peaked briefly at 559. The most exciting news was that despite a slight frequency offset in our crystals, he'd copied me on his RockMite. Steve then switched to the RockMite (at 350mW) for the remainder of our contact! I gave him a 559 (an easy copy here aside from the QSB fades). He copied me at 329. Just amazing...
These great QSO's were the "shot in the arm" I needed to go on calling, even as the band conditions appeared to further deteriorate. Tuning around, I only heard a couple of signals on the band now; besides it was nearly sundown. But you know how it is...just a couple more calls :o)
And suddenly, big as life, I hear FG8AR calling me! I quickly send off a 599 report; feeling my pulse all the way to my sending wrist. FG8AR replies with a 539 report. He says his name is Olivier and asks, "PSE UR PWR?" I reply, and he responds, "GUD GUD WITH 40MW."
It took me a moment to recover after we'd signed; I kept looking at the tiny CK5875 envelope while trying to visualize the geographical location of the island of Guadeloupe.
Most exciting of all is the thought of what will be possible with Solidarity when the conditions on 20m are actually good!
So thanks for your interest, guys. I hope to be QRV on 14.060MHz with this rig come next Tuesday (12/7/10). I hope to hear from you!