MF
Well-known member
The problem:
I was playing the other night with my band and there was a sudden and significant volume drop. The tone after the volume drop was also noticeably crackly/noisy. I immediately checked the power tubes for failure but none of the power tubes were brighter than the rest. So i powered down for the night and took the amp home to swap out some tubes the next day.
My amateur attempt at diagnosis:
I pulled the chassis and removed the preamp covers. I have a couple of spare 12AX7's (electro harmonix) and 6L6's (groove tubes i think) to test with. These were tubes that I had replaced years ago so they should be working fine...but I can't say for certain (though i had previously tested the 12AX7's in an Orange Dark Terror a couple days previous and they both worked fine).
1. Checked the fuse. Intact.
2. Fired up the amp again, but at a much lower volume since i was at home. Hum is very apparent, even with no guitar plugged in. No hum in standby. When guitar is plugged in and standby switched to ON, the crackle/noise from the previous night was still there. I can't confirm if the volume drop is still the same because of the venue switch, but I believe that to be the case. I had to turn the MASTER up far more than I normally would in that setting.
3. I started by doing the pencil/microphonics test. Nothing out of the ordinary here.
4. Tried each channel and the issue was present across all three. Switched channel modes as well, no change.
5. Disabled the FX loop to see if that was the issue. Nothing. Plugged a patch cable into the SEND & RECEIVE jacks, still no change.
6. I then began removing each preamp tube one by one, starting in the V1 slot and replacing with one of the known working 12AX7's. I wasn't sure of the best practice here, so I fully powered down before each swap, though i'm reading that I probably could have just put it in STANDBY. Nothing changed in the V1. But here's where something new happens and i'm a little fuzzy on the sequence of events. At some point when testing the V2 and V3 slots, the amp volume deteriorated even further. I just can't remember if it started when pulling the V2 or V3
- i'm leaning towards V2 though. At this point i'm getting almost no volume from the orange and red channels regardless of MASTER volume, but the green channel seems unchanged from the previous state. Replacing the original preamp tube does nothing to resolve the issue. So I move on to V4 and V5 with no change.
7. Next I swap 2 the outer power tubes with 2 older but believed functioning ones with no change. Then I do the two inner ones. Same result. Then I remove the 2 inner ones completely, switch the speaker out from 8 to 4 ohms and test with no change. Swap the two outer tubes with the 2 inner ones....again no change. Put the tubes all back in their original slot.
8. Finally I flip the chassis over and take a good long look at the PCB for burning or scoring, but it's pristine.
9. Slink away defeated and heartbroken and drown my sorrows in a couple pints of beer
I have a couple of other 12AX7's that i'm considering testing later today but i'm not confident that anything will change. I'm open to other suggestions. The nearest Mesa certified tech is about 10 hours away in Montreal (according to Mesa's website) so that's not really an option. I'll be forced to either ship it or find a uncertified tech closer to home (the amp is an early to mid-2000's so it's long out of it's warranty period. Still i'd prefer to have a certified tech work on it.
Lastly I was reading this in the manual this morning.
Thanks in advance for any advice you can offer.
I was playing the other night with my band and there was a sudden and significant volume drop. The tone after the volume drop was also noticeably crackly/noisy. I immediately checked the power tubes for failure but none of the power tubes were brighter than the rest. So i powered down for the night and took the amp home to swap out some tubes the next day.
My amateur attempt at diagnosis:
I pulled the chassis and removed the preamp covers. I have a couple of spare 12AX7's (electro harmonix) and 6L6's (groove tubes i think) to test with. These were tubes that I had replaced years ago so they should be working fine...but I can't say for certain (though i had previously tested the 12AX7's in an Orange Dark Terror a couple days previous and they both worked fine).
1. Checked the fuse. Intact.
2. Fired up the amp again, but at a much lower volume since i was at home. Hum is very apparent, even with no guitar plugged in. No hum in standby. When guitar is plugged in and standby switched to ON, the crackle/noise from the previous night was still there. I can't confirm if the volume drop is still the same because of the venue switch, but I believe that to be the case. I had to turn the MASTER up far more than I normally would in that setting.
3. I started by doing the pencil/microphonics test. Nothing out of the ordinary here.
4. Tried each channel and the issue was present across all three. Switched channel modes as well, no change.
5. Disabled the FX loop to see if that was the issue. Nothing. Plugged a patch cable into the SEND & RECEIVE jacks, still no change.
6. I then began removing each preamp tube one by one, starting in the V1 slot and replacing with one of the known working 12AX7's. I wasn't sure of the best practice here, so I fully powered down before each swap, though i'm reading that I probably could have just put it in STANDBY. Nothing changed in the V1. But here's where something new happens and i'm a little fuzzy on the sequence of events. At some point when testing the V2 and V3 slots, the amp volume deteriorated even further. I just can't remember if it started when pulling the V2 or V3
- i'm leaning towards V2 though. At this point i'm getting almost no volume from the orange and red channels regardless of MASTER volume, but the green channel seems unchanged from the previous state. Replacing the original preamp tube does nothing to resolve the issue. So I move on to V4 and V5 with no change. 7. Next I swap 2 the outer power tubes with 2 older but believed functioning ones with no change. Then I do the two inner ones. Same result. Then I remove the 2 inner ones completely, switch the speaker out from 8 to 4 ohms and test with no change. Swap the two outer tubes with the 2 inner ones....again no change. Put the tubes all back in their original slot.
8. Finally I flip the chassis over and take a good long look at the PCB for burning or scoring, but it's pristine.
9. Slink away defeated and heartbroken and drown my sorrows in a couple pints of beer
I have a couple of other 12AX7's that i'm considering testing later today but i'm not confident that anything will change. I'm open to other suggestions. The nearest Mesa certified tech is about 10 hours away in Montreal (according to Mesa's website) so that's not really an option. I'll be forced to either ship it or find a uncertified tech closer to home (the amp is an early to mid-2000's so it's long out of it's warranty period. Still i'd prefer to have a certified tech work on it.
Lastly I was reading this in the manual this morning.
It sounds like my issue but i'm not sure what they mean by "small driver tube"...is that the V1 preamp?Dual Rectifier manual said:
Thanks in advance for any advice you can offer.
