JKMV12
Well-known member
Well the worst has happened...my RM80 is malfunctioning...and it is NOT a tube issue. I tried changing all the tubes in the amp and it still had a problem. It happens on both channels so it is definitely within the amp itself. Here is what happened.
Last Wednesday i brought my amp to the university for band practice and it sounded fine. I had it cranked higher than usual b/c i was playing with a drummer and a guitarist in a small room. We played for a good 1.5hrs. I also played it last Friday and it sounded fine (though i didn't have the volume up as high). This Tuesday i bring my amp to the same practice room for band practice but this time the tone sounds like crap. When i turned the volume up and played a chord/note it would cut in and out like crazy. The signal sounded chopped. If you think about it graphically....the signal amplitude vs time...segments of the graph are just missing (or realistically cut down close to zero) and the result is a broken, chopped up signal. It phases in and out at a fairly high frequency and is very noticeable. if i hit the strings harder then there is a longer delay between the first and second audible segments.
So here is a summary of the issue:
1) crappy tone that sounds dull/muffled and presents with a worse high end response than normal
2) chopped/broken signal at high volumes
3) happens on both modules at comparable volume levels (1087 and Super Clean)
4) changed power tubes, ampV1, PI, and FX loop buffer and there was no change (i also tried different tubes in the 1087 with no change)
5) tried with individual speakers and it happened on both (2x12 combo)
6) checked bias and it was fine on both sets i tried (6V6 and EL34)
i noticed that the power tubes emit a blue glow on the outside of the glass and when the signal is getting chopped up this blue glow is cutting in and out as well.
when diagnosing the problem i found that the problem was worse when the preamp was at ~50% and master at ~25% versus the opposite. Those are basically the threshold values under which it is consistent with crappy tone and over which is chopped/broken. My normal bedroom levels are about 25-30% for both volumes. Occasionally i crank it a bit higher when i have the house to myself but it rarely sees anything above moderate levels.
i brought the amp home yesterday and tried it again. the issue persisted. i then changed the FX loop buffer (only time i changed) and after that i tested it. it still had the issue but after a few seconds of testing the signal completely dropped to a extremely low level. the volume and gain knobs had virtually no effect. it was barely audible.
i then proceeded to remove the chassis from the cabinet to see if there were any obvious signs of failure which there were none. my dad and i looked at it for a good 10-15 minutes under good lighting and nothing looked out of order.
i had issues with the 1087 cutting out like this before but it was always only that module and after making sure the tubes were seated properly it would usually work fine.
based on my general working knowledge of tube amplifiers and the problem i observed i think the issue is either in the power supply or the power section of the amp. it could even be the post module section of the preamp (PI/FXloop). I don't use the FX loop at all so i wouldn't suspect that.
If you have had any problems like this or have an idea what the problem might be then please share your knowledge/experiences. I know of a tech i can take it too who might be able to fix it but i want to see what you guys think and see whether or not i can get a better idea of what happened and what caused the issue.
due to the periodic nature of the issue in the beginning i would assume it to be a capacitor failure but that is just a guess. I have a multimeter and access to an oscilloscope so if you can suggest any simple diagnostics i can perform i'd appreciate it.
Thank you for taking the time to read my post and i appreciate help you can provide.
Thanks,
John
Last Wednesday i brought my amp to the university for band practice and it sounded fine. I had it cranked higher than usual b/c i was playing with a drummer and a guitarist in a small room. We played for a good 1.5hrs. I also played it last Friday and it sounded fine (though i didn't have the volume up as high). This Tuesday i bring my amp to the same practice room for band practice but this time the tone sounds like crap. When i turned the volume up and played a chord/note it would cut in and out like crazy. The signal sounded chopped. If you think about it graphically....the signal amplitude vs time...segments of the graph are just missing (or realistically cut down close to zero) and the result is a broken, chopped up signal. It phases in and out at a fairly high frequency and is very noticeable. if i hit the strings harder then there is a longer delay between the first and second audible segments.
So here is a summary of the issue:
1) crappy tone that sounds dull/muffled and presents with a worse high end response than normal
2) chopped/broken signal at high volumes
3) happens on both modules at comparable volume levels (1087 and Super Clean)
4) changed power tubes, ampV1, PI, and FX loop buffer and there was no change (i also tried different tubes in the 1087 with no change)
5) tried with individual speakers and it happened on both (2x12 combo)
6) checked bias and it was fine on both sets i tried (6V6 and EL34)
i noticed that the power tubes emit a blue glow on the outside of the glass and when the signal is getting chopped up this blue glow is cutting in and out as well.
when diagnosing the problem i found that the problem was worse when the preamp was at ~50% and master at ~25% versus the opposite. Those are basically the threshold values under which it is consistent with crappy tone and over which is chopped/broken. My normal bedroom levels are about 25-30% for both volumes. Occasionally i crank it a bit higher when i have the house to myself but it rarely sees anything above moderate levels.
i brought the amp home yesterday and tried it again. the issue persisted. i then changed the FX loop buffer (only time i changed) and after that i tested it. it still had the issue but after a few seconds of testing the signal completely dropped to a extremely low level. the volume and gain knobs had virtually no effect. it was barely audible.
i then proceeded to remove the chassis from the cabinet to see if there were any obvious signs of failure which there were none. my dad and i looked at it for a good 10-15 minutes under good lighting and nothing looked out of order.
i had issues with the 1087 cutting out like this before but it was always only that module and after making sure the tubes were seated properly it would usually work fine.
based on my general working knowledge of tube amplifiers and the problem i observed i think the issue is either in the power supply or the power section of the amp. it could even be the post module section of the preamp (PI/FXloop). I don't use the FX loop at all so i wouldn't suspect that.
If you have had any problems like this or have an idea what the problem might be then please share your knowledge/experiences. I know of a tech i can take it too who might be able to fix it but i want to see what you guys think and see whether or not i can get a better idea of what happened and what caused the issue.
due to the periodic nature of the issue in the beginning i would assume it to be a capacitor failure but that is just a guess. I have a multimeter and access to an oscilloscope so if you can suggest any simple diagnostics i can perform i'd appreciate it.
Thank you for taking the time to read my post and i appreciate help you can provide.
Thanks,
John