Tube question

charmerci · « **on:** 29 Jul 2018, 08:04 pm »

I just bought some Sylvania NOS 6GC7 tubes. Why would one be "lit"/ bright at the bottom while the others are not?

If you need pictures, I'll have to post them later tonight.

FullRangeMan · « **Reply #1 on:** 29 Jul 2018, 08:57 pm »

Apparently the primary cause is hoter filament/cathode than the others tubes or different ones.

charmerci · « **Reply #2 on:** 30 Jul 2018, 01:34 am »

Quote from: FullRangeMan on 29 Jul 2018, 08:57 pm

Apparently the primary cause is hotter filament/cathode than the others tubes or different ones.

I guess that's not a good thing then.

sfox7076 · « **Reply #3 on:** 30 Jul 2018, 01:46 am »

Could also have the heater sleeve slightly higher than the other tubes...

FullRangeMan · « **Reply #4 on:** 30 Jul 2018, 01:51 am »

Quote from: charmerci on 30 Jul 2018, 01:34 am

I guess that's not a good thing then.

Correct. Inspect this tube entirely and closely mainly the filament and compare with the other tubes looking for differences.

There is any dot or unsual mark in the glass of this tube?

charmerci · « **Reply #5 on:** 30 Jul 2018, 02:49 am »

I've looked at them and cannot see any difference. I bought a set on eBay and when I told him about it, he sent me another pair free of charge. I guess I'll just use the two that glow the least and not from the bottom. That's the set I have playing now. I can't really tell but it may sound the best, though I'm not that positive.

FullRangeMan · « **Reply #6 on:** 30 Jul 2018, 03:28 am »

Oh that was a nice gesture from the seller

Do have a tube tester for test emission and short?
I would suggest you dont use this suspect tube until it be tested.
In tube factories it is usual mark a bad tube with a dot or pen risk.

charmerci · « **Reply #7 on:** 30 Jul 2018, 03:31 am »

No tube tester. These are used tubes. Sylvania doesn't exist anymore. I'm using them for my Van Alstine DAC.

FullRangeMan · « **Reply #8 on:** 30 Jul 2018, 03:41 am »

Wow these are used tubes, well so Iam guessing this suspect tube may be with a higher bias than the other tubes.

charmerci · « **Reply #9 on:** 30 Jul 2018, 03:50 am »

They were made in the USA! When did Sylvania stop making tubes in the US? The 60's or the 70's?

FullRangeMan · « **Reply #10 on:** 30 Jul 2018, 04:06 am »

It is said was in the 70, Sylvania from the 70s are know to be less great tubes, the good ones was til the 60.

Gas in excess inside the tube also can show a bright light.

charmerci · « **Reply #11 on:** 30 Jul 2018, 04:19 am »

I stand corrected. The brighter glowing ones sound better. So again the question is, is this a problem? They don't sound like they're destroying the equipment.

FullRangeMan · « **Reply #12 on:** 30 Jul 2018, 04:23 am »

Nice this looks good news, if the bias is OK I see no prob from here.

rollo · « **Reply #13 on:** 30 Jul 2018, 06:35 pm »

Used tubes can have issues. If you like the tube contact Brent Jessie and get a NOS matched pair. Or Andy at Vintage Tube Service. done, no worries.

charles

avahifi · « **Reply #14 on:** 30 Jul 2018, 06:44 pm »

I suggest you try 6DJ8/6922 tubes in your AVA DAC. They are interchangeable with the 6CG7 tubes and to our ears sound better and run cooler to boot.

We have EH 6922 tubes available at $35 a pair plus $10 shipping. These have replaced 6CG7 tubes in all new AVA electronics that did previously use 6CG7 tubes.

I am not comfortable with your use of a tube with a heater element glowing too much. It glows brighter because there is more current going through its filaments than normal. I would hate to see it short out completely and damage the DAC's regulated heater supply.

