I bought this amp used about a years ago and since then I had no problem with it. About a month ago, it started to make some strange sounds when I turn the standby on and off. Now I get these weird noise every time I turn off the standby and it is much louder than it was before. I don't want to use it anymore because I am afraid to blow a speaker in my cab or to damage the amp more than it is right now. I know I should take it to an amp tech but I don't have a big budget for the repair and I can't wait 3 weeks (I called and they told me it's a minimum of 2-3 weeks) or more to get my amp back since I have a couple gigs coming in this month. I recorded a couple of videos of the problems in the hope that maybe someone here will be able to point the problem.

The videos where shooted with no effect in the signal chain. Guitar straight in the amp and the amp straight in the cab.

So here is what happens :

The amp was turned on and I had let it heat for about 5 minutes, when I turn the standby switch to off, this is what happens :
This loud BANG is at the same loudness at any level.

When the standby is off, I can play with the amp without any problem. I don't get any weird noise or hum or anything, it's just working great for about 20 minutes.
After about 20 minutes of playing, the level of the sound drop and the post knob does not change the level of the sound anymore (it will sound the same at 2 and 7) and I get some weird static noise that fade in and out.
I tried to shoot a video of that fading in and out sound but it seems that my phone have a noise gate in it and it did not capture the sound very well. When this problem happens, if I turn the standby on, I get a LOUD static noise fade out.

Post knob not doing anything, weird fading in and out static noise + loud static noise at the end :

If I turn the standby off again, I get a loud static noise fade again and the post knob is still not doing anything. I have to close the amp for about 10 minutes to let it get back to normal (by normal I mean just doing the loud bang thing haha).

Also, I did notice that when the post is at 0, I still can hear my guitar through the amp. Before all these problems started the happen, when the post was at 0 the amp was completely silent.

I hope someone here will be able to help me out ! If you guys know a better place to ask this kind of question please tell me. And by the way, sorry if my english was not perfect, it is not my primary language.

Thanks !
Your power tubes are dying.

Unless your amp has the bias mod, the bias can't be adjusted on the 5150 II enough to damage the amp, in fact, cranked all the way up, it's still not hot enough to properly heat up 6L6s to optimal level - so it's as easy as pulling out the old ones and putting new ones in.
Quote by Dave_Mc
I've had tube amps for a while now, but never actually had any go down on me
Quote by jj1565
maybe you're not saying the right things? an amp likes to know you care.

Thanks a lot for your reply, if it's really just a problem of power tubes, that makes me really happy!
I have a Mesa dual rectifier (old 2 channel) as a backup amp. Can I use these power tube in my 5150 II ? I know both uses 6L6 but im at work at the moment and I can't verify if the tube are the same in both amps. It would be nice to test with tube of my other amp instead of paying for a new set of tube just to test.
Yes, you should be able to. That's a good solution to make sure that it's the tubes, before you spend any money on new ones.

Make sure they're 6L6s, though - Rectos can run EL34s, KT77s, KT66s and KT88s as well
Quote by Dave_Mc
I've had tube amps for a while now, but never actually had any go down on me
Quote by jj1565
maybe you're not saying the right things? an amp likes to know you care.

Yes I am sure they are 6L6, I retubed thst Mesa a couple of times. I was just not sure if there is different type of 6L6 thats why I was asking.
There are different types of 6L6's but the others haven't been made by anybody for decades. So unless they're old NOS tubes (6L6GA, 6L6GB, some really old tubes labeled 6L6 are actually 6L6GA but they'd be really, really old, metal canned 6L6's were 6L6A but I doubt you'd find any of those outside a museum) you'll be fine.
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^I've actually got a set of metal can RCA 6L6's

But yeah, jack the power tubes from the Dual Rec and plug them in the 5150ii and see if that helps. If not we can continue diagnosing the problem
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