I am doing a 1941 Zenith. It is near completion. The 6K7G has a blue tint to it. When I try a nos 6K7G no blue tint is seen. The radio seems to play just as well with either one. The B+ is the same, too.
Is there any reason to replace the 6K7G that has the blue tint? Blue tint is associated with gas.

Best Regards,

Bill Grimm

Blue tube should be OK AFAK. Here's some info I found...
"To summarize, almost any blue glow you see inside a tube is perfectly fine and will not cause any problems in your amplifier. However, if you see bluish/violet glow around a single element such as a wire for example, it could indicate an air leak into the tube. There could be a small crack in the glass or a leak around one of the tube’s pins. A tube showing this needs to be replaced."

???

Blue/purple glow within elements indicates a gassy tube. A gassy tube should not be used as it can draw extra current and eventually fail or damage other components.

Blue just inside the glass isn't a problem. This is sometimes seen on output tubes but doubt a 6K7G.

Norm

:Blue tube should be OK AFAK. Here's some info I found...
:"To summarize, almost any blue glow you see inside a tube is perfectly fine and will not cause any problems in your amplifier. However, if you see bluish/violet glow around a single element such as a wire for example, it could indicate an air leak into the tube. There could be a small crack in the glass or a leak around one of the tube’s pins. A tube showing this needs to be replaced."
:
:

:???
:
: Blue/purple glow within elements indicates a gassy tube. A gassy tube should not be used as it can draw extra current and eventually fail or damage other components.
:
: Blue just inside the glass isn't a problem. This is sometimes seen on output tubes but doubt a 6K7G.
:
:Norm
:
::Blue tube should be OK AFAK. Here's some info I found...
::"To summarize, almost any blue glow you see inside a tube is perfectly fine and will not cause any problems in your amplifier. However, if you see bluish/violet glow around a single element such as a wire for example, it could indicate an air leak into the tube. There could be a small crack in the glass or a leak around one of the tube’s pins. A tube showing this needs to be replaced."
::
::
:
:
Hi Norm and Brian,
It seems this is a prediction of failure.
I recall reading that some blue is normal for power output tubes, as you mentioned, Norm. This tube has a 6F6G next to it that hasn't a hint of blue.
I am thinking that since this is a restoration, not just a fix, the blue tube has to go, even though the radio works fine with it.
Do you think that makes sense?

All the Best,

Bill Grimm

Any BLUE tint on the glass or mica or plate surface is normal. This occurs in output tubes, rectifiers (somewhat rare--usually during turn-on for filament rectifiers) RF tubes, magic eye tubes, and countless other tubes. It occurs most often when the tube is run with higher voltages. The 6K7G often runs with voltages near those of the output tube, and occasionally a blue glow of this kind can be seen in this tube.

If there is a VIOLET HAZE within the tube, then the tube is gassy. This necessitates replacement.

Do not replace your 6K7G.

Thanks

Too bad that tube won't fit any guitar amps that I know of.
You could get big bucks for it if that was a 6V6 or similar. Blue Mojo. :>)

:Any BLUE tint on the glass or mica or plate surface is normal. This occurs in output tubes, rectifiers (somewhat rare--usually during turn-on for filament rectifiers) RF tubes, magic eye tubes, and countless other tubes. It occurs most often when the tube is run with higher voltages. The 6K7G often runs with voltages near those of the output tube, and occasionally a blue glow of this kind can be seen in this tube.
:
:If there is a VIOLET HAZE within the tube, then the tube is gassy. This necessitates replacement.
:
:Do not replace your 6K7G.
:
:Thanks
:

Hi John,
I have some 6V6GT's with blue in them. Some of my favorite radios are Zenith 8H832 and I plan on getting int two Spartan 121's soon.
All have 6V6GT and they are often bad. They test good on the tube tester but sound lousey in the radio, even after the blue glos burns off.
I am sure they wouldn't sound any better in a guitar amp, low emission. Low voltage on the cathode tells the tale.

All the Best,

Bill Grimm

Be sure that the audio bypass capacitors are replaced before blaming tubes. I assume that your radios have been re-capped, but if not, leaky caps cause improper bias and subsequent distortion, but this would actually cause a high cathode voltage, so what you find likely does reveal a weak tube.

