Steven
Thomas
:Usually it will not. If an oscillator tube gets weak, it usually won't oscillate, so the radio just won't work. If the mixer and oscillator sections were separate and used separate cathodes, then this would be more possible, but since many oscillator tubes use the same cathode for both sections (6 and 12SA7, 6A7, 6A8, 1LA6, 1A6, 6K8, etc.), a weak cathode that would normally cause the mixer section to not amplify well would also cause the oscillator section to not oscillate. I have had oscillator tubes that make a radio perform >slightly< worse. You might think that a weak oscillator will produce a weak oscillation, but just like you need to hold a microphone near a speaker to some degree or another before it'll oscillate, you need a certain amount of strength in the tube before it'll oscillate. It is also difficult to tell with most superheterodyne radios, because AVC compensates for weak and strong signals.
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:Thomas
Thomas
:Thanks for the response. My Radiola is using a 12J5GT ocsillator tube. It also uses a 12SG7 for the mixer.
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::Usually it will not. If an oscillator tube gets weak, it usually won't oscillate, so the radio just won't work. If the mixer and oscillator sections were separate and used separate cathodes, then this would be more possible, but since many oscillator tubes use the same cathode for both sections (6 and 12SA7, 6A7, 6A8, 1LA6, 1A6, 6K8, etc.), a weak cathode that would normally cause the mixer section to not amplify well would also cause the oscillator section to not oscillate. I have had oscillator tubes that make a radio perform >slightly< worse. You might think that a weak oscillator will produce a weak oscillation, but just like you need to hold a microphone near a speaker to some degree or another before it'll oscillate, you need a certain amount of strength in the tube before it'll oscillate. It is also difficult to tell with most superheterodyne radios, because AVC compensates for weak and strong signals.
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::Thomas
I notice the characteristics of oscillator tubes quite often when playing with the old superheterodynes, because they have a separate tuning dial for the oscillator. Most of these old sets have a difficult time tuning the modern broadcast band, and I try to find tubes with the least amount of capacity between the elements in order to tune as high as possible. When doing so with the oscillator, I leave the RF tuning dial peaked, and only tweak the oscillator dial after each tube swap. I can tell which tube has the least capacity by noticing which direction the dial is moved. Had the RF tuner been ganged during any one of these dial changes, then it would be de-tuned, and possibly sound as though one of the tubes are weak.
Having a separate dial for the oscillator can also reveal a few things that will back up some of what you have already said. If the oscillator tube has a separate rheostat to control the filament, then it can be changed somewhat either up or down, which will usually reduce the signal level. However, the tuning dial can be touched up afterward and the signal level will usually go right back up where it was before the change. Looked at another way, a weak or strong tube would do about the same thing. Dittos if the plate voltage, grid bias, or pickup coil coupling is changed. All within reason, of course. All these changes may not always do exactly the same thing on all these old superhets, but generally speaking they usually do.
The radio that I am tinkering with right now can be used as an example. The original construction article calls for 90 volts on the plate of the oscillator, but I am running it at 45 volts simply because I do not want to run another bus wire to that part of the circuit. It works about the same with either voltage, and on this particular radio, it does not even change the oscillator dial setting.
Regarding how the dials didn't go up as high back then, though, you must consider that above about 1500 KC was considered the police band. I have many radios that stop at 1500 or 1600 KC. I have others that stop here for one band and continue on upward with another band. The oscillator handles the higher frequencies well. It annoys me, though, when I'm using an old radio and there's some station up around 1650 or 1700. Some of my radios just don't go up there. Since there's nothing worth listening to on AM, though, it doesn't bother me much anymore.
Thomas
:There is one little thing that maybe should be added to what you have already said, Thomas. If a station shifts after an oscillator tube swap, the alignment should be tweaked to make certain that the oscillator is tracking properly with the RF tuner. Some RF amplifiers tune a little broad though, so a slight shift may not reduce the signal level enough to be heard. However, if a reduced signal is heard, it may lead some people to believe that a weak tube is the cause. Of course, an oscillator does not always track precisely all the way across the band, so a certain amount of de-tuning can be expected on certain areas of the dial.
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:I notice the characteristics of oscillator tubes quite often when playing with the old superheterodynes, because they have a separate tuning dial for the oscillator. Most of these old sets have a difficult time tuning the modern broadcast band, and I try to find tubes with the least amount of capacity between the elements in order to tune as high as possible. When doing so with the oscillator, I leave the RF tuning dial peaked, and only tweak the oscillator dial after each tube swap. I can tell which tube has the least capacity by noticing which direction the dial is moved. Had the RF tuner been ganged during any one of these dial changes, then it would be de-tuned, and possibly sound as though one of the tubes are weak.
