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Defective 300B Tubes?

Posted by Devils Advocate on August 15, 2015 at 16:49:53

Hello,

I have a Cary 300SEI integrated amplifier (IMHO a fine amp), and by process of elimination I've narrowed down the problem I'm having to what I suspect is a defective 300B output tube(s).

Here is what's happening... since the tubes are relatively new (approx. 100 hours) I have a Cary bias meter hooked up to check amp bias, and when I turn it on with the suspect tubes installed the meter immediately pegs the needle 'hard & fast' to the end of it's sweep, the needle physically hits the far end until it cannot go any further. Scale markings are graduated to 300mA and the needle swings beyond these markings. I'll also hear a 'pop' coming from the speakers before I can shut off the amp which I do as soon as I see the needle peg.

As I've said, I'll turn the amp off PDQ (within a couple of seconds) for fear of causing damage to the tubes and amp. I've tried removing & reinstalling the tubes and swapping which socket they're installed to see if it's an issue with the amp, I've also tried lightly tapping the tubes with a pencil to see if that might alleviate the problem but to no avail.

I bought the tubes 2 months ago and followed the tube manufacturer's recommendation of setting bias @ 120mA and up till a few days ago they worked perfectly. The problem first emerged when I returned from a week long vacation and I went to turn on the amp and saw the bias meter peg. Luckily the meter was still plugged in or the amp/tubes probably would've blown.

Today I tried once more reinstalling the defective tubes to see if they would cause the same problem and they did. In the meantime I've installed another set of 300B tubes (same manufacturer) and so far have not had any problems (knock wood).

I'm going to send the defective tubes back for a warranty exchange, but in the meantime I'm curious what would cause a set of tubes to all of a sudden go bad like this, I'm relatively new to tubes so please enlighten me.

Thanks.

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