Hi Thomas,
Nice heairng from you. Your comments are always helpful.
However, in this case events have taken a turn. As I run the set the blue glow is fading.
I have seen this in power output tubes in the past. At first turn on they have a blue glow. They will even test weak on a tube tested.
I now recall reading something about this. It happens with tubes that haven't run in a long time. Ions start floating around in the tube but when power is put back on the ions are driven into the elements over time.
With testing and alignment, the blue glow in the 6K7G is just about gone. I am keeping it in the radio. Its a good tube.

All the Best, and thanks to all,

Bill Grimm

: I now recall reading something about this. It happens with tubes that haven't run in a long time. Ions start floating around in the tube but when power is put back on the ions are driven into the elements over time.
: With testing and alignment, the blue glow in the 6K7G is just about gone. I am keeping it in the radio. Its a good tube. All the Best, and thanks to all, Bill Grimm
:
Hey, Bill. I was joking about the guitar amps and blue mojo. But if you can find a way to make good tubes glow blue, I guarantee they'd sell.
I had a similar phenomenon a couple of months ago with a 56 tube. The grid cap on top was pulled off. Warren (or maybe Norm) said just solder it back on. Did that and the tube glowed blue for a while, but is clear now and working well.

:: I now recall reading something about this. It happens with tubes that haven't run in a long time. Ions start floating around in the tube but when power is put back on the ions are driven into the elements over time.
:: With testing and alignment, the blue glow in the 6K7G is just about gone. I am keeping it in the radio. Its a good tube. All the Best, and thanks to all, Bill Grimm
::
:Hey, Bill. I was joking about the guitar amps and blue mojo. But if you can find a way to make good tubes glow blue, I guarantee they'd sell.
:I had a similar phenomenon a couple of months ago with a 56 tube. The grid cap on top was pulled off. Warren (or maybe Norm) said just solder it back on. Did that and the tube glowed blue for a while, but is clear now and working well.
:
Hi John,
Someone was wagging you. Type 56 doesn't have a top cap. You didn't glue one on did you?

Best Regards,

Bill Grimm

::: I now recall reading something about this. It happens with tubes that haven't run in a long time. Ions start floating around in the tube but when power is put back on the ions are driven into the elements over time.
::: With testing and alignment, the blue glow in the 6K7G is just about gone. I am keeping it in the radio. Its a good tube. All the Best, and thanks to all, Bill Grimm
:::
::Hey, Bill. I was joking about the guitar amps and blue mojo. But if you can find a way to make good tubes glow blue, I guarantee they'd sell.
::I had a similar phenomenon a couple of months ago with a 56 tube. The grid cap on top was pulled off. Warren (or maybe Norm) said just solder it back on. Did that and the tube glowed blue for a while, but is clear now and working well.
::
:Hi John,
: Someone was wagging you. Type 56 doesn't have a top cap. You didn't glue one on did you?
:
:Best Regards,
:
:Bill Grimm
:

No, I soldered it because it was actually a 58. The Airline 62-84 has 3 of them.

Ah, good! ;-).

All the Best,

Bill

If the blue glow is on the glass or an element or object within, then there is no problem. If it is a fog, then this indicates gas.

The former blue glow can change as the tube is used and during a single run. It can also sometimes be manipulated around the glass with your fingers.

I think that I'd be inclined to trust the opinion of a good tube tester. If it greenlights the tube, it's probably fine to use. Even if it eventually fails, it can't do so in a destructive manner.

In the distant past I've encountered colorfully glowing tubes (most of which were rectifiers, to the best of my recollection) that seemed to work fine but whose nonconformity bugged me, so my policy then was to replace them. That was when tubes were dirt cheap and before I owned a decent tube tester.

Some tubes that light up colorfully and are SUPPOSED to do so are voltage regulators and thyratrons. VR tubes were used in some postwar ham receivers (my National NC-57 has one) and my 1939 Philco Mystery Radio (first wireless remote control) has a thyratron that "fires" with a beautiful violet glow. Both of these tubes have a small amount of ionizing gas intentionally entrained in them.

I'd still say that any tube that isn't supposed to have a gas inside but which glows is probably "iffy"; but as with many things, it may be something that exists indefinitely without ever becoming a real problem. So, I'd vote for the tube tester to be the final arbiter of goodness.

Metal-jacket tubes were probably invented to solve this problem, sort of like textured steel was introduced into the manufacture of refrigerators in order to reduce the scrap cost of products "dinged" in the factory. (Yes, I'm kidding!)