:
:Having a separate dial for the oscillator can also reveal a few things that will back up some of what you have already said. If the oscillator tube has a separate rheostat to control the filament, then it can be changed somewhat either up or down, which will usually reduce the signal level. However, the tuning dial can be touched up afterward and the signal level will usually go right back up where it was before the change. Looked at another way, a weak or strong tube would do about the same thing. Dittos if the plate voltage, grid bias, or pickup coil coupling is changed. All within reason, of course. All these changes may not always do exactly the same thing on all these old superhets, but generally speaking they usually do.
:
:The radio that I am tinkering with right now can be used as an example. The original construction article calls for 90 volts on the plate of the oscillator, but I am running it at 45 volts simply because I do not want to run another bus wire to that part of the circuit. It works about the same with either voltage, and on this particular radio, it does not even change the oscillator dial setting.
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The oscillator tracking that I was talking about happens because of ganged tuning condensers, and has nothing to do with IF alignment. I used an early superhet as part of the explanation because it has separate tuning condensers, and I thought it may help explain things. Maybe I should not have used the words, “oscillator tracking.” Is that terminology still used today?
With a ganged tuning condenser, the realignment that may be necessary after a tube change is done with the oscillator trimmer on the tuning condenser, and maybe the padder condenser. This realignment would only be necessary if something happens to change the dial setting, such as your statement, “A weak tube may cause stations to shift slightly.” My reasoning is that a stronger tube may also cause stations to shift slightly. In either case, the difference would probably be negligible on most of the cheaper radios, but on some of the better sets with a pre-selector stage, it could be more noticeable.
To explain further, let’s assume that a radio has a two gang tuning condenser, and that the dial is set at 1500kc. Lets also assume that the set is warmed up to prevent any further oscillator drift, and that both the oscillator and antenna trimmers are perfectly adjusted on scale for that frequency. Now lets assume that we replace the oscillator tube with another. It may be weaker or stronger, have more capacity or less, or whatever. Since all tubes are not created exactly equal, lets assume that the replacement tube requires a slightly different dial setting to tune 1500kc. When this slightly different dial setting is made, it also rotates the RF tuning condenser, which was already perfectly tuned by the antenna trimmer. This action doesn’t happen on the older sets with individual dials, and why I wrote all that stuff about them.
Now that the RF is slightly de-tuned, the signal probably won't sound as loud, even with a better oscillator tube. In other words, the oscillator is heterodyning a weaker RF signal. If it tracks the signal by that same difference all the way across the band, then all stations will sound weaker. Of course, it could sound worse or maybe even better as it approaches the other end of the dial, but that would depend on the padder setting.
However, if the oscillator trimmer it readjusted before rotating the dial away from 1500kc, then it will be heterodyning a properly tuned RF signal. This adjustment will make the signal sound louder, which should be about the same as it was with the other oscillator tube. In other words, it doesn’t make much difference how strong or weak the oscillator is. Like you implied in an earlier post, the oscillator either works or it doesn’t.
Since I used a two gang set in the above example, a good bit of what I have written may not apply. Typically, these sets have fairly broad RF tuning, and the dial can be rotated somewhat without a signal loss to speak of. Some of them tune so broad that a padder is not used to set the oscillator at the low end of the dial. When one of these sets have a tracking problem, there ain’t much you can do about it except bend the tuning condenser plates and hope it works.
If you want to demonstrate how well a weak oscillator can work, turn on another AM radio somewhere else it the room. Then slowly turn the dial of either radio. Chances are good that you will hear the oscillator as it approaches the dial setting of the other radio. This was a real problem in the early days in large apartment complexes. In some cases, the oscillator tube could be removed and still receive the station using a neighbor’s oscillator. Later radios do not have the oscillator pickup in series with the loop, so they do not radiate near as much. Since most of my radios are the early type, I don’t have a problem finding out if the oscillator is working in a set that I am troubleshooting. I just turn on another radio and listen for it.
Talking about this kind of thing reminds me of a very early super that I have the plans for. I have not built it yet, but may get around to it if I live long enough. The guy who designed it did not quite understand Armstrong’s paper on the subject. Armstrong used the word “separate” when describing the oscillator, meaning that it was not part of the received signal. This guy took it literately and built the oscillator in a separate box and set it on the table with the receiver. He went on to say that the oscillator could be placed anywhere else in the room.
