Hello,

I have just tried to replace my valves with a new set however i noticed that two of the Marshall EL34s had a blue sticker and two had a yellow sticker on them. I replaced the old ones in my amp and when i switched it on i got a loud hum and one of the tubes glowed up really bright like a light bulb! I knew something wasn't right so I removed and refitted the valves making sure that the numbers on the pins corresponded with the holes in the amp... same thing happened!I've managed to fit the two yellow stickered ones with two old ones but it will not have the blue ones. They keep glowing hotter like they are about to blow.On Marshalls website it says that the valves should be all of the same make... which they are.... and that the amp needs to be biased.??????What is biased... Why does Marshall only have engineers in Milton Keynes????Any ideas what is going wrong here?

Please help

I didn't find the right solution from the internet.

References:
https://www.soundonsound.com/forum/viewtopic.php?f=22&t=1195
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Thanks