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 JapanNauen, Germany
Voice of TurkeyEmirler,Turkey
BBCKishinev-Grigoriopol, Moldova
BBCTalata-Volonondry, Madagascar
BBCTinang, Philippines
KCBS Voice of KoreaPyongyang, North Korea
WBCQMonticello, Maine
Radio Habana CubaHabana, Cuba
Radio RebeldeBauta, Cuba
WTWWLebanon, Tennessee
WRMIOkeechobee, Florida
WEWNVandiver, Alabama
Voice of AmericaCantagalo, Sao Tome
Helleniki RadiophoniaAvlis, Greece
Radio RepublicanIssoudun, France
Radio MartiGreenville, North Carolina
Radio France InternationalIssoudin, France
Radio Voz Missionara (ZYE 890)Comboriu, Brazil
Radio Saudi InternationalRiyadh, Saudi Arabia
Radio Free AsiaAgignan Point, N. Mariana Islands
CNR1 Voice of ChinaBeijing, China
CRI China Radio InternationalXianyang, China
KBS World RadioGimjae, 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
WBCQMonticello, Maine
WHRICypress 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.

550KARIBlaine, WA2.5kW
580KIDONampa, ID5kW
600CJWWSaskatoon, SK25kw
630CHEDEdmonton, AB50kW
630KCISEdmonds, WA2.5kW
650CISLVancouver, BC4kW
660CFFRCalgary, AB50kW
670KBOIBoise, ID50kW
660KAPSMt. Vernon, WA10kW
690CBUVancouver, BC50kW
730CHMJDelta, BC50kW
750CKJHMelfort, SK10kW
790KGMIBellingham, WA1kW
810KTBIEphrata, WA,23kW
810KGOSan Francisco, CA50kW
820KGNWSeattle, WA5kW
830WCCOMinneapolis, MN50kW
850KOADenver, CO50kW
880KIXIMercer Island, WA10kW
900CKBIPrince Albert, SK, BC10kW
910CKDQDrumheller, AB50kW
930KBAIBellingham, WA500W
950KJRSeattle, WA50kW
960CFACCalgary, AB50kW
980CKNWSurrey, BC50kW
1000KOMOSeattle, WA50kW
1010CBRCalgary, AB50kW
1040CKSTLangley, BC25kW
1060CKMXCalgary, AB50kW
1070CFAXVictoria, BC10kW
1090KFNQSeattle, WA50kW
1130CKWXRichmond, BC50kW
1140CHRBHigh River, AB50kW
1160KSLSalt Lake City, UT50kW
1200CJRJRichmond, BC25kW
1250KKDZSeattle, WA5kW
1260CHMBRichmond, BC50kW
1260CFRNEdmnton, AB50kW
1320CFTERichmond, BC50kW
1340KWLEAnacortes, WA1kW
1380KRKOEverett, WA50kW
1410CJVBRichmond, BC50kW
1430KBRCMt. Vernon, WA5kW
1450KONPPort Angeles, WA1kW
1510KGASpokane, WA50kW
1520KKXASnohomish, WA50kW
1530KFBKSacramento, CA50kW
1550KRPIFerndale, WA10kW
1600KVRIBlaine, WA10kW
1640KDZRLake Oswego, OR1kW
1680KNTSSeattle, WA10kW


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.