View Full Version : CW Transmitter
06-24-2003, 08:22 AM
Hi, I'm trying to get info for a cw transmitter project. I want to use it on the 80 meter band. I want to make use of a vfo from a ts-520 if its possible, the 6146 tube for the PA, (perfered 2x6146 in the PA) if easier a 6L6GC tube. I have a small collection of tubes,3 x 6146, 2x 6L6GC, 12AX7A, 2x 12BY7A, 2x 6BA6,4x 6BK7b, 6BQ7A and 6x 6GH8A. I would like to use these for a driver stage or something. Most likely the 12BY7A tube. Any information or web links will be appreciated.
PS, If possible I want break in operation.
Go to a hamfest and find any of several of the ARRL handbooks from the mid 60s to mid 70s, and you should find all kinds of construction projects for the type of rig you are thinking of. #There was a fair amount of overlap from year to year, but some of the books might contain just EXACTLY what you are looking to build. #Some might have a good break-in circuit while others might have the best power supply solution. #But you should be able to get pretty close with just one or two of the books, and the cost should not be all that high!
Keep us posted. #We would all love to hear your results! #The VFO from a 520 is a great idea, (in my humble opinion) and should eliminate a lot of problems in getting a good, stable signal on the air.
73 from Jim AG3Y
06-24-2003, 08:23 PM
The problem with the VFO from the TS520 is it is either around 5 or 8 MHz. All it does is heterodyne with the het oscillator selected for whatever band is switched in. Thus the VFO alone will be of little help.
I like this guy. Great idea for a project. The Handbooks mentioned will have great ideas, great projects and VFOs as well. I would even suggest an old commercial VFO and some work on stabilising it. Fun project, one I think Glen K9STH has gone over here. As long as you're going to have a HV supply anyway, there's no reason not to take this tack.
As for the 6146 vs a 6L6, they're very different tubes. The 6146 should be good for around 50-60 watts out - each. The 6L6 (my first transmitter was a crytal controlled 6L6 oscillator on 40m) is more in the range of 5-8 watts.
Good luck and keep us posted.
ZDY, you can look at the frequency of the 520 VFO as either a curse, or a blessing ! #By using a 5 Mhz ( or 8 Mhz , not sure which without researching ) VFO, hetrodyned with a crystal oscillator to get the right frequency, you can make the output work on any band you choose, providing you pick the right frequency crystal and coils, etc. #The VFO will always work in that narrow range of frequencies that it is best designed for. #
That was the major thing that made radios using the hetrodyning principle so superior to radios that used one oscillator either oscillating on the output frequency, or some sub-multiple of the desired frequency. #A hetrodyned radio was as stable on 10 meters as it was on 80 ! #A multiplied oscillator radio would warble around like crazy on 15 and 10, and was pretty noticable on 20, for that matter.
For a good example of a hetrodyned radio, check and see how popular the old Drake 2B still is ! #Think about how many 520s, 820s, and similar rigs are still on the air!
Forge ahead with that project. #We will be rooting for you all the way !
73 from Jim AG3Y
06-24-2003, 11:09 PM
AG3Y, you are of course correct about the advantages to a heterodyne transmitter. I was merely thinking of this being a beginning homebrew project, and the fact the original poster did not mention the added complexity, I wanted to be sure he 1) knows what he's adding to his project, and 2) that there are other easier, albeit not as ideal, ways to get his project going.
For a start, I think a multiplied oscillator is the way to go. Modify as you learn and move forward. Just my opinion of course. I was actually thinking for a first time project, crystals could be an ok idea. Hams survived on HF with crystals for a lot of years.
06-25-2003, 12:31 AM
The 520 VFO operates in basically the 5.0 - 5.5 MHz range and will have to be heterodyned to get to most of the amateur frequencies although it would hit 15 meters by quadrupling.
As for QSK ("break in") operation: You will either have to use separate antennas on your receiver and transmitter or else build what is called a "TR switch" into your transmitter. A TR switch is basically a single tube that isolates the transmitted r.f. from the receiver while allowing instant switching between transmit and receive. There are several circuits that can take power up to around 100 to 150 watts r.f. in several sources including some of the ARRL Handbooks of the early 1960s. I used one back around 1959 / 1960 that used a 6AH6 with my DX-100 and Hallicrafters S-85 receiver. It worked fine. However, you definitely need to use a low pass filter between the transmitter / TR switch and the antenna because they can generate harmonics.
