RE: [AMRadio] FW: Homebrew receiver
Don, ...the Scott had push pull 25L6 (6L6 with 25 volt filaments), ... The tube is a beam power tetrode like the 6L6, but two are quite different. The 25L5, 35L6 and 50L6 were designed for ac/dc radios, they are physically much smaller and have nowhere near the scrote of a 6L6. They are more like a 6V6 than a 6L6. A later miniature version is the 50C5, which is similar to a 6AQ5. Thanks for pointing that out, I never looked up the tube, and assumed it was just like a 6L6! The Scott only runs about 120 volts on things, so power output is low, but its very clean, about 3 watts likely. I have a Scott SLRM with pushpull 25L6's, and it sounds very good on AM. It is not a real communications receiver, though. It is more like a high quality late 1930's broadcast radio. Most of the precision technology went into careful shielding to prevent the local oscillator from radiating. The rx was used on board the Liberty Ships during WWII as an entertainment radio. They didn't want the enemy to home on the signal from the oscillator and discover the location of the ship. I knew it was designed to prevent anything radiating out of the receiver. They really went to extreme methods to prevent any LO radiation, completely closed cabinet, not even a single vent hole, total shielding between the rf coils in the RF amp, etc. I knew it was designed just before WW2, and also knew it was designed for use on ships, but I did not know about the liberty ships. Its only real drawback is the frequency resolution, the stability is very good, the noise is very low, good bandwidth choices. I had problems with the magic eye tube, it was too dim till I increased the voltage to it...I think I built a little separate high voltage supply and got the plate voltage up to 200 volts, and the tube is nice and bright. The mixer design they used looks very interesting, and works very well. Its a pentode, and they use one grid for rf, another for the LO injection. I have not seen that circuit design in any of the handbooks or other receivers, they use multigrid converter tubes (4 grids) which are very noisy, or use tubes with 3 grids but inject the LO into the rf grid or the cathode circuit, not another grid like the Scott. The circuit Scott used seems to be very quiet, but isolates the LO very well from the tuned circuits in the mixer RF input. The Scott I have is just like new,, and in 10 years or so has not had a single problem other than the dim magic eye tube. Radio shack made a good foam suspension speaker with wizzer cone that fits in, and sounds very good! Another great thing about the Scott SLRM is that both 80 and 40 meters are on the same band (2). Tuning is a little touchy, but no band changing between 80 and 40 meters! Its quite a good receiver for AM when things are not crowded, and when the band is packed, I don't want to listen anyway, too much of a chore on any receiver. Its nice to use when hanging around the shack building things, a real fine sounding classic! Brett N2DTS Don K4KYV _ Add MSN 8 Internet Software to your current Internet access and enjoy patented spam control and more. Get two months FREE! http://join.msn.com/?page=dept/byoa ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
[AMRadio] RE: new home brew receiver progress
I am making good progress on the new receiver. I have tried lots of circuits for the local oscillator, and have come up with a good circuit. Its very hard to build a good stable oscillator without complex coils with feedback windings. I found a circuit in the Bill Orr handbook, out of the C-W man's receiver that uses one untapped coil, one side goes to ground, a variable cap across the coil to ground, an RF choke in the cathode, and a pair of caps in series from the grid to ground, the center (between the caps) goes to the cathode for feedback. I tried various coil types, various size ceramic slug tuned forms with various gauge wire on them, pre made phenolic slug tuned forms that I unwound some wire from till I got to the correct frequency, large ceramic coil forms, and B+W coil stock. The best results are from the B+W coil stock, a much higher Q coil, gives less harmonics out. I changed the design some, instead of using the 6U8, I used a 6C4, since the ARRL handbook says a separate oscillator tube results in a more stable receiver. 2nd harmonics are down more than 45 DB with the B+W coil, much less with some of the others, some generate harmonics out to 60 Mhz or so! The oscillator keeps working down to as little as 10 volts on the plate, and I may run it at 75 volts. I just tack parts together on the workbench, hooked up to the old heathkit variable high voltage power supply, so the finished product should actually work better, with short wires and a sensible layout. To switch between 80 and 40 meters, all I need to do is short part of the coil to ground. One end is always connected to ground, and a tap to ground will give 40 meters, I tested it, and it works well, no drop in output, no increase in harmonics. I plan on putting the coil and switch in a small box to isolate it. I think the mixer tube will be a 6AH6, and I will try to inject the osc into the cathode. If that does not work well, I can inject it into the grid. The ARRL 1967 handbook has lots of good info about this sort of mixer stuff on page 99. I think the IF amps will be 2 or 3 6BZ6 tubes. The same handbook gives circuits and values for various tubes. I have a circuit for the low distortion detector, and the S meter