On the subject of bandwidth, the Police Band did not exist in the early days. The amateurs were allocated 200-250 meters (1200-1600kc), because everyone thought it was useless. The amateurs proved them wrong, and are responsible for broadcast radio as we know it today. On the other end of the band, ships at sea worked at 600 meters (500kc), and this was considered to be the highest frequency that was useful. These bands were code, not broadcast. When broadcast radio finally came along, it was only allocated two frequencies, 360 and 400 meters, depending on the transmitting power. Talk about a mess. All the broadcasters in a large city were competing at the same frequency and all the regenerative sets were squealing all over town trying to receive them.
Most of the early listeners only wanted to listen to broadcast, but some of them also wanted to receive code. Therefore, the early manufacturers tried to design radios to receive these bands. The high frequency was cheated somewhat because it was difficult to tune without resorting to tapped windings. The Grebe MU-1 had taps, but it was one of the few exceptions. The Grebe was a well designed radio, by the way, and was sold for several years before it became obsolete. Anyhow, the above is the main reason why the next generation of radios only tuned to 1500kc. Some of the less efficient designs could not even tune that high. When the broadcast band was extended and the Police Band established, the amateurs got the short end of the stick again. When I was a kid, my dad tweaked my cousin’s car radio to receive the police band in Beaumont so he could drive around town and listen to them.
Back when radios were designed to receive 500kc, a few more turns of wire were on a tuning coil. I have removed these extra turns on several of the early pukeadynes and received 550-1600kc with them. Sort of. Just barely.
You think there's nothing worth listening to on AM? I’ve been living in the redneck country of Southeast Texas all these years and have probably been thinking that longer than you. While I was still making the AWA meets in Rochester a few years ago, there were a few AM stations in that region that played classical music. I haven’t heard that kind of thing around here on AM radio since about the 1950’s.
Very interesting about the whole separate oscillator thing. When I think about it I realize that it is entirely possible. Like you said, you could tune the old radios off of a neighbor's oscillator. I bet if you pulled the oscillator out of one radio (an AC radio) and then tuned another next to it, you'd get the stations to shift. I've actually done that. I've also made a radio transmitter out of a radio temporarily. I wired audio through a .01 MFD condenser into the grid cap of the 6A7 tube in my Philco 60. The grid cap also had a 1 meg resistor going to the chassis. Then I connected an antenna to the 1st IF tube grid connection (removed from the tube). Then I tuned the radio until the oscillation came in on another radio. Worked well.
I think the reason why the signal shifts slightly with weak or strong tubes is because the stronger a tube is, the higher the frequency it produces for any given combination of component values and voltages. It would make sense that a weaker tube would push a slightly lower frequency for the same given settings. Your explanation of weaker reception makes sense, too.
Regarding Texas and radio reception, well, I'd say that, but I was always afraid that I'd start an argument with someone in here. I guess everyone else here feels the same way I do (for the most part). We have such blood thirsty, blunt, savage (literally, when it comes to "Savage Nation,") broadcasters here in Milwaukee on AM. One station offended our entire Mexican district with the use of the word "wetbacks." That's not the worst of it, really, but I guess it isn't really a preferable word to be using on the radio. I have many Mexican friends who weren't too offended, but I thought it was poor taste.
We have lies from stations, too. Recently, after Hurricane Katrina, an announcer stated that no other countries were helping us. I wrote a nasty letter (shame on me) to the station telling them to open their eyes and see the 17 other nations which were offering us aid, some of them poorer than us (like Mexico, for instance).
AM is just bad. I like N.P.R. I wish it was on AM. Some people think that it is ultra liberal. It isn't, really. The news is about the most well balanced there is, and it tends to lack sensationalism.
Anyway, if I didn't start a political war on this site a long time ago, I might do so right now with what I wrote, because I'm sure there are some who like what is on AM. I think I'm going to drop the subject. Still, I think we can all agree that AM lacks music, especially the kind that sounds ideal on our radios. Classical is better on FM, though.
Thomas
:What I wrote was probably confusing, Thomas. The things I write or say out loud don’t always gee haw with what is inside my head. Regardless of what it sounded like, part of what I said was meant to back up some of your statements, and I hope you don’t think I was being critical.
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:The oscillator tracking that I was talking about happens because of ganged tuning condensers, and has nothing to do with IF alignment. I used an early superhet as part of the explanation because it has separate tuning condensers, and I thought it may help explain things. Maybe I should not have used the words, “oscillator tracking.” Is that terminology still used today?
:
:With a ganged tuning condenser, the realignment that may be necessary after a tube change is done with the oscillator trimmer on the tuning condenser, and maybe the padder condenser. This realignment would only be necessary if something happens to change the dial setting, such as your statement, “A weak tube may cause stations to shift slightly.” My reasoning is that a stronger tube may also cause stations to shift slightly. In either case, the difference would probably be negligible on most of the cheaper radios, but on some of the better sets with a pre-selector stage, it could be more noticeable.
