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 Post subject: Re: Tube tester showing "false" short
PostPosted: Jan Sun 21, 2018 3:13 am 
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Any leads on this one yet? Could it be that this is normal for these tubes, and is an error on the roll chart? Is there something in common between the cathodes of these tubes that would cause this? But why only when the cathodes are hot? If I turn off the filament to the tube, the "short" fades away. I have tested pretty much all the 12A*7 and such tubes that I own, most NOS and some used, and they ALL do this. So they all can't be defective, right? Even the octal tubes, such as a 6SN7 do it, another dual triode. It also happens with some dual diodes, such as the 6AL5. All of these tubes work in a circuit, so they can't be bad in theory.

I actually recall this happening on another tester I own, my PACO model T-60. The shorts lamp lit up, but when I turned off the filament, as soon as the cathode began to cool off, the shorts light gradually faded away, just like on the TV-11. So what is causing this? Is this normal for some emission tester circuits? It does NOT happen with my Precision 10-12, which uses a "dynamic" circuit. This is just puzzling to me. Any ideas?


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 Post subject: Re: Tube tester showing "false" short
PostPosted: Jan Sun 21, 2018 4:14 am 
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This one is baffling, but to answer one of your questions, the cathodes do not have anything in common. The 12A*7 family of tubes are all just a pair of 6 volt tubes in the same glass. The only thing common between them is that their filaments are tied together, which is connected to pin 9. That's why if you have a 6 volt filament transformer, you can connect pins 4 and 5 (the other ends of the filaments) to one side of the transformer filament secondary, and pin 9 to the other. Or you can apply 12 volts between pins 4 and 5, and leave 9 disconnected.

But, the light should not come on. The point is showing if there is a short in the tube between the heater and another element, which I guess would pretty much have to be the cathode. Something that throws this off for me is that both the heater and the plate to cathode circuit all run off the same transformer, and the same common, which you don't have in a normal circuit. So trying to figure out how to test for this is a bit harder, but I think I have an idea.

Unplug the tester, then throw all of the levers to N. That should disconnect them from everything in the tester. Depending on the socket that's used, your multimeter probes may or may not be able to make contact in the 9 pin socket, but the they should fit into the octal socket, and all of them are tied together anyway. The only thing is, if you use the octal socket, then stick a piece of bare wire into pin 9 of the 9 pin. Start with one probe in pin 1, the other in pin 2, checking for continuity, then move the probe from 2 the 3, then 4, etc. Once you get to 9, then move the first probe to pin 2, and the other pin, start back over at pin 3.

Ultimately you are checking for continuity between all pin combinations, but obviously everytime you move the first probe you only have to then use the other probe on the higher number pins. But there should be no continuity between any pins. If there is, you have a short in the tester somewhere.

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 Post subject: Re: Tube tester showing "false" short
PostPosted: Jan Sun 21, 2018 4:30 am 
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TPAairman wrote:
This one is baffling, but to answer one of your questions, the cathodes do not have anything in common. The 12A*7 family of tubes are all just a pair of 6 volt tubes in the same glass. The only thing common between them is that their filaments are tied together, which is connected to pin 9. That's why if you have a 6 volt filament transformer, you can connect pins 4 and 5 (the other ends of the filaments) to one side of the transformer filament secondary, and pin 9 to the other. Or you can apply 12 volts between pins 4 and 5, and leave 9 disconnected.

But, the light should not come on. The point is showing if there is a short in the tube between the heater and another element, which I guess would pretty much have to be the cathode. Something that throws this off for me is that both the heater and the plate to cathode circuit all run off the same transformer, and the same common, which you don't have in a normal circuit. So trying to figure out how to test for this is a bit harder, but I think I have an idea.

Unplug the tester, then throw all of the levers to N. That should disconnect them from everything in the tester. Depending on the socket that's used, your multimeter probes may or may not be able to make contact in the 9 pin socket, but the they should fit into the octal socket, and all of them are tied together anyway. The only thing is, if you use the octal socket, then stick a piece of bare wire into pin 9 of the 9 pin. Start with one probe in pin 1, the other in pin 2, checking for continuity, then move the probe from 2 the 3, then 4, etc. Once you get to 9, then move the first probe to pin 2, and the other pin, start back over at pin 3.

Ultimately you are checking for continuity between all pin combinations, but obviously everytime you move the first probe you only have to then use the other probe on the higher number pins. But there should be no continuity between any pins. If there is, you have a short in the tester somewhere.

