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Slight hum doesnt raise or lower with volume knob on my fisher allegro

tonyg88

tonyg88

Super Member
Slight hum doesnt raise or lower with volume knob on my fisher allegro. It is not that loud but my fisher 800c is dead quite when on... Which caps cause a hum? Is the power section or the caps on the output tubes?
 
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Not sure, but if you have a 3 prong plug, you could have a ground loop somewhere. I had this on a set up I had. Pass Labs amp. They told me to use a cheater plug. (3 prong to 2 prong)
It cured the problem.
Later on, I moved things around in my set up and tried the 3 prong plug again. No hum that time.
Still don’t know what it was, or what I did to fix it.
 
trinhsman said:
Not sure, but if you have a 3 prong plug, you could have a ground loop somewhere. I had this on a set up I had. Pass Labs amp. They told me to use a cheater plug. (3 prong to 2 prong)
It cured the problem.
Later on, I moved things around in my set up and tried the 3 prong plug again. No hum that time.
Still don’t know what it was, or what I did to fix it.
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I;'ve tried reversing plug in the wall and that didn't change anything. Adding a 3 prong to an amp not designed for it will cause more hum from my readings.
 
tonyg88 said:
I;'ve tried reversing plug in the wall and that didn't change anything. Adding a 3 prong to an amp not designed for it will cause more hum from my readings.
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Then your amp is a 2 prong plug?
I agree, adding a 3 prong could cause even more issues.
Tube going bad? Cap out of spec?
I’m not good at that stuff, so it could be anything.
I wish you luck with it.
 
power supply caps and/or poorly balanced output tube sections are the usual reasons. If its louder in one channel I'd go for tubes, if its equal then my vote is supply caps. As old as it is, those caps are pretty well past their expiration date anyway.
 
gadget73 said:
power supply caps and/or poorly balanced output tube sections are the usual reasons. If its louder in one channel I'd go for tubes, if its equal then my vote is supply caps. As old as it is, those caps are pretty well past their expiration date anyway.
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I tried swapping output tubes and no change. Will switch some of the 12ax7 out just encase but yeah i am thinking power supply caps well. I was hoping it was because the el95 tubes were closer to the transformers vs the ell80 but it did the same hum volume with both kinds of tubes.
 
Damn judging by my past projects I am better off waiting until its the motor boat sound before touching it. haha. I wish they made these cap cans as replaceable as tubes as they usually out last the tubes that are in these units!
 
cademan said:
It may never ever motorboat however the hum may get worse and worse until you fix the problem.
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You are right about the hum slowly increasing... It was very low ( had to put my ear pretty close at first ) and has risen slightly over the last week or so. Thanks
 
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gadget73 said:
or they might short and make it so you can't fix the problem
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Could take a transformer out right? Anyways not going to use it for now until I have time to check it out. Prepare for an interesting thread! get the popcorn ready folks!
 
yeah, less of a worry if there is a mains fuse but some gear was built without that. I think Fisher generally did have a fuse though.
 
gadget73 said:
yeah, less of a worry if there is a mains fuse but some gear was built without that. I think Fisher generally did have a fuse though.
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Unfort this is the only fisher model I can think of that doesn't have a fuse. I'm not sure what avery was thinking that day, Must of had a hang over from drinking and listening to his jazz music the night before.
 
still annoying and not cheap. Its easier to sub but unless it matches the original pretty well you have to figure out how to mount it, and how to correct the voltages so it works like it should.
 
gadget73 said:
still annoying and not cheap. Its easier to sub but unless it matches the original pretty well you have to figure out how to mount it, and how to correct the voltages so it works like it should.
Click to expand...
I wonder if I can inject 60hz hum in the opposite polarity of what I am hearing if it would cause the hum to cancel out?
 
Question I wonder if this was an early symptom.... Before this happened I would only hear a hum while it started up (the first 5 seconds or so) but then it would go away. Is that an early symptom for failing filter caps? Just want to know for the future.
 
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