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Posting this for anyone's future benefit.
Blues Jr III comes in with a nasty, distorted sound that sounds just like a bad speaker, but confirmed not as it does it into a different cab. Fellow had lightning in one of his EL84s, so he put in a new set. Didn't have this problem before. First thing when I power it up is notice a pretty sizable hum, which he said always did that from new 4 years ago. I measure the hum at the speaker, and scratch my head because it was coming out as 40KHz. I see a 50vp-p sine on the scope that is not effected by any of the controls. I double checked it on my Fluke, and sure enough it reads 38.5KHz.
Thing really strange thing is when playing through it, the amplitude of the hum sine does not increase with the notes, it just sort of widens the trace to the right until it breaks up, which really has me puzzled. Oscillation?
What I have done so far is probe and flex, change all the tubes, moved the ribbons per Bill m. clipped fresh caps one at a time on the filters, and clipped in a different 15 watt OT, which was even worse, maybe from all the clip leads. The original OT passes the neon test.
What is up with this thing?
I finally did what I probably should have done sooner, compare power tube voltages. I had 294 volts on one screen pin 9, and 16 volts on the other, AHAH! Sure enough R35 screen fuse was open, likely taken out by the shorted tube. Replaced it, and it is fixed. Not sure why the high speaker reading, or exactly why it scoped the way it did, but I do understand only one conducting tube here is a bad thing.
Another mystery solved.
https://www.thetubestore.com/lib/the...atic-Rev-D.pdf
002.mp4
Blues Jr III comes in with a nasty, distorted sound that sounds just like a bad speaker, but confirmed not as it does it into a different cab. Fellow had lightning in one of his EL84s, so he put in a new set. Didn't have this problem before. First thing when I power it up is notice a pretty sizable hum, which he said always did that from new 4 years ago. I measure the hum at the speaker, and scratch my head because it was coming out as 40KHz. I see a 50vp-p sine on the scope that is not effected by any of the controls. I double checked it on my Fluke, and sure enough it reads 38.5KHz.
Thing really strange thing is when playing through it, the amplitude of the hum sine does not increase with the notes, it just sort of widens the trace to the right until it breaks up, which really has me puzzled. Oscillation?
What I have done so far is probe and flex, change all the tubes, moved the ribbons per Bill m. clipped fresh caps one at a time on the filters, and clipped in a different 15 watt OT, which was even worse, maybe from all the clip leads. The original OT passes the neon test.
What is up with this thing?
I finally did what I probably should have done sooner, compare power tube voltages. I had 294 volts on one screen pin 9, and 16 volts on the other, AHAH! Sure enough R35 screen fuse was open, likely taken out by the shorted tube. Replaced it, and it is fixed. Not sure why the high speaker reading, or exactly why it scoped the way it did, but I do understand only one conducting tube here is a bad thing.
Another mystery solved.
https://www.thetubestore.com/lib/the...atic-Rev-D.pdf
002.mp4
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