It sure is! And that's what I thought, thanks for clearing that up. This seems like a good test, and I will try it! So I put my multimeter in the octal socket pins? Or one lead to a pin in the octal socket, and the other to a pin on the 9 pin socket? This happens with some octal tubes too, including 6U8 miniature tubes. This is just baffling to me. Also, when I remove the tube from the tester and test for shorts, no short shows up. Does this prove there are no shorts within the tester? Also, why would it happen on only some tubes, and not another tube, such as a 50C5 or a 6J6 for example? And I swear this happened on my other tester too. Also, what would cause the short to be on pin 3 on the other side of lets say, a 12A*7, as supposed to following the same pin (lever)? For example, on a 12AU7 side 1, there is the short on pin 8, and on the other side it's pin 3. And I still don't understand how turning down the filament voltage makes the short go away. It doesn't seem like it's an actual short circuit within the tube or tester, but i'm not sure. I know thermal shorts can happen, but not with ALL of my 12A*7 tubes, or other tube types I own. And it doesn't happen on all tubes, which to me, would rule out any possible short within the tester.


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 Post subject: Re: Tube tester showing "false" short
PostPosted: Jan Sun 21, 2018 7:19 am 
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On your tester, as is the case with most, (but not all) pin 1 of each socket is wired to each other socket pin 1, pin 2's are all wired together, etc. So if you put a probe in the octal socket on pin 1, it's the same as putting it on pin 1 of the 9 pin socket. The problem with the 9 pin socket is you probably cannot get your probes to go down in the socket. So that's why you would use the octal. But since the octal socket has no pin 9, just stick a piece of bare wire into pin 9 of the 9 pin socket so you can include it in the test. You could stick bare wires into all of them, but using the octal socket would be quicker and easier.

If there is some issue going on with the tester, it very well may not happen on other tubes since their pinouts are different.

As far as the shorts light during the shorts test, that possibly could be an indicator of something, but I think first you need to rule out a short on the socket pins.

CrankTelephone101 wrote:
Also, what would cause the short to be on pin 3 on the other side of lets say, a 12A*7, as supposed to following the same pin (lever)? For example, on a 12AU7 side 1, there is the short on pin 8, and on the other side it's pin 3. And I still don't understand how turning down the filament voltage makes the short go away.


Those are the cathode pins on the respective sides. The whole thing is goofy, and if there is a fault in the tester, then it means there's two faults or a single fault that is affect both pins somehow. Which is getting far fetched, but so far, that's all that I can see that's in common. But one possibility is a short in the tester between pins 8 and 9. I'm not certain how that would carry over to the other cathode, but the reason that comes to mind is that it because the two pins being next to each other would make it easier for there to be a short.

If there is a short from the filament somewhere, then when you turn down the voltage, you are cutting the power that is lighting the light.

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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Fri 02, 2018 8:22 pm 
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Those are the cathode pins on the respective sides. The whole thing is goofy, and if there is a fault in the tester, then it means there's two faults or a single fault that is affect both pins somehow. Which is getting far fetched, but so far, that's all that I can see that's in common. But one possibility is a short in the tester between pins 8 and 9. I'm not certain how that would carry over to the other cathode, but the reason that comes to mind is that it because the two pins being next to each other would make it easier for there to be a short.

If there is a short from the filament somewhere, then when you turn down the voltage, you are cutting the power that is lighting the light.[/quote]
Sorry I haven't gotten back on this sooner, I have been very busy. Anyway, yes I agree the whole thing is very goofy! I pulled out my meter today and the tester and I am getting continuity between what appears to be all the pins, even on different sockets. This happens when all levers are in the "K" position. When set up for a 12AU7 tube, there is no short between pins 8 and 9, however all other pins seem to have continuity to pin 8, and I can change that by moving the levers around. I'm not too familiar with exactly how the sockets are wired, so let me know if there seems to be a problem here. I also made a short video showing the problem, that can be viewed here: https://www.youtube.com/watch?v=kXKau82HJyg
Let me know if you get any more ideas here, I appreciate the help! This really has me curious and confused at the same time.


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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Sat 03, 2018 1:54 am 
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First of all, let me say that I don't know anything about the details of your tester nor do I have a schematic, so all of the following is speculation.

Do you have the manual for your tester? Have you read it? My Knight 600B behaves exactly as yours does. The roll chart is marked to show that these pins SHOULD show shorts. The manual explains what these markings are; in my case they are parentheses.

Here is what is happening in MY tester; if yours has a similar circuit, it explains yours too. In the Knight tester, the short indicator is a neon bulb. It has a resistor in series to limit the current and a capacitor to block DC. There may be other parts to control the sensitivity but that is the basic circuit. When a tube is tested for shorts, there is a series circuit of the transformer secondary, the circuit described above, and the elements of the tube to be tested for shorts. My tester doesn't have a disconnected position on the selector switches so everything is connected all the time, either to common, the heater supply, or the short test circuit. Since the switches have been set for the particular tube, the plate and maybe the grid(s) are connected to the short test circuit and the cathode is connected to the common circuit. This applies an AC voltage between the plate (and grids?) and all the other elements at once. Since a normally operating tube only passes current in one direction, it rectifies the AC. The capacitor in the short test circuit blocks the resulting DC and the lamp does not light. If there is an actual short, the short passes AC and the lamp lights. The charging and discharging of the capacitor causes the lamp to flash as the switches are moved but there is no steady glow.

