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Old 16th January 2013, 06:37 PM   #1
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Default 300B Tube Failure

Today one of the tubes in my amp failed, didn't see it coming, no flashes, no smoke, buzz, hiss, just died quietly. Upon inspecting it closely I see the cloudiness on the glass envelope that looks like the result of getters spewing beams of electrons. There are also cracks in the glass base, where the connecting wires go from the glass envelope to the pins. Could it be that cracks in the glass compromised the vacuum in the tube causing it to fail? The mesh plate looks spotless - no browning, discoloration or deformations. I don't have a tester that could test 300B.

When in the circuit the filaments were not glowing, there are no shorts between filaments and anode/cathode, and filament itself seems to be conducting (according to my DMM). Replacement tube seems to be working fine, heater voltage measured 5VDC exact.

What would cause a tube failure like this? Could it be manufacturing defect? The tube in question is not exactly cheap and has good reputation.
Attached Images
File Type: jpg 300B-Failed 004.jpg (715.0 KB, 366 views)
File Type: jpg 300B-Failed 001.jpg (754.3 KB, 344 views)
File Type: jpg 300B-Failed 002.jpg (720.6 KB, 335 views)
File Type: jpg 300B-Failed 003.jpg (708.4 KB, 331 views)
File Type: jpg 300B-Failed 005.jpg (749.8 KB, 322 views)
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Old 16th January 2013, 07:01 PM   #2
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Looking at ur nice well focused pix, I can't clearly see a stress crack through the base envelope, to the outside air. If it is there, even a micro path, that would kill the tube asap.

Looks like there has been stress cracking in two places?

I'd say this is a defect, but if the tube has many hours on it, I doubt the mfr will make good, no harm in asking, and sending along the pix.

If the tube shows no resting current and the filament lights properly, then it is definitely not conducting.

Since the replacement tube does work, it's a pretty safe assumption that it has failed, and likely that those cracks are the cause.

Do your other equal or similar hour tubes show any sign of cracking?
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Old 16th January 2013, 07:13 PM   #3
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Quote:
Originally Posted by bear View Post
Looks like there has been stress cracking in two places?

I'd say this is a defect, but if the tube has many hours on it, I doubt the mfr will make good, no harm in asking, and sending along the pix.

If the tube shows no resting current and the filament lights properly, then it is definitely not conducting.

Since the replacement tube does work, it's a pretty safe assumption that it has failed, and likely that those cracks are the cause.

Do your other equal or similar hour tubes show any sign of cracking?
The other tube from the pair shows no cracking, this pair had approx 80 hrs on it. I also checked another pair of tubes (EH) that was used for a couple of years with this amp - no signs of damage.

I will send the tube to Mr. Wugang for them to review it. Knowing their reputation (Sophia) I would expect them to come up with something. Tube manufacture date is 12/16/2011.

Last edited by ClefChef; 16th January 2013 at 07:26 PM.
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Old 16th January 2013, 07:34 PM   #4
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The cloudiness around the getter on the glass indicates air has entered the tube. .
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Old 16th January 2013, 07:47 PM   #5
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I called Sophia and was surprised to get Richard Dwu himself on the phone. I explained the matter to him and he asked me to send the pair back for them to look at. I would not expect them to replace the failed tube for free, maybe a discount on a matching tube to make a pair...

In regard to the cracks - in picture nr. 2 the crack is clearly visible. It is formed along the wire that runs from the base of the tube to the inside of the glass envelope and is baked into the inside of the glass pillar. There is a second, smaller crack that doesn't seem to run all the way.
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Old 16th January 2013, 07:57 PM   #6
cotdt is offline cotdt  United States
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It is clear that the vacuum got compromised from the crack. The glass was not tempered properly on this one. I have the same tubes and they are reliable so far.
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Old 16th January 2013, 07:59 PM   #7
cotdt is offline cotdt  United States
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Nice pictures, btw.
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Old 16th January 2013, 09:26 PM   #8
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Easily could have been from shipping stress.
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Old 16th January 2013, 09:39 PM   #9
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Easily could have been from shipping stress.
I agree. If this tube used the thick glass as 6C33 and GM70 this fail not happen.
Military tubes has many advantages...
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Old 16th January 2013, 10:05 PM   #10
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Sophia 300b uses thick glass, much thicker than EH or ShuGuang. These tubes were "sitting" in my amp for quite a while before one of them quietly died. I am inclined to think that cracks developed from heating/cooling process of normal tube operation. shipping stress would manifest right after the shipping.
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