Bob Weaver
Stations Heard: 109
Large, detailed schematic (600k)
6JH8 ONE TUBE DIRECT CONVERSION RECEIVER
This strange circuit is the result of my fascination with beam deflection tubes, also known as sheet beam tubes. They were designed to be used as balanced demodulators, and are noted for having an exceptionally linear transfer characteristic between the deflectors and the plates.
While not wanting to get into too many details of this tube's operation, the most important thing to note is that any signal applied to grid 1 will be amplified and appear as an in-phase (common mode) signal at the two plates. On the other hand, any signal applied between the deflector plates will be amplified and appear as an out of phase (differential mode) signal at the two plates. This allows the different signals to be separated and redirected as necessary. This unique characteristic allows reflexing the audio back through the tube without it breaking into oscillation.
To work as a direct conversion receiver, the RF signal passes through a tuned RF stage consisting of T1 and one section of the tuning capacitor, and is applied to grid 1. A signal from the local oscillator running at the same RF frequency is applied to the deflectors. The result of this mixing operation is an out of phase audio signal at the plates. The out of phase audio is detected in T3 and from the secondary is routed back to grid 1. This audio signal is amplified and appears at the plates as an in-phase signal which passes through the primary of T3, this time with no effect, and then to T4 where it is impedance matched to the headphones.
Feedback for the local oscillator is taken as a differential signal between both plates through a DC blocking capacitor to the primary winding of the oscillator coil T2. Half of the secondary winding forms a parallel tank circuit with one gang of the tuning capacitor, and feeds back to one deflector. The other half of the secondary winding provides an equal but out of phase signal for the other deflector. For best operation the deflectors need a slight negative bias. With a B+ of about 160 volts, the optimum deflector bias is about -9 volts, and is provided by a standard 9V battery permanently wired into the circuit. As there is no current drawn, the battery will last as long as its shelf life.
Because the oscillator circuit operates on differential signals, it seems to be extremely immune to influence from the incoming RF signal which is common mode. The result is that the oscillator hardly pulls at all, even when tuned near a very powerful local station. This in fact, is almost a detriment, because the tuning has to be precisely on frequency before the oscillator locks in. To help, the tuning capacitor has a 6:1 reduction drive, and an additional 5 pF variable capacitor in parallel with the oscillator section of the tuning capacitor, is mounted on the front panel for fine tuning. On the other hand, the stability of the oscillator, allows accurate calibration of the dial and provides immunity from adjacent stations.
For RF regeneration, feedback for the tickler winding of T1 is taken from the cathode circuit. By taking the signal from the cathode, only common mode RF should be present, and none of the oscillator signal (which should be entirely differential).
All of this sounds good in theory, but how does it work in practice? I have to admit, I wasn't expecting this to be a very well behaved circuit. I had expected that a certain amount of unbalance in the differential circuits would lead to some spurious common mode audio cropping up where it wasn't wanted and would lead to oscillation. However, I also figured that this could be cancelled out by proper phasing of the secondary of T3 so that it would be fed back negatively and disappear. It turned out that the circuit was surprisingly well behaved, and worked pretty much according to plan. Maybe better. There was a difference in operation depending on the phasing of the T3 secondary, but it was hard to tell which way was actually better. By the time I had desoldered the transformer leads, flipped them and resoldered, then powered up the receiver, listening conditions would have changed enough that a proper comparison could not be done.
Finally, I wired up a DPDT switch to reverse the polarity on the fly. The result was that in one position (which I will refer to as negative mode) the audio signal level was at a reasonable level and frequency response was quite flat. With the switch in the other position (positive mode), the audio level was a bit greater, and there was some discernible positive audio feedback which did not necessarily lead to oscillation. It actually provided a bit of extra audio gain and a bandpass filter effect which sometimes helped intelligibility of the received signal. With the volume control turned all the way up, it could go into oscillation. However, the transition was very smooth as the audio bandpass became very narrow and began to ring a bit and then finally oscillate. This behavior was affected by the amount of RF regeneration. With the regen set at a modest level, the operation was as described. At the high end of the band, it was possible to turn up the regen quite a bit to get more RF gain, at the expense of audio stability. As a result, the audio gain had to be reduced somewhat when the RF regen was cranked up (especially with the audio phase switch in positive mode). (It is important to make sure that the regeneration is strictly working to provide amplification, and not detection.) In the end, I decided that the DPDT phasing switch should remain in the circuit permanently in order to pick the best operating mode according to signal conditions.
