Exploding 300B

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AndrewR
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#1 Exploding 300B

Post by AndrewR »

Hi all,

One of my 300B valves just exploded from inside. Initially there was some noise, then a white/yellow flash inside, followed by loud ping!

A chunk of glass blew out inside the valve at the base (see pic below). The valve is about 6-7 years old, with on-off usage. I swapped the working valve into the other side and seems to work fine. Operating voltages measure fine, however I did just modify the driving 5842 (with interstage transformer) to battery bias on the cathode.

Coincidentally I bought an hour before a pair of EH Gold Grid 300Bs and would like a bit of reassurance that the problem is with the old valve and not the amp

ImageWP_20140728_001(1) by andrew.randle, on Flickr

Andrew
steve s
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#2

Post by steve s »

Its hard to say without checking the amp out Andrew, i would suggest to check the actual valve bias volts the amp is working at, and check the voltages are all where they should be with the amp well warmed up, and of course double check all your work, us humans are not as infalable as we think we are ,
Easy checks are ensuring the tubes and the amp is not getting warmer than normal, check all the resistors for signs of overheating too.
But the chances are its a duff valve...
Best of luck..
AndrewR
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#3

Post by AndrewR »

Thanks Steve. Yeah it happened at switch-on, which to me is highly indicative of valve failure - which happened quite dramatically with the white/yellow flash inside and loud ping

The fact that the same channel now works with the other valve and quiescent voltages are okay, seems to indicate valve failure. Just hoping that there was no catastrophic inrush from my new battery biased driver - which I doubt would happen anyway.

Andrew
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IslandPink
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#4

Post by IslandPink »

You'd have to think something shorted, from anode to cathode, in this case, from your description . Maybe there was something loose in the supports & connections at the bottom, which was disturbed when you did the amp mods.
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Alex Kitic
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#5

Post by Alex Kitic »

That is definitely some "mechanical" or "construction" issue with your particular tube, and not your amp. A manufacturing defect.

It can be seen as obvious on the picture that the tube has lost its vacuum (white getter). Thus it was an implosion that you have heard. Since the implosion happened when the tube was "powered on" this explains the fireworks effects.

Why has the implosion happened? I am almost 100% certain that it was not your amp that caused it (what could it do to cause it? overvoltage at start-up... well, usually the tubes survive it, and if they do not, the glass still remains intact).

This has happened because of a weak spot in the lower part of the tube, where the wiring for the electrodes passes. Just take a look at how tubes are made (there is a nice video of it even on YouTube) and you will see for yourself how a weak spot might remain during the final stage when the tube is being closed (although the video was taken in the premises of a different manufacturer, the basic technique is the same).

I believe by now you have replaced the tubes and checked again the operating points and all relevant values, and that everything is as you meant it to be when building the amp?
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Mike H
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#6

Post by Mike H »

Wow it lasted 6 - 7 years! :shock:

:lol:


IslandPink wrote:You'd have to think something shorted, from anode to cathode, in this case, from your description . Maybe there was something loose in the supports & connections at the bottom, which was disturbed when you did the amp mods.
Some makes have a tendency (spelling?) to filament snapping on power up. Russian ones can be prone to it apparently. Could be was that, and a loose end flew off and shorted across onto something else internally. Maybe.
 
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