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Old 13th October 2007, 06:37 PM   #1
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Default Hiss but no hum...

I built a p-p Aikido pre-amp and it pretty quiet as far as hum is concern but I do have a fair amount of hiss. I'm not to sure where the sourse of it is coming from. Is it my tubes or the way I built it. Its on both channels and swapping tubes does not improve things. Tubes are 6CG7and 12BH7. Are these tubes noisey by nature?

Short of a o-scope, how do I troubleshoot this? Also, what techniques/tips should I incorporate if I were to rebuilt this? TIA...
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Old 13th October 2007, 07:38 PM   #2
Sheldon is offline Sheldon  United States
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Does it happen with the inputs shorted?
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Old 14th October 2007, 01:26 AM   #3
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Its still hisses when the inputs are shorted. Its also not volume dependent but the hum goes up a bit more at full volume.

I am using a 6X4 in the power supply and was wondering if that could be an issue. I think I read somewhere that this was not the best tube to use but can't remember if that was true or not. Any opinions on this rectifier tube? I could try some SS diode if need be but like the look of that tiny tube. Anybody have any luck with another smallish type tube?
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Old 14th October 2007, 02:32 AM   #4
SY is offline SY  United States
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Hissy noise is often a symptom of oscillation. If you don't have a scope, bring your preamp to Burning Amp and we'll have one there to check it out for you. It's possible that it's power supply, but unlikely.
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Old 14th October 2007, 04:59 AM   #5
Sheldon is offline Sheldon  United States
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SY would be one to listen to. My experience is much more limited, but I can tell you that the 6X4 is unlikely to be the issue. It may not be considered the "best" rectifier, but that's probably due to its relatively high impedance. Regarding noise, I've used them with no problems at all.

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Old 14th October 2007, 05:36 AM   #6
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I am not familiar with the Aikido amplifier, but with a hissy amp I always try the easiest: swap the first preamp tube before doing anything else.
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Old 14th October 2007, 01:17 PM   #7
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Quote:
Tubes are 6CG7and 12BH7. Are these tubes noisey by nature?
Based on my own experiences with my dual mono Aikido line stage I formed the opinion that the 6CG7/6FQ7's need to be hand picked for the lowest noise and hiss. I finally ended up using clear top RCA as my input tube of choice. Mind you I still haven't been able to score a pair of NOS 6FQ7's since we started talking about them for the Aikido. It seems that the ebay price has shot skyward since then.

Yes, I have a very slight amount of hiss if I put my head right up next to the speaker and listen. It exhibits no signs of oscillation on the scope.

I have enjoyed mine since I put it together and no issues have come up.

My hand wired 6sn7/6sn7 Aikido has no hiss or noise and I used a 5Y3 as the rectifier tube.
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Old 14th October 2007, 03:19 PM   #8
KP11520 is offline KP11520  United States
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Could anything like too high a value resistance on the input side cause thermal noise? I beleive it happens with the Phono Stage in SS preamps. Not sure about tubes.
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Old 14th October 2007, 03:35 PM   #9
SY is offline SY  United States
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Different thing. In a solid state phono stage using bipolars, the current noise dominates. For tube input stages run well away from grid current, voltage noise dominates. Generally, the high value grid leak resistors are shunted by a much lower driving source impedance so they do not contribute significantly to the noise.
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Old 14th October 2007, 03:59 PM   #10
cuibono is offline cuibono  United States
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the two times I've had hiss as a problem in amps (one ss, one tube), it turned out my enclosure was the problem - I had made wood enclosures. It seems electronics really need conductive cases to ground stray fields...

Good luck.
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