Frank Van Alstine

charmerci · « **Reply #15 on:** 30 Jul 2018, 06:53 pm »

Actually Frank, the 6922 Sovtek tubes that came originally with the Ultra DAC from you glow brighter than the 6GC7's that I am trying. So there's that!

avahifi · « **Reply #16 on:** 30 Jul 2018, 07:24 pm »

Well then, one of your 6CG7 tubes is not glowing as brightly as the other. My bad.

Frank

sfox7076 · « **Reply #17 on:** 31 Jul 2018, 12:02 am »

Do you have a multimeter? Measure the filament pins if you do. What do they measure?

These are all indirectly heated tubes. The filament is placed inside a tube that is heated by the filament, the tube is the cathode. The cathode sleeve sits around the filament (thus why the filament on an indirectly heated tube is called a heater). The filament consists of a fine piece of nichrome resistance wire that is folded on itself a number of times (the longer the wire, the higher the voltage of the filament). The nichrome is coated with an oxide insulation which keeps it from shorting to itself (lowering resistance) or the cathode sleeve. But sometimes that insulation goes bad, due to defect or improper operation. Then they usually burn out completely. Sometimes the cathode sleeves are not positioned the same on each tube, so there is a difference in look. In that instance, the glow will be different from the tube because the sleeve will show a different amount of "filament". If the tubes are two different structures, there will always be a difference in the shine because the filaments are folded differently or are exposed differently. If the filaments light up, there is relatively little risk. There may be heater-cathode leakage, but there is no way to check that safely without a tube tester. Regardless, I don't think you are in any danger using the tubes. If there was heater/cathode leakage, you would eventually get hum and you would know to change the tube. Catastrophic failure taking out a winding is really really rare. Ask us radio guys, H-K leakage is only really a concern on big power tubes... The hum lets you know on the preamp/signal tubes...

rollo · « **Reply #18 on:** 31 Jul 2018, 12:12 am »

Quote from: avahifi on 30 Jul 2018, 06:44 pm

I suggest you try 6DJ8/6922 tubes in your AVA DAC. They are interchangeable with the 6CG7 tubes and to our ears sound better and run cooler to boot.

We have EH 6922 tubes available at $35 a pair plus $10 shipping. These have replaced 6CG7 tubes in all new AVA electronics that did previously use 6CG7 tubes.

I am not comfortable with your use of a tube with a heater element glowing too much. It glows brighter because there is more current going through its filaments than normal. I would hate to see it short out completely and damage the DAC's regulated heater supply.

Frank Van Alstine

Even better yet from Avahifi.

charles

charmerci · « **Reply #19 on:** 31 Jul 2018, 04:09 pm »

Quote from: sfox7076 on 31 Jul 2018, 12:02 am

Do you have a multimeter? Measure the filament pins if you do. What do they measure?

These are all indirectly heated tubes. The filament is placed inside a tube that is heated by the filament, the tube is the cathode. The cathode sleeve sits around the filament (thus why the filament on an indirectly heated tube is called a heater). The filament consists of a fine piece of nichrome resistance wire that is folded on itself a number of times (the longer the wire, the higher the voltage of the filament). The nichrome is coated with an oxide insulation which keeps it from shorting to itself (lowering resistance) or the cathode sleeve. But sometimes that insulation goes bad, due to defect or improper operation. Then they usually burn out completely. Sometimes the cathode sleeves are not positioned the same on each tube, so there is a difference in look. In that instance, the glow will be different from the tube because the sleeve will show a different amount of "filament". If the tubes are two different structures, there will always be a difference in the shine because the filaments are folded differently or are exposed differently. If the filaments light up, there is relatively little risk. There may be heater-cathode leakage, but there is no way to check that safely without a tube tester. Regardless, I don't think you are in any danger using the tubes. If there was heater/cathode leakage, you would eventually get hum and you would know to change the tube. Catastrophic failure taking out a winding is really really rare. Ask us radio guys, H-K leakage is only really a concern on big power tubes... The hum lets you know on the preamp/signal tubes...

Yeah no, I have no multi-meters or tube testers.

I just happened to read an old post of mine where someone said that the Sylvania tubes sounded best, so I just tried them for $16 for the heck of it.