Now consider a two or more section tube WITH SEPARATE CATHODE CONNECTIONS FOR EACH SECTION. When you test the cathode of the second section, you are connecting it to the plate of the first section. The cathode of the first section is connected to the plate of the second section to common. This puts the two sections in parallel IN OPPOSITE DIRECTIONS. On one half cycle, one section passes current, on the other half cycle, the other does. The result is that current flows in both directions; this is AC and the short lamp lights. Since this isn't a real short but is a result of normal current through the tube, it doesn't happen if the tube is cold.

Also note that some tubes have multiple pins connected to the same element; these pins will also show shorts on a good tube. They will also be noted on the roll chart.

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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Mon 05, 2018 12:23 am 
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Here is a schematic for my tester (last page) : https://stevenjohnson.com/manuals/mfile ... manual.pdf

And I do, and have read it. The manual nor the roll chart say anything about pin 8 or any other pin with this issue. Wow, well then maybe it is normal for these circuits, as a lot of emission testers use very similar circuit designs. I know it was common for roll chart errors too so maybe Superior didn't discover this issue so they didn't note it. My PACO model T-60 doesn't note it, either.

And thanks for the explanation! That does seem pretty correct that this is indeed a normal occurrence. It baffled me for the longest time! I hope it has been solved, but you can take a look at my schematic.
Thanks for your time and your help and input on this issue!


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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Mon 05, 2018 11:54 pm 
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You're welcome.

Your tester does indeed use the short test circuit I described and the manual doesn't say anything about this condition. If there is any note about it anywhere it would be in the filament short column of the roll chart. But it probably isn't since you didn't see anything.

I did see one VERY IMPORTANT thing in the schematic, however. The power transformer in this unit is an autotransformer. This means that nothing is isolated from the AC line, including the top cap connector and the noise jack. This device can be a shock hazard any time it is plugged in, whether it is turned on or off, sometimes depending on which way the plug is inserted.. It is best to use it with an isolation transformer or be VERY careful what you touch when using it. The manual makes no mention of this either.

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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Tue 06, 2018 12:58 am 
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Jim Mueller wrote:
You're welcome.

Your tester does indeed use the short test circuit I described and the manual doesn't say anything about this condition. If there is any note about it anywhere it would be in the filament short column of the roll chart. But it probably isn't since you didn't see anything.

I did see one VERY IMPORTANT thing in the schematic, however. The power transformer in this unit is an autotransformer. This means that nothing is isolated from the AC line, including the top cap connector and the noise jack. This device can be a shock hazard any time it is plugged in, whether it is turned on or off, sometimes depending on which way the plug is inserted.. It is best to use it with an isolation transformer or be VERY careful what you touch when using it. The manual makes no mention of this either.

Thanks again! I thought something was wrong with my tester! I wonder why this isn't mentioned anywhere. Like some tubes say "should show short on lever 3" or whatever, but tubes like this don't. And it indeed does not mention anything about this on the roll chart.

And thank you for pointing that out, I learned of the tapped autotransformer when I got the unit, and I wonder, why did they use this design? I guess just because it was cheaper. It just doesn't seem very safe to me. Is the chassis of the tester always going to be at line potential, like in a hot chassis radio? I have gotten a good tingle from the plate cap connector before, quite a few times. I just wonder why this was able to pass production this way, if indeed it is a hot chassis design with the autotransformer.


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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Tue 06, 2018 1:43 am 
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Is the chassis of the tester always going to be at line potential, like in a hot chassis radio?

That depends on how it was made. I assume it is in a wooden or plastic cabinet so that part can't be a problem. Is the panel metal? If so, whether it is a hazard depends on if it is connected to part of the circuit or if there is significant leakage from the circuit. Unlike a radio, there is no particular electrical benefit to using the panel as a "ground" and with the messy circuit, there probably wouldn't be much, if any, saving in wire or labor either. Perhaps it is connected at the noise jack to save insulating washers.

Quote:
I just wonder why this was able to pass production this way, if indeed it is a hot chassis design with the autotransformer.

In the "old days" people didn't care as much about safety as they do now.

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 Post subject: Re: Tube tester showing "false" short
PostPosted: Feb Fri 09, 2018 5:02 am 
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That's very true! I agree with that!!


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