Thanks
:Joe,
:The "12" in the tube number means it needs 12.6 volts to function. I would check to see if a previous owner just plugged that tube in the socket to fill a hole, when what is supposed to be there is the proper tube with a 6.3 volt filament voltage requirement. You can look the schematic up on this site. Good luck.
:Jim
This is a series wired filament radio right across the AC line. Have you tested the 12SK7? When a tube loses vacuum it can act this way. Try another 12SK7 or use 12SG7 or 6SS7 as replacement. Yes, 6SS7 as it has a .15 amp filament like the others.
A hum, not changed by the volume control, is usually caused by bad electrolytic caps. The 12SK7 is an IF amplifier and wouldn't cause the hum.
http://www.nostalgiaair.org/PagesByModel/995/M0014995.pdf
Norm
:Jim---The radio in question is an R.C.A. Victor, Model 56X3.This is the tube numbers with the filament voltage readings I'm getting in brackets....35Z5 (31v), 12J5 (13v), 12SG7 (13v), 12SK7 (6.5v), 12SQ7 (16v), 35L6 (38v). I am getting a steady hum which is not controlled by the voluum control, with no stations being picked up. I am assuming it is caused bt the electrolytic caps but can't be sure until I have them replaced....waiting on my order....I really don't know if the low reading on the filaments of tube 12SK7 would cause a hum or not.
:Thanks
:
:
::Joe,
::The "12" in the tube number means it needs 12.6 volts to function. I would check to see if a previous owner just plugged that tube in the socket to fill a hole, when what is supposed to be there is the proper tube with a 6.3 volt filament voltage requirement. You can look the schematic up on this site. Good luck.
::Jim
Keep in mind that if the tube has a filament short, but otherwise works, you will blow it if you test it on your tube tester with a 12.6 volt setting (if you care...the tube is faulty, so you may not). Tubes with filament shorts often don't function normally anyway, because many filament shorts will cause only half of the cathode to light, so you may want to simply discard the tube in any case. I just explained all of this so that you get the full picture, not necessarily so that you could save the tube, though you can if you can get it to work this way (it won't work at all if the vacuum is lost).
Thomas
2. to prevent the filament from oxidizing quickly and burning out. (Just as in an incandescent lamp)
Rich
:The tube can also act that way if it has an internal filament short. All 12 volt tubes in your radio will have the same resistance readings when cold (and I mean COLD, not just 5 minutes after the set was turned on....wait an hour) regardless of whether their vacuums have been lost or not. If the questionable tube has a lower resistance, then its filament has a short. It may still operate if this is the case (though it's better to replace). If it has the same resistance as all the other tubes, then it definitely has lost its vacuum (filament can't heat when there's air inside, because air carries away the heat too quickly.......the filament won't reach operating temperature, and its resistance won't increase properly, so the voltage drop won't be as much as it should be).
:
:Keep in mind that if the tube has a filament short, but otherwise works, you will blow it if you test it on your tube tester with a 12.6 volt setting (if you care...the tube is faulty, so you may not). Tubes with filament shorts often don't function normally anyway, because many filament shorts will cause only half of the cathode to light, so you may want to simply discard the tube in any case. I just explained all of this so that you get the full picture, not necessarily so that you could save the tube, though you can if you can get it to work this way (it won't work at all if the vacuum is lost).
:
:Thomas
Now, regarding radio tubes, I know for a fact that they won't light when the seal is broken. They already light rather dimly compared to an incandescent lamp. When air is introduced, they can't heat up at all. The heat is carried away. If you use a higher voltage you can get them to light to ordinary brilliance, but, when the voltage is removed, you'll notice that the filament goes dark much more quickly. Again, this is because the air carries away the heat quickly. If you continue to light the tube so that it glows normally, it will eventually burn out. If you feed it normal voltage, though, and it doesn't glow, it can't burn out because it isn't getting hot enough for the chemical reaction to take place.
The reason why a lightbulb burns out when the glass is broken is because it is being lit to a much brighter amount than a radio tube. When the bulb is broken, it will still light up, but less brightly, but still enough to cause a chemical reaction. It is because the filament is lit so much more brightly to begin with that it will still glow when the bulb is broken. If you were to apply a voltage to the bulb that would light it only as bright as a radio tube, it would not light much if at all when the glass was broken. Try it.
