Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.

Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
These can be smoked out often by testing the cathode to grid voltage. It will test low.

I have two that are more subtle. One just happened.
The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.

The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.

I was wondering what other tests good but is really bad tubes you have found.

Best Regards,

Bill Grimm

Bill

The best tube tester is a radio.

Tube testers don't detect a gassy 80 unless emission is also low. Tube testers need a grid to check for gas. An 80 is only a diode. Don't use a gassy 80. In time it can destroy a power transformer.

Since tube testers don't check filament current uneven lighting will also pass as long as emission is good. The bright section might burn out soon?

Emission testers don't check for elements that are intermittent. Most reading on an emission tester is between cathode and grid #1. If a 3rd grid or even plate doesn't make contact with the lead a tube may still pass. An element in your 6K7 most likely made intermittent contact?

Don't think forward voltage drop alone was responsible for the bad 6T8. Maybe great differences between 3 diodes in the 6T8?

Agree with you. There are all kinds of cases where tubes won't operate but test good. This is common with oscillators in radio and TV.

Norm

:Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
:More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.
:
:Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
:These can be smoked out often by testing the cathode to grid voltage. It will test low.
:
:I have two that are more subtle. One just happened.
:The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.
:
:The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
:Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
: Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.
:
:I was wondering what other tests good but is really bad tubes you have found.
:
:Best Regards,
:
:Bill Grimm

:Bill
:
: The best tube tester is a radio.
:
: Tube testers don't detect a gassy 80 unless emission is also low. Tube testers need a grid to check for gas. An 80 is only a diode. Don't use a gassy 80. In time it can destroy a power transformer.
:
: Since tube testers don't check filament current uneven lighting will also pass as long as emission is good. The bright section might burn out soon?
:
: Emission testers don't check for elements that are intermittent. Most reading on an emission tester is between cathode and grid #1. If a 3rd grid or even plate doesn't make contact with the lead a tube may still pass. An element in your 6K7 most likely made intermittent contact?
:
: Don't think forward voltage drop alone was responsible for the bad 6T8. Maybe great differences between 3 diodes in the 6T8?
:
: Agree with you. There are all kinds of cases where tubes won't operate but test good. This is common with oscillators in radio and TV.
:
:Norm
:
:
:
::Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
::More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.
::
::Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
::These can be smoked out often by testing the cathode to grid voltage. It will test low.
::
::I have two that are more subtle. One just happened.
::The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.
::
::The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
::Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
:: Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.
::
::I was wondering what other tests good but is really bad tubes you have found.
::
::Best Regards,
::
::Bill Grimm
Hi Norm,
Thank you for the comments. I didn't know that a gassy rectifier could damage the power transformer. Does the filament take too much current?
The 12AL5 indeed did eventually flash its filament.
The 6K7G remains a puzzle. I have tested the other elements. They stay active.

Best Regards,

Bill Grimm

Hi Bill

A gassy rectifier can arc over in time and destroy a transformer.

There are certain gas rectifiers such as 83 and OZ4 that are made to operate this way. A gassy 80 usually has a blue or pink color in operation. At first the color is stable between plate & filament. Later it can move around and arc over. This is a common cause for burned out power transformers.

Norm

Norm

::Bill
::
:: The best tube tester is a radio.
::
:: Tube testers don't detect a gassy 80 unless emission is also low. Tube testers need a grid to check for gas. An 80 is only a diode. Don't use a gassy 80. In time it can destroy a power transformer.
::
:: Since tube testers don't check filament current uneven lighting will also pass as long as emission is good. The bright section might burn out soon?
::
:: Emission testers don't check for elements that are intermittent. Most reading on an emission tester is between cathode and grid #1. If a 3rd grid or even plate doesn't make contact with the lead a tube may still pass. An element in your 6K7 most likely made intermittent contact?
::
:: Don't think forward voltage drop alone was responsible for the bad 6T8. Maybe great differences between 3 diodes in the 6T8?
::
:: Agree with you. There are all kinds of cases where tubes won't operate but test good. This is common with oscillators in radio and TV.
::
::Norm
::
::
::
:::Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
:::More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.
:::
:::Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
:::These can be smoked out often by testing the cathode to grid voltage. It will test low.
:::
:::I have two that are more subtle. One just happened.
:::The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.
:::
:::The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
:::Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
::: Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.
:::
:::I was wondering what other tests good but is really bad tubes you have found.
:::
:::Best Regards,
:::
:::Bill Grimm
:Hi Norm,
: Thank you for the comments. I didn't know that a gassy rectifier could damage the power transformer. Does the filament take too much current?
: The 12AL5 indeed did eventually flash its filament.
: The 6K7G remains a puzzle. I have tested the other elements. They stay active.
:
:Best Regards,
:
:Bill Grimm

Hi Norm,
That makes sense now. I suppose tha arc over doesn't do much for the filters, either. Once the power tansformer is ruined, I suppose those don't matter much.