A 6146 / 6146A / 8298 is good for 90 watts maximum input which will give around 50 watts output, a little less on 10 meters. The 6146B / 8298A, which is really a different tube from the 6146 / 6146A / 8298, is good for 120 watts maximum input or about 65 to 70 watts output, again a little less on 10 meters. However, the 6146 is much happier running about 75 watts or less input and the 6146B at 90 watts or less input.
The 6L6 is good for about 35 watts input or about 22 watts output maximum. Again, less on 10 meters.
The 12BY7 will work fine as a buffer and as a driver. The Heath DX-35 used the 12BY7 as the crystal oscillator/VFO buffer and as the driver for a single 6146. The 12BY7 and the 6CL6 are close relatives and can be used basically interchangeably at least up to 30 MHz and even on 6 meters.
You can use a 12BY7 as a mixer/oscillator with the TS-520 VFO and then a second one as a driver. However, it will be easier to key the transmitter without a "chirp" if you use a different tube for the heterodyne crystal oscillator. The output of both 12BY7 tubes should be at the operating frequency. The 12BY7 will drive a pair of 6146 tubes without any problems. You need to keep the grid drive on a single 6146 series tube to between 1.75 and 2.5 mA and a pair between 3.5 and 5 mA even though the tube specifications call for a maximum of 3 mA per tube. You will get full power output and the tubes will last much longer with the lower grid drive.
For keying, you can key the cathode of the mixer tube, letting both oscillators run, and you will get no chirp on the transmitted signal. This is what is done in the Collins 32S-3 series transmitters for CW operation.
You will have to find crystals that allow the 5.0 MHz signal from the VFO to be mixed to give the amateur band frequencies.
Otherwise, there is nothing critical about constructing a CW transmitter for 160 through 10 meters. Just keep the component leads fairly short and do a good job of soldering.
06-25-2003, 01:19 AM
Well this is fun repling to my own topic. Anyhow I know that adding the vfo will complicate matters, im still going to go ahead with that idea. The breakin op might be omitted, even though it is a nice feature. It might be an addition later but very unlikely due to the fine tuning that most likely will be involved to keep the chirps out. I'm pretty confident in a 6146 pa. Later I plan to used a matched pair. Since the post I have been reading in a couple old books. The 1971 Radio Amatuer Handbook and an 1959 Radio engineering hand book that has enough information to make even an engeers brain go numb(almost too complicated but im understanding more and more). I went to a couple used book stores today, but my little venture didn't pull out anything good this time, oh well. I wanna thank you guys for the replys.
73s You never know you might hear this little rig on the air one day.
06-25-2003, 02:07 AM
When you run tubes in parallel, matching doesn't really do much for you although when operated in push-pull they have a slight advantage. Even though the tubes start out "matched", within a few hours of operation they normally age at a different rates and thus become mismatched very soon.
The major "boat anchor" manufacturers like Collins, Heath, Johnson, etc., that used parallel 6146 tubes in the final amplifiers didn't bother to match the tubes and that gear works fine!
Some audio people think that they can hear the differences between matched pairs and non-matched pairs when used in push-pull audio circuits. There may be a few who can hear the difference, but the vast majority of people cannot tell the difference in performance. Again, after several hours of operation most of the tubes are no longer matched anyway.
I know that there are some people who claim that they have "burned up" tubes in linear amplifiers that were not "matched". This may be true if an almost "dud" is put in with new tubes. But, in general, buying matched pairs only adds to the price. There is nothing wrong with using them. However, as I said before, after the tubes have been in use for a little while they are usually no longer matched anyway.
06-25-2003, 02:59 AM
Thanks for that follow up. I've been going over my ideas and I want to know if you have any added info. Im thinking about using a 9 mhz crystal and mixing it with the vfo 5-5.5mhz for 3.5-4 mhz and also be able to switch it to 14-14.5. Im now thinking about keying the transmitter in the PA stage. What do you think about this idea?
06-25-2003, 03:18 AM
That will work. However, if you use cathode keying on the final then you will have high voltage on the key. If you bias the tubes to "cut off" when the signal is not applied, and key the mixer, you will usually have cleaner keying and no where near as much voltage on the key.
