Tim Kilboy
Jackson, TN
Stations Heard: 112
The Direct Reflex is a unique design which builds on the 1920's technique of reflexing RF and AF through a single grid on a triode.
In the old triode reflex receivers, a 2nd tuned RF tank was placed in the plate circuit to capture amplified RF. This signal was then rectified and passed through an AF transformer. The AF transformer secondary was then put in series with the 1st tank, in the grid circuit. Now both the original RF as well as the detected AF are both impressed on the grid. The set is now a two stage receiver using one tube.
This old design has the distinct advantages of good 'two stage' sensitivity, and good selectivity due to being double tuned, since it incorporates two tuned tanks. On the other hand, the second tank was not very selective since it was feeding the relatively low impedance of the 3:1 AF transformer primary.
Reflexing died out fairly quickly since it was only really effective on single tube circuits. Although it was tried on 3 and 4 tube sets, to get double the gain, there were endless problems with keeping the set stable across the frequency range that were difficult to overcome. So since reflex technology was best suited for one tubers, and the market was quickly moving toward 5 tube neutrodyne sets, reflexing became extinct.
Enter the 'multi-grid' vacuum tube. During the reflex heyday, only low gain triodes were available and by the time the screen grid tube came along, reflexing was a forgotten fad. However, taking a more modern tube like the 12BA6 Pentode RF Amp with its higher gain (4400 uMhos) and using its suppressor grid as a second input grid (for audio), provides a new spin on an old idea. The anticipated benefit is that both tanks may now run with very light loading, which should lead to optimal sensitivity and selectivity.
The 'Direct Reflex' set demonstrated some excellent traits, but also had some problems. The performance can be broken into ranges. 530-1250 KHz - Superb sensitivity and selectivity, except for 850-950 which was somewhat 'dead'. 1400-1700 KHz - reflexing was not as effective, seemed to work best with reflex gain turned very low. Tube selection may be an issue, since the 12BA6 specs the suppressor (grid 3) as also being an internal shield. Use of the suppressor grid is probably not the optimal way to get a second input. The set could be pushed to oscillation at high gain settings. I improved this problem by putting the second tank in the cathode rather than the plate circuit. Even with this, it seemed apparent that the suppressor grid was 'talking' to the No 1 grid - perhaps due to the shielding function of the suppressor.
Despite its shortcomings, the Direct Reflex made a very respectable DX showing. Station separation was very crisp and sensitivity was as good enough to make it impossible to find a dead frequency. Building this set has probably raised more questions than it answered. I think with selection of a more appropriate tube, it would make it a home run.
Log
Homebrew DX Contest 2007
Tim Kilboy
Jackson, TN
Seq UTC Call Khz
7/29/2007
1 4:45 KTRS 550
2 4:51 WSM 650
3 5:01 CMKA 600
4 5:02 WMT 600
5 5:11 KJSL 630
6 5:16 KCSP 610
7 5:21 WCRV 640
8 5:29 WSCR 670
9 5:34 WLW 700
10 5:36 WGN 720
11 5:40 WIRJ 740
12 5:43 WBBM 780
13 5:48 WHAS 840
14 5:50 WBAP 820
15 5:54 WTAM 1100
16 5:57 KMOX 1120
17 6:00 KWKH 1130
18 6:04 WRVA 1140
19 6:08 WHAM 1180
20 6:15 WOAI 1200
21 6:19 WIND 560
22 6:26 WLS 890
23 6:33 WRJZ 620
24 6:45 KLIF 570
25 6:55 WDXI 1310
26 7:07 WTJS 1390
27 7:16 WJAK 1460
28 7:25 CJCL 590
29 7:30 WLNO 1060
30 7:38 WHO 1040
31 7:52 CFRB 1010
32 7:59 KDKA 1020
33 15:22 WREC 600
34 15:25 WCTA 810
35 15:30 WMUF 1000
36 15:34 WGSF 1030
37 15:37 KWAM 990
38 15:40 KOTC 830
39 15:48 WHBQ 560
40 15:52 WSMB 680
41 15:54 KLCN 910
42 23:15 WJHX 620
43 1:30 WYFN 980
44 1:31 KMBZ 980
45 1:33 WGTK 970
46 1:44 WBT 1110
47 1:48 KAAY 1090
48 2:05 KWHN 1650
49 2:07 WTNI 1640
7/31/2007
50 0:05 WDIA 1070
51 0:29 WAPI 1070
52 0:32 WCRT 1160
53 0:40 WIST 690
54 0:45 WMLB 1690
55 1:12 CMEA 570
56 1:40 WDXL 1490
57 2:00 KFAB 1110
58 2:08 WSB 750
59 2:18 WYLL 1160
60 2:30 WKFN 540
61 3:04 WMVP 1000
62 3:10 CBW 990
63 11:37 WGOL 920
64 11:50 WAKM 950
65 23:57 WLDX 990
8/1/2007
66 0:05 WSMT 1050
67 0:16 WGGH 1150
68 0:56 WCNN 680
69 1:02 WOWO 1190
70 1:30 WLAC 1510
71 1:34 WCKY 1530
8/2/2007
72 0:54 WWLX 590
73 1:01 WAGG 610
74 1:11 CKOC 1150
75 1:28 KRLD 1080
76 1:41 WTWG 1050
77 1:50 WWNC 570
78 1:54 WNSR 560
79 2:10 WSDZ 1260
80 10:30 KCJJ 1630
81 10:34 KATZ 1600
82 11:00 XEWA 540
83 11:02 WKYX 570
84 11:05 WVLK 590
85 23:45 WERC 960
86 23:49 WWL 870
87 23:55 WERH 970
8/3/2007
88 0:05 WELO 580
89 0:10 WZQZ 1180
90 1:32 KFUO 850
91 1:42 WBZ 1030
92 1:45 KYW 1060
93 2:00 KXEN 1010
94 2:09 KFXR 1190
95 2:44 WZRX 1590
96 10:20 WFWL 1220
97 10:30 WMAY 970
8/4/2007
98 0:30 WMAC 940
99 0:34 WSFZ 930
100 0:39 KRJO 1680
101 0:49 WHBY 1150
102 0:54 WGOW 1150
103 1:02 KFAQ 1170
104 1:07 WLLI 1190
105 1:23 WNML 990
106 9:20 WCHS 580
107 9:45 WLOC 1150
108 10:00 KCTA 1030
109 10:34 WWJ 950
110 11:06 KXJK 950
111 11:22 WKAC 1080
112 11:30 WGUN 1010