:
:To explain further, let’s assume that a radio has a two gang tuning condenser, and that the dial is set at 1500kc. Lets also assume that the set is warmed up to prevent any further oscillator drift, and that both the oscillator and antenna trimmers are perfectly adjusted on scale for that frequency. Now lets assume that we replace the oscillator tube with another. It may be weaker or stronger, have more capacity or less, or whatever. Since all tubes are not created exactly equal, lets assume that the replacement tube requires a slightly different dial setting to tune 1500kc. When this slightly different dial setting is made, it also rotates the RF tuning condenser, which was already perfectly tuned by the antenna trimmer. This action doesn’t happen on the older sets with individual dials, and why I wrote all that stuff about them.
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:Now that the RF is slightly de-tuned, the signal probably won't sound as loud, even with a better oscillator tube. In other words, the oscillator is heterodyning a weaker RF signal. If it tracks the signal by that same difference all the way across the band, then all stations will sound weaker. Of course, it could sound worse or maybe even better as it approaches the other end of the dial, but that would depend on the padder setting.
:
:However, if the oscillator trimmer it readjusted before rotating the dial away from 1500kc, then it will be heterodyning a properly tuned RF signal. This adjustment will make the signal sound louder, which should be about the same as it was with the other oscillator tube. In other words, it doesn’t make much difference how strong or weak the oscillator is. Like you implied in an earlier post, the oscillator either works or it doesn’t.
:
:Since I used a two gang set in the above example, a good bit of what I have written may not apply. Typically, these sets have fairly broad RF tuning, and the dial can be rotated somewhat without a signal loss to speak of. Some of them tune so broad that a padder is not used to set the oscillator at the low end of the dial. When one of these sets have a tracking problem, there ain’t much you can do about it except bend the tuning condenser plates and hope it works.
:
:If you want to demonstrate how well a weak oscillator can work, turn on another AM radio somewhere else it the room. Then slowly turn the dial of either radio. Chances are good that you will hear the oscillator as it approaches the dial setting of the other radio. This was a real problem in the early days in large apartment complexes. In some cases, the oscillator tube could be removed and still receive the station using a neighbor’s oscillator. Later radios do not have the oscillator pickup in series with the loop, so they do not radiate near as much. Since most of my radios are the early type, I don’t have a problem finding out if the oscillator is working in a set that I am troubleshooting. I just turn on another radio and listen for it.
:
:Talking about this kind of thing reminds me of a very early super that I have the plans for. I have not built it yet, but may get around to it if I live long enough. The guy who designed it did not quite understand Armstrong’s paper on the subject. Armstrong used the word “separate” when describing the oscillator, meaning that it was not part of the received signal. This guy took it literately and built the oscillator in a separate box and set it on the table with the receiver. He went on to say that the oscillator could be placed anywhere else in the room.
:
:On the subject of bandwidth, the Police Band did not exist in the early days. The amateurs were allocated 200-250 meters (1200-1600kc), because everyone thought it was useless. The amateurs proved them wrong, and are responsible for broadcast radio as we know it today. On the other end of the band, ships at sea worked at 600 meters (500kc), and this was considered to be the highest frequency that was useful. These bands were code, not broadcast. When broadcast radio finally came along, it was only allocated two frequencies, 360 and 400 meters, depending on the transmitting power. Talk about a mess. All the broadcasters in a large city were competing at the same frequency and all the regenerative sets were squealing all over town trying to receive them.
:
:Most of the early listeners only wanted to listen to broadcast, but some of them also wanted to receive code. Therefore, the early manufacturers tried to design radios to receive these bands. The high frequency was cheated somewhat because it was difficult to tune without resorting to tapped windings. The Grebe MU-1 had taps, but it was one of the few exceptions. The Grebe was a well designed radio, by the way, and was sold for several years before it became obsolete. Anyhow, the above is the main reason why the next generation of radios only tuned to 1500kc. Some of the less efficient designs could not even tune that high. When the broadcast band was extended and the Police Band established, the amateurs got the short end of the stick again. When I was a kid, my dad tweaked my cousin’s car radio to receive the police band in Beaumont so he could drive around town and listen to them.
:
:Back when radios were designed to receive 500kc, a few more turns of wire were on a tuning coil. I have removed these extra turns on several of the early pukeadynes and received 550-1600kc with them. Sort of. Just barely.
:
:You think there's nothing worth listening to on AM? I’ve been living in the redneck country of Southeast Texas all these years and have probably been thinking that longer than you. While I was still making the AWA meets in Rochester a few years ago, there were a few AM stations in that region that played classical music. I haven’t heard that kind of thing around here on AM radio since about the 1950’s.