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Old 14th October 2011, 03:04 PM   #1
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Default Temporary hum

Hi, I had some search issues using my phone so I'm posting this question.
I have a jolida 502brc and when I turn it on now, I get hum in the right channel through the speaker. It goes away in about 30 seconds or so. It reminds me of old Tvs warming up. The tubes were new about 10 months ago and this issue is new. Amp gets used for many hours daily.

Is this the sign that it's time to replace tubes? I guess I'm pushing the 3000 hour mark on these 6550s (Tung sol reissues).

Thank you!
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Old 14th October 2011, 03:12 PM   #2
DF96 is offline DF96  England
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My Mullard 5-20 hums for a few seconds when warming up. It means that an output pair are not heating up and emitting electrons at exactly the same rate, so the output stage is temporarily unbalanced. P-P amps rely on stage balance to reduce hum, so they can get away with less smoothing than an SE amp would need. Once the cathodes are at full temperature, their emission comfortably exceeds the circuit requirements so balance is restored.

It is probably an early sign that one of the valves is nearing the end of its life. Not necessarily an output valve, as an unbalanced LTP phase splitter can do the same, but small-signal valves generally seem to last for ever so probably an output one.
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Old 14th October 2011, 03:33 PM   #3
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Thanks for the quick answer. I guess with the kind of usage I put this through, I might need to change output tubes annually or close to it.
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Old 15th October 2011, 10:31 PM   #4
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You should test your tubes. I've got some re-isssue KT-88s that I listen to daily like you and 3 out of 4 just tested at 100% + and one was at 98%. Daily use for hours at a time.
Maybe its a passive component that is giving you noise are you sure it's hum?

Cheers,

Bob
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Old 7th November 2011, 11:37 PM   #5
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Hmm.. Haven't noticed this for like a couple weeks now. And, furthermore I just posted that a peak distortion in my preamp also has gone away. I'm starting to think that I may have some potential electrical issues in this place, especially when gear acts up and then suddenly no issues. I've had huge trucks drive by my place and bounce hard on the street enough that my lights slightly flicker, the crazy pole outside has been worked on repeatedly.

I hate to say it but maybe I should invest in some sort of voltage regulating protection/filter. Hate to spend a lot on that though.
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Old 29th March 2012, 03:36 AM   #6
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I know it's been awhile but I wonder if adding too much voltage to bias would have also caused the same thing? These amps come with an "EZ" bias which is set visually with an led, one for each output tube. The thing is, is that they do flicker on and off which is fine. However, every once in awhile, when they flickered off, I would turn the bias up to the point where the light would come back on. So I had been doing this for about a year, without testing the actual voltage lol....

So I picked up a new multimeter, turned the setting all the way down (I don't even know how high they were really...) and then set the EZ bias again as suggested (until the lights just come on.) At that point, I tested the voltage on each one and they were not too far off from the recommended 500mv. I then adjusted them to be all nearly the same. Still no further issues, and I would also say that the performance has been better and also very consistent. The lights do flicker here and there, but I figure it's better to just leave them alone at the voltage I set them at and be done with it. Oh and according to the manual, they will flicker so it's not a problem.

At some point, around the time of this original post, I did actually swap the pairs of output tubes (and then drivers) to different channels. This may also be why I haven't seen any issues but I suspect the biasing.

Another question: Can anyone recommend a reasonable way to test the tubes?
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Last edited by Pure_Brew; 29th March 2012 at 03:39 AM.
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Old 29th March 2012, 04:59 AM   #7
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it's a good practice to test your tubes and grade them, putting matched pairs in as P-P partners helps minimize hum due to unequal tube strenth. The nature of tube PP cancels power supply ripple......you can always do things to reduce the power supply ripple as well.
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