Strange issue with my 5150 III

Bishop5150

Bishop5150

Well-known member
Last night at practice my amp made this howling feedback type noise when I took it off standby. So I unplugged everything from it so it was just my guitar and it was still doing it. Messed with changing to the other speaker jack and that seemed to get rid of it. Bad speaker cable maybe? I hope not. It's a brand new Evidence Siren II speaker cable. Maybe the Jack? Your thoughts are appreciated. Then, later on it gets more interesting. We're practicing our set and my amp switches itself to the 2nd (blue) channel, locks on that channel and mutes itself. So I turned the amp off and turn it back on. Same drill after about a minute or so. We were basically done with rehearsal so I called it so I could try to see what was going on with the amp. I shut it all down for about 10 minutes and fired everything back up and played it for a few switching between all three channels and it was fine. WTF? Any ideas what might cause the amp to default to the 2nd channel, lock up and mute itself? Almost sounds like a protection feature or something. Cant be having this going on live. :confused:
 
Sounds like a bad tube.
Change out V8 and/or V7 tubes.
They are the 2 small ones next to the power tubes.
V7 is the loop driver/recovery tube
V8 is the PI
 
Is it cool to just swap out the the v7 and v8 tubes or would I have to swap them all? Would I have to worry about any kind of biasing by swapping out just the 2? Please excuse my ignorance in this matter.
 
Ok thanks. I will give it a try. Another thing I noticed about it was that when I'm using just the 1st and 2nd channels there seems to be no problem. But when I would use that 2nd channel at all even if I switched to it then went back to the 1st or 3rd channel it shortly thereafter would default itself back to the 2nd channel, lock up and mute. Weird.
 
Ancient Alien":3iww72q3 said:
Sounds like a bad tube.
Change out V8 and/or V7 tubes.
They are the 2 small ones next to the power tubes.
V7 is the loop driver/recovery tube
V8 is the PI
Click to expand...

What is the driver/recovery tube and PI? Just curious. This is all Greek to me.
 
That is a crazy amp build wise.
It could be as simple as by the switch jack.
But there is way too much going on inside that amp to just guess.
It has 6 gain stages for god sakes :lol: :LOL:
I fixed one of them, and hope to never see another :no:
Just a lot of stuff going on in there
 
victim5150":2rhgeruc said:
Ancient Alien":2rhgeruc said:
Sounds like a bad tube.
Change out V8 and/or V7 tubes.
They are the 2 small ones next to the power tubes.
V7 is the loop driver/recovery tube
V8 is the PI
Click to expand...

What is the driver/recovery tube and PI? Just curious. This is all Greek to me.
Click to expand...

V7-AKA Effects Loop Drive/Recovery Tube
V8- Phase Inverter Tube
 
Yeah, there is a lot going on in there. Well, I'll go grab a couple of preamp tubes today from guitar center today and start from there. The ones that are in the amp are stock. Guitar Center will probably only have Groove Tubes. I'll guess that'll work. Thanks for your insight and for the the V7 and V8 explanation. I have the tube layout schematic but it doeasnt tell what is what. Much appreciated. :rock:
 
Well, last night the amp crapped out on me. I turned it on and saw a small flash of bluish light inside the amp through the front grill and now there's no sound. All the lights light up, tubes are lit up but no sound. What do y'all think? Output transformer? :confused: It seemed like the flash of light came from that area. Very bummed. :cry: Thankful that it happened at practice and not at a show but very bummed. I got 2 big shows the next 2 weeks. So I finished practice with my original 5150 that I've had since 93'. I've gotten really spoiled with the 5150 III's clean channel though. I forgot how bad the originals clean channel is. :no: Gotta find an authorized Fender repair shop tomorrow. Any of you guys have any thoughts on what might be wrong and what I might possibly be looking roughtly cost wise to get it fixed? Thanks for your input ahead of time. It's not a matter of if shits gonna happen it's just a matter of when. :doh:
 
Check your HT fuse. If its blown, one of your power tubes is probably blown and is probably what you saw arc. The amp will still appear to be "on" and the power tubes will still appear ready to go even with that blown. Good luck with it.
 
Fuse is good. Still think it could be a bad power tube? Man, I hope thats what it is. That would be a quick fix. I'm usually not that lucky though :doh:
 
I would swap the power tubes before taking it somewhere. 99% of tube amp failures are simply related to the tubes and fuses.
Did you check the fuse with a meter? Sometimes it looks good by looking at it but its actually blown.
 
Well, I swapped the fuse and power tubes and even bought a couple of spare preamp tubes and swaped them out in each one and still no sound. I checked for any kind of scarring at each of the sockets and didn't notice anything. I didn't smell anything last night or today from the amp so I'm hoping it's an easy type of fix on the board. With all the lights lighting up hopefully that's a good sign. I just hope it's not the output or power transformer. :scared:
 
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