My ENGL Fireball had been creating unwanted noise for quite some time, loud hissing and cracking, even through the guitar sound, increasing with increasing volume, so I thought something was wrong, took it to an amp builder. A week later he had fixed it, by placing new tubes in it. Today I picked it up, tested it at his place and the hissing was gone, amp fixed.

So I took my amp home, plugged it into my own cab, everything was alright. No hissing. 2 hours later, the goddamn hissing and cracking returned! For no apparent reason. Needless to say, I was pretty pissed and I have no clue what the hell went wrong. Does anyone have any idea what might be the case? What to do now?

Thanks very much in advance, I'm in a quite desperate and annoyed mood atm, beacuse my equipment keeps failing on me

tl;dr: amp hissed, new tubes, hissing gone. hissing returned 2 hours later. what now?
I do not want to have a signature anymore.
That's some effing bad luck.. How long have you had the amplifier for? There's more to go wrong in an amp than just tubes.
Gear pics

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I'll take a guess and say you've the impedance wrong? Just a guess.
Did a tube blow before? That might have taken out the grid resistor with it....

More info please? ie. Cab, previous blown tubes etc.
Tubes should not create a hissing sound. Tried using another cable between guitar and head, between head and cabinet? Tried another cabinet and another guitar?
Impedance can't go wrong, I always check that to be sure. As for cable between the head ad the guitar, I was talking about the amp without anything plugged in, so it can't be a cable defect. I don't know if a tube blew before, so I guess not, but as stated in my previous post, they have just been replaced. My cab is a Marshall MF400, I don't have any other cab, so I can't test that. I only have two speaker cables and it's hissing on both. The sound that comes out reminds me of a seashore and it's ridiculously loud, definitely something wrong..
I do not want to have a signature anymore.
Maybe a resistor or capacitor blew? Best bet is to take it to a tech, explain the problem very clearly and hope (s)he'll fix it.