Thomas
It would seem like a tube should flash and burn out but in this case other tubes are in series. In this case a tube that loses vacuum doesn't have enough available current to burn out and just drops less voltage.
Norm
:I would think that if a tube has a broken seal, ie., air inside, the moment you energize the filament, you will get a bright glow and the filament will burn up. Tubes have a vacuum for two reasons:
:1. to minimize the number of gas molecules between the plate and cathode, so that electron flow isn't impeded.
:
:2. to prevent the filament from oxidizing quickly and burning out. (Just as in an incandescent lamp)
:
:Rich
:
:
::The tube can also act that way if it has an internal filament short. All 12 volt tubes in your radio will have the same resistance readings when cold (and I mean COLD, not just 5 minutes after the set was turned on....wait an hour) regardless of whether their vacuums have been lost or not. If the questionable tube has a lower resistance, then its filament has a short. It may still operate if this is the case (though it's better to replace). If it has the same resistance as all the other tubes, then it definitely has lost its vacuum (filament can't heat when there's air inside, because air carries away the heat too quickly.......the filament won't reach operating temperature, and its resistance won't increase properly, so the voltage drop won't be as much as it should be).
::
::Keep in mind that if the tube has a filament short, but otherwise works, you will blow it if you test it on your tube tester with a 12.6 volt setting (if you care...the tube is faulty, so you may not). Tubes with filament shorts often don't function normally anyway, because many filament shorts will cause only half of the cathode to light, so you may want to simply discard the tube in any case. I just explained all of this so that you get the full picture, not necessarily so that you could save the tube, though you can if you can get it to work this way (it won't work at all if the vacuum is lost).
::
::Thomas
Norm, I too thought that as soon as air was introduced it would burn up... thans for the info.
...and Sir Thomas... how the heck do you know all this ?! .. did you sit around testing it all as a kid?... wait.. you're still a kid... lol
Bill
The 12SK7 draws .15 amps, the 6SK7 draws .3 amps. In a series circuit that calls for a 6SK7 would probably supply close to the 12 volts, or more than the 12SK7 requires. Remember, you are working with series circuits where the amperage of the tube is important, not the voltage.
Dave
Now I'm not math whiz or anything ..so let me think out loud here...and stumble through this..
For simplicity...Even though we have AC here we can maybe try to use ohms law for DC (if the AC current and voltage are in phase) or just as a relative camparison here:
...At 12 volts pushing 150 ma it would produce 1.8 watts. (12v x .15a = 1.8watts)
...and 6 volts on the same tube heater only pushes 75ma producing only .45 watts or less than about 1/3 the power and heat. (6 v x .075a = .45 watts)
Did I do that right?
And, regarding sitting around trying out broken tubes, yes, I did do weird things like that when I was a kid. Now I don't so much because I lack time. I have important things that won't go away, because I don't live with my parents anymore. I now know why my mother disliked our walking on the floors with shoes. They get dirty extremely fast. ANNOYING! However, I had a tube from an old car radio that I fooled around with. Had no idea how to make the radio work because I didn't know what a vibrator supply or B battery was. I just kept on feeding it 12 volts, trying to get some sound out of it. Well, it never did work, and I accidently broke the audio output tube. Sad I was, and I also noticed that the tube would no longer light at 12 volts (it would light if I fed it more voltage, but the filament would turn green). However, I got this great idea to circulate water past the heater and use it to heat my pool. I removed all of the glass and immersed the innards in my pool. Well, even if the pool was perfectly insulated (which it isn't at all), it would have taken a year and a day to raise the water temperature by one degree. And on that day I learned that just because tubes get hot doesn't mean that they can heat a swimming pool.
Thomas
Thomas
:I have the famous "HUM" in a R.C.A. Victor and waiting for caps to arrive so that I can replace them. Although all the tubes are checking satisfactory on my tester the reading across the filaments on tube 12SK7 is only 6.5 volts. Can this tube operate effectively with this low reading or should it be replaced?
:
:Thanks