Failing Output Tube??

Audible Nectar

New Mac City
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One of my output tubes (RCA 6L6GC) in one of my MC30's started "dimming" - diminished glow after a few minutes of being powered on. I initially thought this could be an out of tension socket - but after swapping the front and back power tubes, the same tube exhibits the same problem.

Is this the initial signs of a failing tube?
 
I'd get both on a tester and see what gives. Where are you located? Might be a member nearby that could help, since tube testers at the local grocery stores are a thing of the past...
 
Not even close to there - I'm in western IL.

I just had these tubes tested (as I did my whole stash) on a Chris Haedt calibrated Hickok. I've been using this quad as my "normal quad", but had them tested for strength, just for my knowledge, because I had access to that tester and someone who knew how to use it.

I also had considered the idea that this could be the amp, but it was just serviced by Terry, and when reversing the 2 6L6 outputs front to back, the dimming problem stayed with the tube (not with the socket).

Having those tested was a seven hour one way drive....so no such luck having them tested again in any quick fashion.

Complicating matters is the fact that these are my only output tubes.
 
Well without testing them it would be difficult to tell if the dimming meant anything.

I just started a thread for people with testers and willingness to help AK members to list their location

Maybe someone is closer than 7 hours. I'm in CA so thats no help... :no:

If you get access to testing, test both since you know where you were starting, and can use the other as a reference. (not all testers will read the same numbers, or so I've been led to belive. In any case some use different scale methods.)
 
Tube testing is a good place to start but don't forget a tube could test good on a tester and not work properly in the amp. As you said:

"One of my output tubes (RCA 6L6GC) in one of my MC30's started "dimming" - diminished glow after a few minutes of being powered on."

The tube might not be put under enough strain on a tester to get to your diminished glow condition.

Just something to think about. I have had tubes test good that did not work in my equipment and was only able to find the weak tube by swapping out tubes.

Victor
 
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I also have had tubes test "good" when in fact they were not. And this was on two different makes of tube testing machines.
 
OK -

Found an ebay seller in my area with good testing equipment, who tested ALL of my output tubes for transconductance and current draw. Apparently he has a machine that will test current draw from the tube when operating at 400 volts.

The current draw on the "failing" tube seems to be an issue - it read 14.5. My other tubes read 17.5, 25.5 and 24.5 ma. He recommended replacing both low reading tubes....and does have a pair of similar testing/matching tubes available.

Apparently transconductance (micromhos) and current draw are not necessarily related, as all of these tubes tested from 5600-5900.

Any thoughts? Thanks for the replies, BTW.
 
Sounds like a problem with the filament of the tube...if the filament is not glowing hot enough you will get the symptoms of a tube with low emission (weak tube)...I would just go ahead and replace it.

The filament glow of a tube should not change...it should be the same regardless of load or how long it's been on...again it sounds like a bad tube.
 
This appears to be solved. I ended up with 4 tubes all within 3ma of each other (30-33ma). My last post was incorrect - I had two testing around 31-33ma, and picked up two more testing at about the same strength to make a foursome.

Thanks folks!
 
Victor said:
Tube testing is a good place to start but don't forget a tube could test good on a tester and not work properly in the amp. As you said:

"One of my output tubes (RCA 6L6GC) in one of my MC30's started "dimming" - diminished glow after a few minutes of being powered on."

The tube might not be put under enough strain on a tester to get to your diminished glow condition.

Just something to think about. I have had tubes test good that did not work in my equipment and was only able to find the weak tube by swapping out tubes.

Victor

Bingo....tube testers even the 400V version (sounds like a maxi matcher) are not 100% reliable the true test is in a amplifier especially with output tubes and this is why I'm a huge advocate of Fixed adjustable biased amps. This way you the user have a great way to know the health of your output tubes. Even a McIntosh can only make up for slightly imbalanced output tubes. This tube sounds like its actually having a problem with the heaters. The Glow in a tube is not caused by the high voltage you can pull your 5U4 completely out of your MC30 and the tube will glow and warm up to near full temp without any high voltage present.

By the way what happened with the GEC KT66 you took home? Remember me saying I confirmed the operation of the GEC's in my amps.

Craig
 
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Hey,
I had something similar happen to me. I purchaced 2 "new" Sovtek tubes from Music Direct for my Bottlehead Seduction. Played them both about 3 hours and one channel started to fade then went very low on gain. Swapped tubes and it followed. Cold they both have equal output but after a few moments of on time the same one fades. Swapped in my old HP's and bingo, problem solved. Now, does anyone have a set of 6dj8's or equivalant to sell me? I dont want to go thru this again and no one locally has any tubes for sale. MD wouldn't exchange the bad one...dissapointing considering the dough I drop there on a yearly basis. The Sovteks sounded much better than the HP's so I really want to replace them.

Artie
 
If the heater is dim the tube will be weak. The cloud of needed electrons will be insufficient. Many times this is due to oxidation on the tube pins and a wire brush and some alcohol are the fix. If the tube pins are lose in the socket these socket may need the female sections tightened up with a scribe.
You may in fact have a tube with a weak heater or high resistance heater. Rare but can happen.

Ron-C
 
ron-c said:
If the heater is dim the tube will be weak. The cloud of needed electrons will be insufficient. Many times this is due to oxidation on the tube pins and a wire brush and some alcohol are the fix. If the tube pins are lose in the socket these socket may need the female sections tightened up with a scribe.
You may in fact have a tube with a weak heater or high resistance heater. Rare but can happen.

Ron-C

I belive he mentioned moving the tube to a different location and the problem moved with it, so it's unlikely to be a socket, but the dirty tube pins could be an issue and worth checking.

I have a noisy tube in 2 units, and am going to check what you memtioned. One at least I suspect contact problems to be the issue as I moved it and it was noisy, then got quiet. I suspect I scrubed past some corosion, a temp fix at best...
 
The usual way I fix a tube like this is to take it outside and step on it. Problem solved! Ha.

Ron-C
 
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