This spring I finally decided to tackle a project that had been on my ‘must build’ list for over 20 years! I was looking for inspiration in one of the several reference notebooks that I fill with circuits of interest and came across the ‘NEOPHYTE 1’ ... once again!
The circuit originally appeared in the 1968 Fall-Winter edition of ‘Electronic Hobbyist’ and in the Jan-Feb 1968 edition of it’s sister publication, 'Elementary Electronics'.
The NEOPHYTE is a simple one-tube regenerative receiver that uses a 6U8A triode-pentode, a tube that found widespread use in the television industry as a VHF oscillator (triode) and mixer (pentode). The NEOPHYTE uses the pentode section as the regenerative detector and the triode for the audio.
Can you spot the error in the schematic? A forage through my junk box revealed that all the necessary parts were on hand and once gathered, the circuit was breadboarded on my aluminum sheet testbed.
After winding a coil, the circuit came to life as expected. Over the course of several weeks and discussions with others on ‘The Radio Board’ forum, a few subtle changes were made that improved its performance.
The original circuit used a 100K pot for regeneration control of the screen voltage but the value is far too high. It was reduced to 10K backed with fixed resistors on each end. This allows a much finer tuning of the regeneration compared with the original design. As well, the grid leak resistor was increased from 1M to 2.2M ohms which removed an annoying short-period cyclical frequency variance in the oscillator.
The other changes made related to the audio quality. The detector plate bypass capacitor was reduced to provide less bass response and the triode stage was given a small amount of cathode bias to improve linearity.
Another new feature was the addition of a 1-turn antenna coil link to the tank coils which gives me the option of using my large 50 ohm coaxial-fed HF antennas rather than a high-impedance random length of wire clipped to the top of the detector coil as shown in the schematic diagram.
The original circuit used a 47pf antenna coupling capacitor which worked nicely for short antennas (the first evening’s quick tune across the broadcast band found KFBK in Sacramento and KOA in Denver using a 10” clip lead antenna!), it was much too large for bigger or resonant antennas. This was reduced to 5pf.
Once rebuilt into its final form, the lightly modified NEOPHYTE is a good little performer and, like most optimised regens, is a pleasure to use.
All of the changes are shown in my re-drawn schematic below. The parts shown in BLUE are original components where values have been changed. Parts in RED are newly added to the circuit. As well, DC voltage measurements at various points are shown in the red boxes.
The original circuit used a single fixed coil while I chose to make two plug-in coils, one to cover the AM Broadcast band and a second one covering the lower SW bands. The SW coil will tune from 2.8 to 10.2 MHz with the main tuning capacitor (410-15pf) but with the ‘HI-LO’ switch, a 50pf capacitor is connected in series with the main tuning capacitor, effectively lowering its range to 50-15pf. This provides a form of band-spreading by reducing the SW tuning range to 7.3 - 10MHz.
Did you find the error in the original schematic showing the ‘HI-LO’ switch labelling reversed?
If really fine tuning of any particular band such as the 31m or 40m band was desired, an appropriate series-connected capacitor in combination with an appropriate coil could effectively spread the entire band across the capacitor’s tuning range. I may eventually do this for either the 80 or 40m CW band and mate the NEOPHYTE 1 with a simple one-tube (6AQ5?) 'NEOPHYTE 2' of similar physical appearance, to produce a small CW station to play with.
The original NEOPHYTE has a built-in half-wave power supply in the same enclosure but I chose not to do this. With the power transformer so close to the detector and the poor quality of DC produced by half-wave rectification, I feared that the hum level might be objectionable. Instead, I run the receiver from a separate regulated supply and find that best performance is achieved with 50 volts for the B+.
