So I bought a 6U5 tube off of eBay. I tested it on my Heathkit TC-2 and the eye opens and closes fine, everything is nice and bright. But when I do the emission test the needle drops all the way to the left in the "Bad" zone. I've never tested an eye tube on the TC-2 before. Is it normal for the tube to show zero emission but still glow nice and bright on the "Eye Closed", "Eye Open" tests?

Thanks,
Geoff

I am not familiar with the TC-2. But, eye tubes usually fail because the
phosphor is damaged, and this happens way before emission falls off.

The fact that the tube is bright, and opens and closes, would indicate,
to me at least, that your tube is fine. The proof is trying it in a
radio.

Pete

Unfortunately the only radio I have that uses a 6U5 is in the middle of a restoration and it's going to be awhile before it is ready. However, I just tried a different 6U5 tube that was very, very dim. The emission test came back as just barely in the "Good" zone so I should definitely get some deflection of the needle for the emission test. The new one I got has zero deflection but is nice and bright. I'll try the Short test tonight when I get home from work.

- Geoff

This is moving into the realm of the Test Equipment Forum.
Since we aren't getting many responses here, I will move the topic
and hope Alan Douglas or someone more familar with tube testers
will reply.

Pete

To Geoff ,

The 6U5 , like many other eye tubes , is actually two tubes in one enclosure . One section is the target which glows , and the other is a separate triode that amplifies the AGC signal to the level required to fully deflect the target . I recall testers having two tests , one to light the target and one to measure conduction of the triode .

Hopefully , both sections of the new tube are OK ......

It's the way that the new tube came back with zero emission that leads me to wonder about the health of that tube's triode section , , , Presumably the element to be measured on an emission test ?

( the target test being , of course , how well it lights up )

I also tested a good 6E5 tube and it passed the "Eye Open" test, the "Eye Closed" test, and the "Emission" test which went well into the green "good" section on the meter. Sounds like you are right about the triode in my new 6U5. It is probably bad. Shame on me for buying a tube on eBay. Well, I'll contact the seller and hope there aren't any issues with returing it.

By the way, if a tube is supposedly NOS and has never been used is it typical for them to still have that shiny silver coating on the inside of the tube or does that only come from using the tube?

Thanks for the feedback!

- Geoff

Hi Geoff ,

Yes , the silvered area of glass is totally normal in tubes that are truly NOS . That silver deposit is a vestige from the tube's birth and is the deposit of the "getter" having been "flashed" during the tube's evacuation of air before it's sealed off to hold it's vacuum . It is only a problem when it is a milky white translucent color , since that is what happens to it when air invades the tube's vacuum . If the getter deposit is nice and silvery , it means the tube is most likely still holding it's vacuum , but it doesn't really indicate anything else as to tube condition .

The misconception comes to the general public that the silver = a bad (or well used and about to fail) tube because of the way that incandescent light bulbs fail . Once the filament in an incandescent bulb gets thin and weak enough from prolonged use , it vaporizes and leaves a dark spot on the glass . Well , folks see the silvered area on a tube and , knowing not of the getter , automatically assume the tube is bad from their experience of the dark spot = bad bulb .

I hope my description helps

I contacted the seller and gave him some feedback based on what was discussed in this thread and he has agreed to send me a refnd after I send the tube back. So thanks for the help.

Intersting though. I looked up the test parameters for the 6U5 on a Hickok 533A and that unit only performs 2 tests; eye open and eye closed. No test of the triode section as in my Heathkit TC-2.

- Geoff

Hi Geoff ,

Funny thing is , I have had the same experience ! Ive got a kinda cheap "supreme" emission tester , and it has separate tests for both the Triode and Target sections . And yet my best tester I ever had (a "cardmatic" type that was more expensive) only had the target test . I wonder why the better testers ignored the triode section ? One possible thought could be that , as Peter mentioned , the target failed first in just about every tube , and so long as the triode had ANY emission , it was enough to give full deflection of the beam . In your case , where a separate test does indicate zero emission of the triode , I am left to suspect a failure such as a badly soldered pin connecting to the triode's grid ? I question the triode's grid connection because of two reasons . First , that the tube still lights the target(meaning the tube's cathode is fine) . And second , that most emission only testers will engage the grid instead of the plate , especially if the plate happens to be tied in with other tube elements (as the triode plate and target grid appear joined in the 6U5) .

What puzzles me here most , is that in the opening post , you mentioned that the target display opens and closes OK in the tester . In practicality , this would indicate a good tube . My only thoughts around this are that maybe the tester is using an over abundance of voltage to open and close the target , and the defective triode wouldn't be noticed till it was asked to work at the low AVC voltage levels that the radio would require it to ? If they are , for the target test , just blasting a huge jolt through the target's grid (triode's plate) to slam open and shut the target , the lack of triode sensitivity would not be evident .

That is what is confusing. Usually the target phosphor fails much before
the tube loses emission. I've seen numerous projects that used the triode
sections in eye tubes for other applications. We don't know if this is
normal for this tester, a problem with the tube tester, or a bad section
in the tube itself.

I'm inclined to think it is a bad solder connection or section in the tube. I thoroughly went over the tube tester a couple of years ago so I have no concerns there. And again to clarify, the TC-2 performs 3 tests for the 6U5:

Test 1 for Emission
Test 2 for Eye Open
Test 3 for Eye Closed

For the tube in question I pass tests 2 & 3 but fail test 1.

For the other bad 6U5 I have I pass test 1 but fail tests 2 & 3. So again, getting needle deflection on this tube for test 1 should be expected and I get zero deflection on the tube in question.

Again, thanks for the help guys!

- Geoff

Your Welcome Geoff ... And yea , with 0 emission on the triode , I'd have sent it back too .... I was just trying to come up with ideas as to why the triode had 0 emission , and a "cold joint" in the tube's grid pin was all I could come up with . I was not inclined to suspect your tube tester due to the other tube , even with weak target , giving a better than 0 reading on emission .

Now , Get that radio restored , so we can see photos of it !