Regarding the RF regeneration, only a single turn was required for the tickler winding, but in retrospect this probably makes sense considering the degree of coupling that comes from using a toroid core. In operation, there was a very abrupt increase in gain as the resistance of the regen pot was increased just a bit above zero. With further rotation of the regen control, the gain remained about the same and then decreased just a bit before finally breaking into oscillation. The optimum position remained the same for frequencies below about 1400 kHz. And so I normally left it in this position and didn't touch it again. Above 1400 kHz, if I was trying to pick up and especially weak signal, I found that the regen could be turned up quite a bit higher to get more gain without breaking into oscillation.
In operation, I found this receiver to be comparable in performance to my previous homebrew receiver which is a two tube superhet with reflexed audio. The new set was a bit more complicated to operate however. It was also somewhat more susceptible to overloading from powerful local stations. Additionally, my antenna matching unit which has a significant effect on the amount of received signal, tended to be more critical with this receiver. As the antenna matcher was adjusted near to the optimum position, it could cause the receiver to break into oscillation. I suspect this may be due to the matcher (being just slightly off frequency) causing uneven attenuation of upper and lower sidebands which resulted in unbalanced signals passing through the receiver, and messing up the common/differential mode signal behavior. This was somewhat annoying, but not an insurmountable problem. With a bit of practice, I got the hang of tuning the antenna, and found the receiver quite easy to operate. I just had to make sure to keep the audio gain turned down until the everything was tuned in.
Being a direct conversion receiver, there was never any question about being on frequency. There was always a clear beat note as each station was tuned in. Even in situations where no station could be heard, there was almost always enough of a carrier from some distant station, to get an audible beat note, and hence there were markers every 10kHz virtually across the entire band. (I was able to use a second receiver with digital tuning to check the frequency when necessary. The second receiver would easily pick up the local oscillater frequency.)
For headphones, Sony MDR-W08 Sport Walkmans were used. Initially, a pair of homebrew piezo phones were tried, but these required the use of an extra matching transformer which added enough insertion loss to negate any benefit of their extra sensitivity. I could have wired the output transformer as an autotransformer to get a better match, but I wasn't crazy about having the B+ that close to my ears. I've come to the conclusion, after using the MDR-W08s in a couple of DX contests, that they are surprisingly sensitive, not at the level of sound powered phones, but seemingly better than standard high-Z magnetic phones. To use the piezo phones in a tube circuit like this one, I've found that the output transformer needs to have about a 2:1 ratio. A probable future modification will be to find a good 2:1 output transformer, and go back to the piezo phones.
The antenna used in the contest was a relatively short long wire, about 35 feet strung out in a horizontal L (with each leg about equal length), running above my deck on the shore of Lake Katepwa. Being almost right on the water, seems to help bring in those DX signals. There was also very little local interference, so I was able to make out quite a few weak signals that would have been otherwise obliterated.
The antenna matcher is an L network consisting of a tapped coil wound on a ferrite core, and a 100pF variable capacitor. An SPST switch allows connection of the variable capacitor to either the antenna or the receiver side of the coil. The matcher makes a significant difference in the amount of received signal, especially when the antenna is as short as this one was.
