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Discussion Starter #1
I just fired up my amp after a jam yesterday, and Im now getting a periodic thump coming from the speakers. It starts every 5 seconds or so, then after a few minutes of playing becomes more of a pulse like a bass drum. I thought it was maybe power as I was using a plug I never used but that wasn't it.

I turned off my lights to check the tubes and noticed a bit of a blue arc in one of the power tubes when it thumps.

Do you guys think its just the tube? I have to do some digging to see if I still have some spares.
 

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Discussion Starter #2
Never mind. Must have been the tube. I swapped their places and the same tube was 'arc'ing. Swapped them with some old ones from a previous amp and all is good.

Is this a symptom of other issues?

Does anyone know of a good tutorial for biasing amps? I have a multimeter and know how to discharge caps, but have always owned cathode bias amps so never had the need to learn.
 

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I'm no amp tech, but I don't think it's a symptom of other issues. Tubes die. And with no warning. Has happened to me a few times recently, after years of no issues at all. Moral of the story: have a backup plan for the gig.
 
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