amp, but have to change from octal tubes to 7 or 9 pin tubes, from a 6SN7 for the S meter circuit, and the detector needs a diode, and a triode, like the 6SN7. I may be able to drive the S meter directly with an IF amp plate current, like the Gonset G76 does. I found a nice S meter from an old heathkit receiver on E bay a while ago. It can be lit up from the back, a nice touch, and its only an S meter, not out of a transceiver with plate current and so on.. I have the chassis, the side supports and front panel, the IF transformers, the digital frequency display, the tuning cap and various reduction drives, nice ceramic tube sockets, the antenna tuning coil forms, loads of VR tubes, and all the other tubes. The Kiwi filters have not arrived yet, and I may need to buy a power transformer, to give 200 volts out with choke input, and filament windings. Its really getting to be ham radio season, with building and operating, and I am having FUN! Brett N2DTS
Re: [AMRadio] RE: new home brew receiver progress
Bret, I think what you are doing is GREAT! Really enjoy reading all of your email notes in the theory, design, construction (and hopefully not too much trouble shooting on the bench HI). Keep up the good work. I look forwards in reading your post, as it is actually a tutorial for us all. Of note, I am not an engineer; I am a retired mental health psychologist. Don't let the think they know it all, but their wives know better types change your course in this project as this retarded shrink wishes to be on for the bus ride in the back seat enjoying the view with you driving. HI bst 73 de Robert WPE4FGR W4RL Pensacola Florida. Brett Gazdzinski wrote: I am making good progress on the new receiver. I have tried lots of circuits for the local oscillator, and have come up with a good circuit. Its very hard to build a good stable oscillator without complex coils with feedback windings. I found a circuit in the Bill Orr handbook, out of the C-W man's receiver that uses one untapped coil, one side goes to ground, a variable cap across the coil to ground, an RF choke in the cathode, and a pair of caps in series from the grid to ground, the center (between the caps) goes to the cathode for feedback. I tried various coil types, various size ceramic slug tuned forms with various gauge wire on them, pre made phenolic slug tuned forms that I unwound some wire from till I got to the correct frequency, large ceramic coil forms, and B+W coil stock. The best results are from the B+W coil stock, a much higher Q coil, gives less harmonics out. I changed the design some, instead of using the 6U8, I used a 6C4, since the ARRL handbook says a separate oscillator tube results in a more stable receiver. 2nd harmonics are down more than 45 DB with the B+W coil, much less with some of the others, some generate harmonics out to 60 Mhz or so! The oscillator keeps working down to as little as 10 volts on the plate, and I may run it at 75 volts. I just tack parts together on the workbench, hooked up to the old heathkit variable high voltage power supply, so the finished product should actually work better, with short wires and a sensible layout. To switch between 80 and 40 meters, all I need to do is short part of the coil to ground. One end is always connected to ground, and a tap to ground will give 40 meters, I tested it, and it works well, no drop in output, no increase in harmonics. I plan on putting the coil and switch in a small box to isolate it. I think the mixer tube will be a 6AH6, and I will try to inject the osc into the cathode. If that does not work well, I can inject it into the grid. The ARRL 1967 handbook has lots of good info about this sort of mixer stuff on page 99. I think the IF amps will be 2 or 3 6BZ6 tubes. The same handbook gives circuits and values for various tubes. I have a circuit for the low distortion detector, and the S meter amp, but have to change from octal tubes to 7 or 9 pin tubes, from a 6SN7 for the S meter circuit, and the detector needs a diode, and a triode, like the 6SN7. I may be able to drive the S meter directly with an IF amp plate current, like the Gonset G76 does. I found a nice S meter from an old heathkit receiver on E bay a while ago. It can be lit up from the back, a nice touch, and its only an S meter, not out of a transceiver with plate current and so on.. I have the chassis, the side supports and front panel, the IF transformers, the digital frequency display, the tuning cap and various reduction drives, nice ceramic tube sockets, the antenna tuning coil forms, loads of VR tubes, and all the other tubes. The Kiwi filters have not arrived yet, and I may need to buy a power transformer, to give 200 volts out with choke input, and filament windings. Its really getting to be ham radio season, with building and operating, and I am having FUN! Brett N2DTS ___ AMRadio mailing list AMRadio@mailman.qth.net http://mailman.qth.net/mailman/listinfo/amradio
[AMRadio] FS: Swan 350 with matching Power Supply
Hi all Nice old radio. The Swan 350.. Has the matching power supply with speaker.In pretty nice shape for the age, a little surface rust on the top of the chassis,But very minor, bottom of chassis is clean. The radio works!! Put out over 200 watts on 40 and 75 meters. I have talk on it for hours. Covers have been professionally repaint by a friend at auto shop and look good. Comes with Astatic hand mic and the all important manual with schematic $135.00 plus shipping or pick up in central Florida thanks john KI4NR