The following international SW stations were heard during a few hours of early evening listening:
31m BAND (9MHz)
NHK World Radio Japan Nauen, Germany Voice of Turkey Emirler,Turkey BBC Kishinev-Grigoriopol, Moldova BBC Talata-Volonondry, Madagascar BBC Tinang, Philippines KCBS Voice of Korea Pyongyang, North Korea WBCQ Monticello, Maine Radio Habana Cuba Habana, Cuba Radio Rebelde Bauta, Cuba WTWW Lebanon, Tennessee WRMI Okeechobee, Florida WEWN Vandiver, Alabama Voice of America Cantagalo, Sao Tome Helleniki Radiophonia Avlis, Greece Radio Republican Issoudun, France Radio Marti Greenville, North Carolina Radio France International Issoudin, France Radio Voz Missionara (ZYE 890) Comboriu, Brazil Radio Saudi International Riyadh, Saudi Arabia Radio Free Asia Agignan Point, N. Mariana Islands CNR1 Voice of China Beijing, China CRI China Radio International Xianyang, China KBS World Radio Gimjae, South Korea
49m BAND (5-6MHz)
WWCR Nasville, Tennessee CFRX Toronto, Canada (1kW) WTWW Lebanon, Tennessee WRMI Okeechobee, Florida Radio Habana Cuba Titan Quivican, Cuba Radio Habana Cuba Bauta, Cuba WBCQ Monticello, Maine WHRI Cypress Creek, South Carolina
Some recent daytime and early evening checks on the AM BROADCAST BAND revealed the following stations, all heard before 10PM local time.
550 KARI Blaine, WA 2.5kW 580 KIDO Nampa, ID 5kW 600 CJWW Saskatoon, SK 25kw 630 CHED Edmonton, AB 50kW 630 KCIS Edmonds, WA 2.5kW 650 CISL Vancouver, BC 4kW 660 CFFR Calgary, AB 50kW 670 KBOI Boise, ID 50kW 660 KAPS Mt. Vernon, WA 10kW 690 CBU Vancouver, BC 50kW 730 CHMJ Delta, BC 50kW 750 CKJH Melfort, SK 10kW 790 KGMI Bellingham, WA 1kW 810 KTBI Ephrata, WA, 23kW 810 KGO San Francisco, CA 50kW 820 KGNW Seattle, WA 5kW 830 WCCO Minneapolis, MN 50kW 850 KOA Denver, CO 50kW 880 KIXI Mercer Island, WA 10kW 900 CKBI Prince Albert, SK, BC 10kW 910 CKDQ Drumheller, AB 50kW 930 KBAI Bellingham, WA 500W 950 KJR Seattle, WA 50kW 960 CFAC Calgary, AB 50kW 980 CKNW Surrey, BC 50kW 1000 KOMO Seattle, WA 50kW 1010 CBR Calgary, AB 50kW 1040 CKST Langley, BC 25kW 1060 CKMX Calgary, AB 50kW 1070 CFAX Victoria, BC 10kW 1090 KFNQ Seattle, WA 50kW 1130 CKWX Richmond, BC 50kW 1140 CHRB High River, AB 50kW 1160 KSL Salt Lake City, UT 50kW 1200 CJRJ Richmond, BC 25kW 1250 KKDZ Seattle, WA 5kW 1260 CHMB Richmond, BC 50kW 1260 CFRN Edmnton, AB 50kW 1320 CFTE Richmond, BC 50kW 1340 KWLE Anacortes, WA 1kW 1380 KRKO Everett, WA 50kW 1410 CJVB Richmond, BC 50kW 1430 KBRC Mt. Vernon, WA 5kW 1450 KONP Port Angeles, WA 1kW 1510 KGA Spokane, WA 50kW 1520 KKXA Snohomish, WA 50kW 1530 KFBK Sacramento, CA 50kW 1550 KRPI Ferndale, WA 10kW 1600 KVRI Blaine, WA 10kW 1640 KDZR Lake Oswego, OR 1kW 1680 KNTS Seattle, WA 10kW
Although tuning can be managed using the original knob size, I've found it much easier to use a larger knob when searching for signals. The picture below shows the NEOPHYTE set up in the DX mode, ready to go. The graduated dial plate also provides a rough frequency calibration scale once a few signals are located.
To download a pdf of the original article as published, CLICK ON THE ABOVE IMAGE.