:I think that I'd be inclined to trust the opinion of a good tube tester. If it greenlights the tube, it's probably fine to use. Even if it eventually fails, it can't do so in a destructive manner.
:
:In the distant past I've encountered colorfully glowing tubes (most of which were rectifiers, to the best of my recollection) that seemed to work fine but whose nonconformity bugged me, so my policy then was to replace them. That was when tubes were dirt cheap and before I owned a decent tube tester.
:
:Some tubes that light up colorfully and are SUPPOSED to do so are voltage regulators and thyratrons. VR tubes were used in some postwar ham receivers (my National NC-57 has one) and my 1939 Philco Mystery Radio (first wireless remote control) has a thyratron that "fires" with a beautiful violet glow. Both of these tubes have a small amount of ionizing gas intentionally entrained in them.
:
:I'd still say that any tube that isn't supposed to have a gas inside but which glows is probably "iffy"; but as with many things, it may be something that exists indefinitely without ever becoming a real problem. So, I'd vote for the tube tester to be the final arbiter of goodness.
:
:Metal-jacket tubes were probably invented to solve this problem, sort of like textured steel was introduced into the manufacture of refrigerators in order to reduce the scrap cost of products "dinged" in the factory. (Yes, I'm kidding!)
:
Hi Tom,
I take a tube tester result with a grain of salt.
Many a times I have had a power output tube that tests good on a tester but provides lousey sound in the radio. I have one other that dos not test short but in the radio delivers a nice 60 cycle hum.
My tube tester isn't the greatest. I suppose there are good ones out there that minimize test escapes.
On the other hand I have had power output tube that test marginal that sound great in the radio. Many of those have the blue glow that fades over time, like my now famous 6K7G.
I even have a 6K7G that tests good in the tube tester and then fails in the radio. It comes up and sounds great at first then I her a click after about three minutes of operation and all the distant stations go away and only local ones can be received. I take it out and puzzle over it a while but never figure it out.
It doesn't have a blue glow.

All the Best,

Bill Grimm

I wouldn't leave a gassy tube in a radio. In some cases they will draw excessive current and damage the power transformer. In the case of 6K7G being gassy AVC voltage may go positive in a radio. This causes other tubes on the AVC line to draw more current and there won't be AVC action. Strong stations will be very loud.

Norm

::I think that I'd be inclined to trust the opinion of a good tube tester. If it greenlights the tube, it's probably fine to use. Even if it eventually fails, it can't do so in a destructive manner.
::
::In the distant past I've encountered colorfully glowing tubes (most of which were rectifiers, to the best of my recollection) that seemed to work fine but whose nonconformity bugged me, so my policy then was to replace them. That was when tubes were dirt cheap and before I owned a decent tube tester.
::
::Some tubes that light up colorfully and are SUPPOSED to do so are voltage regulators and thyratrons. VR tubes were used in some postwar ham receivers (my National NC-57 has one) and my 1939 Philco Mystery Radio (first wireless remote control) has a thyratron that "fires" with a beautiful violet glow. Both of these tubes have a small amount of ionizing gas intentionally entrained in them.
::
::I'd still say that any tube that isn't supposed to have a gas inside but which glows is probably "iffy"; but as with many things, it may be something that exists indefinitely without ever becoming a real problem. So, I'd vote for the tube tester to be the final arbiter of goodness.
::
::Metal-jacket tubes were probably invented to solve this problem, sort of like textured steel was introduced into the manufacture of refrigerators in order to reduce the scrap cost of products "dinged" in the factory. (Yes, I'm kidding!)
::
:Hi Tom,
: I take a tube tester result with a grain of salt.
: Many a times I have had a power output tube that tests good on a tester but provides lousey sound in the radio. I have one other that dos not test short but in the radio delivers a nice 60 cycle hum.
: My tube tester isn't the greatest. I suppose there are good ones out there that minimize test escapes.
: On the other hand I have had power output tube that test marginal that sound great in the radio. Many of those have the blue glow that fades over time, like my now famous 6K7G.
: I even have a 6K7G that tests good in the tube tester and then fails in the radio. It comes up and sounds great at first then I her a click after about three minutes of operation and all the distant stations go away and only local ones can be received. I take it out and puzzle over it a while but never figure it out.
: It doesn't have a blue glow.
:
:All the Best,
:
:Bill Grimm
:

Are you sure it's the tube? What you are describing sounds like you have a cold solder joint on the tube's cathode-to-ground connection; or, if a cathode resistor is used, maybe the resistor is opening up when its body reaches a certain temperature.

I suppose that it's possible for tube internal elements to expand under heat and short together.

With flaky tubes, sometimes the best solution is to wrap them in newspaper and take a hammer to them.