Log
Time is GMT. Location: Katepwa Lake, SK
Time Freq. Stn. Dist
No. Date UTC kHz ID km Location
=== ========== ===== ==== ==== ==== ========
001 2008-08-01 09:45 1690 WVON 1573 BERWYN, IL, US
002 2008-08-01 09:56 1700 XEPE 2284 TECATE, BN, MX
003 2008-08-01 10:20 1680 KDOW 1409 SEATTLE, WA, US
004 2008-08-01 10:26 1660 KXOL 1227 BRIGHAM CITY, UT, US
005 2008-08-01 10:26 1660 KQWB 660 WEST FARGO, ND, US
006 2008-08-01 10:58 1630 KRND 1066 FOX FARM, WY, US
007 2008-08-01 11:02 1620 KYIZ 1396 RENTON, WA, US
008 2008-08-01 11:20 730 CKDM 245 DAUPHIN, MB, CA
009 2008-08-01 11:20 800 CHAB 128 MOOSE JAW, SK, CA
010 2008-08-01 11:25 750 CKJH 224 MELFORT, SK, CA
011 2008-08-01 11:26 980 CJME 78 REGINA, SK, CA
012 2008-08-01 11:28 620 CKRM 80 REGINA, SK
013 2008-08-01 11:30 540 CBK 169 WATROUS, SK
014 2008-08-01 11:30 690 CBKF1 220 GRAVELBOURG, SK, CA
015 2008-08-01 11:35 940 CJGX 109 YORKTON, SK, CA
016 2008-08-01 11:47 650 CKOM 253 SASKATOON, SK, CA
017 2008-08-01 11:48 600 CJWW 270 SASKATOON, SK, CA
018 2008-08-01 11:53 710 KXMR 484 BISMARCK, ND, US
019 2008-08-01 11:58 860 CBKF2 273 SASKATOON, SK, CA
020 2008-08-01 12:00 880 CKLQ 297 BRANDON, MB, CA
021 2008-08-01 12:12 660 KEYZ 271 WILLISTON, ND, US
022 2008-08-01 12:22 570 CKSW 302 SWIFT CURRENT, SK
023 2008-08-01 12:44 680 CJOB 473 WINNIPEG, MB, CA
024 2008-08-01 13:01 1190 CFSL 135 WEYBURN, SK, CA
025 2008-08-01 13:05 1280 CJSL 187 ESTEVAN, SK, CA
026 2008-08-01 13:51 590 KGLE 407 GLENDIVE, MT, US
027 2008-08-02 09:05 700 KALL 1263 NORTH SALT LAKE CITY, UT, US
028 2008-08-02 09:08 740 CBX 732 EDMONTON, AB, CA
029 2008-08-02 09:25 720 WGN 1532 CHICAGO, IL, US
030 2008-08-02 09:30 770 CHQR 732 CALGARY, AB, CA
031 2008-08-02 09:40 780 WBBM 1534 CHICAGO, IL, US
032 2008-08-02 09:59 830 WCCO 979 MINNEAPOLIS, MN, US
033 2008-08-02 10:01 850 KOA 1245 DENVER, CO, US
034 2008-08-02 10:29 880 KRVN 1177 LEXINGTON, NE, US
035 2008-08-02 10:33 890 CJDC 1237 DAWSON CREEK, BC, CA
036 2008-08-02 11:00 960 CFAC 717 CALGARY, AB, CA
037 2008-08-03 06:15 1620 KOZN 1207 BELLEVUE, NE, US
038 2008-08-03 06:24 1600 KPNP 974 WATERTOWN, MN, US
039 2008-08-03 06:32 1590 KVGB 1427 GREAT BEND, KS, US
040 2008-08-03 06:45 1590 KLFE 1409 SEATTLE, WA, US
041 2008-08-03 07:16 1590 KCNN 573 EAST GRAND FORKS, MN, US
042 2008-08-03 07:50 1490 CJSN 364 SHAUNAVON, SK
043 2008-08-03 08:00 1460 KLTC 430 DICKINSON, ND, US
044 2008-08-03 08:12 1450 KQDI 664 GREAT FALLS, MT, US
045 2008-08-03 08:19 1430 KLO 1251 OGDEN, UT, US
046 2008-08-03 08:42 1390 KRRZ 324 MINOT, ND, US
047 2008-08-03 08:45 1380 KLIZ 847 BRAINERD, MN, US
048 2008-08-03 08:46 1370 KXTL 843 BUTTE, MT, US
049 2008-08-03 09:07 1310 KNOX 574 GRAND FORKS, ND, US
050 2008-08-03 09:15 1300 KGLO 1159 MASON CITY, IA, US
051 2008-08-03 09:18 1290 CFRW 463 WINNIPEG, MB, CA
052 2008-08-03 09:37 1260 CFRN 752 EDMONTON, AB, CA
053 2008-08-03 09:41 1220 CJRB 301 BOISSEVAIN, MB, CA