If you use cathode keying of the final make sure that you add a capacitor across the key to cut down on key clicks. There are circuits in all of the older ARRL Handbooks that help eliminate clicks and also help "shape" the CW signal for best copying at the other end.
The reason that lower sideband is "the" mode on 75 meters and upper sideband is "the" mode on 20 meters is because back in the late 1940s and into the 1950s, amateurs could buy the BC-457 command transmitters for very little since they didn't cover any amateur band. Then by heterodyning that with a 9 MHz sideband generator they could get SSB on both 75 and 20 meters with the same basic rig. But, the sideband was inverted. Thus, LSB became common on 75 meters and USB became common on 20 meters.
Of course, today, the conversion "schemes" are different in most rigs. But, the convention of LSB on 75 and USB on 20 is still around.
I forgot that you can double the VFO frequency and get 30 meters without having to use a mixer.
06-25-2003, 03:49 AM
Thats what I was thinking that also. It just seemed easier to key in the PA stage but im probably just not as educated on this as I would like to be. I know that am modulation is often in the mixer stage but can also be in the PA stage. Im not too worried about it though seeing as LSB is most of 80 meter telephony.
06-25-2003, 04:00 AM
I stumbled across this web site while doing my own research...
The Tube Manual is especially useful. I'll second the use of the old ARRL manuals. I picked up the 1959 and 1969 (among others) real cheap and the information is priceless.
One question: I've got a handful of 6GH8As lying around that I'd love to be able to use in a CW transmitter. According to what I've found, these are typically used as "receiving tubes" (not surprising since I think I salvaged them from an old TV set), although they seem to be pretty common in audio amps too.
Obviously you can use solid-state parts like a 2N2222 for a transmitter or a receiver; is there any limitation on using a "receiving tube" in a transmitter? (aside from the obvious limitations on power output)
06-25-2003, 04:32 AM
Many, if not most, of the tubes used in the lower stages of a tube-type transmitter were designed originally for receiving!
Then, there are the television sweep tubes that were used from the 1950s until the 1980s as transmitter finals. Back when they were first used, every television around had a sweep tube in it and they were extremely cheap. However, now days, since televisions no longer use tubes, the sweep tubes are much more expensive than the power tubes that are in the same power range.
I would have to pull my manuals, but I believe that the 6GH8 series tubes were used by Heathkit in a number of their transceivers.
Believe it or not, the "lowly" 12AT7 tube was used as a low-powered, "free running oscillator" transmitter on the 420 - 450 MHz band back in the 1950s and into the 1960s, especially as a "normal" fast scan television transmitter. There are all sorts of circuits around using the 12AT7 as a crystal controlled transmitter for 6 and 2 meters.
Audio output tubes like the 6L6, 6V6, 6AQ5, etc., have all been used for relatively low powered transmitters for many years. Of course, tubes like the 6BQ6, 6DQ6, 6LQ6, etc., sweep tubes were used in thousands of transmitters and even in high powered linear amplifiers like the GLB-1000 series (as well as thousands of illegal "CB" type amplifiers).
The use of many tubes for transmitting functions as well as receiving functions made the old tube-type televisions a "treasure house" of parts. You could literally build a complete, up to 50 watt input CW and AM transmitter and a matching receiver from the parts from a single TV set. All that was required besides the TV set was a crystal (if you didn't build a VFO from TV parts), a microphone, a key to send CW, and an antenna. In fact, there usually were several chokes, etc., in the television that you could unwind to get enough wire for even an antenna!
I know that back when I was in junior high school, and even after I got my license as a freshman in high school, that there was a "garage shop" television repair shop located about 1/2 block from my parents house. The owner gave me many old televisions to take apart for the parts that I had a pretty good stock of components to build just about anything (and I did build a lot of things back then!).
I tell you, when Glen gets on a roll like this, I just sit back in awe and absorb all the information ! #Great discussion guys!
Oh, BTW, you could use a keying relay to keep that cathode voltage off the external key. That keyup voltage can really bite! If you don't use some kind of isolation, at least make sure the frame of your key is on ground and only the contact is hooked to the cathode. It all sounds like it is getting more and more complicated, but you can build this thing up one step at a time, as has been mentioned!
73 from Jim AG3Y keep in touch !