Best Regards,

Bill Grimm

I know what you are saying about testing tubes. It can also depend on the quality of the tube tester. I have an old Hickoc mutual conductance. This one seems the most dependable. However it's not perfect, tubes that test good will not perform as well as they should. It's better to just sub out a tube with a known good one, if you are having a problem there. I too have seen tubes that glow bright. There is a heater to heater short. Very risky to use any of those. I have also seen the duel diode triode ones arc in the diode section when testing the diodes. In any case though, testing the tube gives you an idea of the life left. Gassy, heater to cathode shorts, low emission.

: I know what you are saying about testing tubes. It can also depend on the quality of the tube tester. I have an old Hickoc mutual conductance. This one seems the most dependable. However it's not perfect, tubes that test good will not perform as well as they should. It's better to just sub out a tube with a known good one, if you are having a problem there. I too have seen tubes that glow bright. There is a heater to heater short. Very risky to use any of those. I have also seen the duel diode triode ones arc in the diode section when testing the diodes. In any case though, testing the tube gives you an idea of the life left. Gassy, heater to cathode shorts, low emission.

Hi Warren,
Subbing only works if you have a known good tube, else it can lead you a stray.
I had two (and only 2) bad 42's. They were bad in the same way, too. Messed up troubleshooting something fierce.

Best Regards,

Bill Grimm

I can relate to this for sure.
like you mentioned the notorious power output types really are up there for false readings.
in my case,seems like, i'm forever pulling out and replacing 42's and 6f6's after they read good on two or three testers.
also some pentagrid converters such as 6a7's have giving the same false positives to me.
recently I was working on a Zenith 805 replaced all caps and o,o,t resistors,the 6f7 was reading good ,yet after about ten minutes I would lose the upper band,tried another 6f7, same happened but after about 5 mins.
checked all my work , did another chassis inspection,finally put in another 6f7 been working fine ever since.

Bill Nielson
b2nielson at yahoo.com

:Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
:More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.
:
:Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
:These can be smoked out often by testing the cathode to grid voltage. It will test low.
:
:I have two that are more subtle. One just happened.
:The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.
:
:The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
:Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
: Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.
:
:I was wondering what other tests good but is really bad tubes you have found.
:
:Best Regards,
:
:Bill Grimm

:I can relate to this for sure.
:like you mentioned the notorious power output types really are up there for false readings.
:in my case,seems like, i'm forever pulling out and replacing 42's and 6f6's after they read good on two or three testers.
:also some pentagrid converters such as 6a7's have giving the same false positives to me.
:recently I was working on a Zenith 805 replaced all caps and o,o,t resistors,the 6f7 was reading good ,yet after about ten minutes I would lose the upper band,tried another 6f7, same happened but after about 5 mins.
:checked all my work , did another chassis inspection,finally put in another 6f7 been working fine ever since.
:
:Bill Nielson
:b2nielson at yahoo.com
:
:
::Don't you love tubes with open filaments? Eventhough it is bad, it is honest.
::More of a nuisance is a tube that tests good but is really bad. I have a type 80 which tests OK, but glows blue. No wonder the radio sounded bad. Also a 12AL5 which tested OK, however one filament glowed very brightly and the other hardly at all. It's hard to call that one good.
::
::Perhaps the most common example are power output tubes. Tube testers can't emulate the current draw through them, so ones with weak emission still test good. Usually they cause distorted sound, as though the announcer has cotton in his ears.
::These can be smoked out often by testing the cathode to grid voltage. It will test low.
::
::I have two that are more subtle. One just happened.
::The first was a 6K7G which comes up fine and runs well for 20 minutes. I then hear an audible click. Gain plummets and only local stations can then be received. I can run the tube all day on the tester and it tests OK. Filament current is right, too.
::
::The second just happened. It is a 6T8. I was aligning the FM and couldn't get the secondary to null. No positive voltage was possible.
::Since this type of radio is susceptable to silver mica disease, I changed out the discriminator. The problem was unchanged. I then changed the 6T8 (no soldering necessary) and got plenty of positive and a good null at alignment.
:: Both 6T8s, the bad and the good, check identically on the tester, so I checked further. I ran the tube filament and checked the diodes like they were semiconductor diodes. 6T8 has a diode section between pins 2 and 3. On the good tube the forward voltage read 0.3 volts. On the bad tube the forward voltage was 0.6 volts. I suppose it is hard for a tube tester to catch that.
::
::I was wondering what other tests good but is really bad tubes you have found.
::
::Best Regards,
::
::Bill Grimm
Hi Bill,
Check on the 42's. I had a radio here with distorted audio. I changed out the 42 with the only other I had and there was no change. I trouble shot that like crazy. At the time I didn'r know about checking the bias.
I sold it to a guy (cheap) who must have suspected the problem. He had an NOS 42 waiting when it got there. He says it plays great.

Best Regards,

Bill Grimm