054 2008-08-03 09:50 1160 WYLL 1569 CHICAGO, IL, US
055 2008-08-03 10:06 1140 KSOO 957 SIOUX FALLS, SD, US
056 2008-08-03 10:06 1140 CHRB 717 HIGH RIVER, AB, CA
057 2008-08-03 10:07 1110 KFAB 1215 OMAHA, NE, US
058 2008-08-03 10:11 1090 KBOZ 788 BOZEMAN, MT, US
059 2008-08-03 10:15 1050 CKSB 471 ST. BONIFACE, MB, CA
060 2008-08-03 10:25 1040 CKST 1390 VANCOUVER, BC, CA
061 2008-08-03 10:28 1040 KCBR 1322 MONUMENT, CO, US
062 2008-08-03 10:37 1010 CBR 726 CALGARY, AB
063 2008-08-03 10:40 1030 KTWO 893 CASPER, WY, US
064 2008-08-03 10:42 1000 KOMO 1412 SEATTLE, WA, US
065 2008-08-03 10:53 950 KJR 1415 SEATTLE, WA, US
066 2008-08-04 08:47 1330 WLOL 1009 MINNEAPOLIS, MN, US
067 2008-08-04 08:50 1400 KXGN 406 GLENDIVE, MT, US
068 2008-08-04 08:59 1540 KXEL 1280 WATERLOO, IA, US
069 2008-08-04 09:03 1570 CKMW 435 MORDEN, MB, CA
070 2008-08-08 09:27 550 KFYR 482 BISMARCK, ND, US
071 2008-08-08 09:28 560 KMON 664 GREAT FALLS, MT, US
072 2008-08-08 10:11 580 CKUA 734 EDMONTON, AB, CA
073 2008-08-08 10:32 570 WNAX 988 YANKTON, SD, US
074 2008-08-08 10:33 580 KIDO 1246 NAMPA, ID, US
075 2008-08-08 10:34 590 KQNT 1045 SPOKANE, WA, US
076 2008-08-08 10:52 610 CKYL 1084 PEACE RIVER, AB
077 2008-08-08 11:14 630 KFXD 1245 BOISE, ID, US
078 2008-08-08 11:16 630 CHED 732 EDMONTON, AB, CA
079 2008-08-08 11:19 640 KGVW 782 BELGRADE, MT, US
080 2008-08-08 11:26 670 KBOI 1252 BOISE, ID, US
081 2008-08-08 11:28 680 KNBR 2071 SAN FRANCISCO, CA, US
082 2008-08-08 11:31 690 CBU 1408 VANCOUVER, BC
083 2008-08-09 10:21 710 KIRO 1414 SEATTLE, WA, US
084 2008-08-09 10:32 900 CKBI 307 PRINCE ALBERT, SK, CA
085 2008-08-09 10:40 910 KCJB 323 MINOT, ND, US
086 2008-08-09 10:56 910 CKDQ 679 DRUMHELLER, AB, CA
087 2008-08-09 11:05 920 KXLY 1054 SPOKANE, WA, US
088 2008-08-09 11:07 920 CFRY 382 PORTAGE LA PRAIRIE, MB, CA
089 2008-08-09 11:26 930 KSEI 1089 POCATELLO, ID, US
090 2008-08-09 11:48 1040 WHO 1276 DES MOINES, IA, US
091 2008-08-10 06:59 820 KUTR 1323 TAYLORSVILLE, UT, US
092 2008-08-10 08:36 890 WLS 1579 CHICAGO, IL, US
093 2008-08-10 09:26 1070 KVKK 796 VERNDALE, MN, US
094 2008-08-10 09:30 1070 KNX 2219 LOS ANGELES, CA, US
095 2008-08-10 09:55 1100 KNZZ 1357 GRAND JUNCTION, CO, US
096 2008-08-10 10:21 1230 KXLO 582 LEWISTOWN, MT, US
097 2008-08-10 11:06 1270 KIML 722 GILLETTE, WY, US
098 2008-08-10 11:10 1320 CHMB 1396 VANCOUVER, BC, CA
099 2008-08-10 11:13 1350 KTIK 1247 NAMPA, ID, US
100 2008-08-10 11:25 1550 KRPI 1374 FERNDALE, WA, US
101 2008-08-11 08:43 1510 KCKK 1242 LITTLETON, CO, US
102 2008-08-11 08:44 1510 KGA 1060 SPOKANE, WA, US
103 2008-08-11 09:14 1420 KIGO 996 ST. ANTHONY, ID, US
104 2008-08-11 09:25 1410 CFUN 1396 VANCOUVER, BC, CA
105 2008-08-11 09:30 1450 KMMS 778 BOZEMAN, MT, US
106 2008-08-11 09:34 1470 KKTY 890 DOUGLAS, WY, US
107 2008-08-11 09:45 1470 CJVB 1393 VANCOUVER, BC, CA
108 2008-08-11 11:06 1530 KFBK 1918 SACRAMENTO, CA, US
109 2008-08-11 11:15 1580 WTTN 1399 WATERTOWN, WI, US