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Old 10th February 2006, 08:03 AM   #1
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Default noisy noisy noisy noisy noisy tube

yes, really, that noisy.

I finally got my preamp up and running. this is my first scratch build. It is based on pete millett's low mu preamp, but uses the bottlehead c4s instead of the pentode and a 6080/6as7g tube.

Anyway, it works, the sound it makes is quite good, but there is a problem. It makes a lot of noise. I have 3 different 6080 tubes and 1 6as7. each makes a unique noise -- some have a whooshing sound, some hum really badly, some sound like I recorded the radiators. Some of them even make noise with the power amp off -- sort of like banging springs. Also, if you just snap your fingers near the tubes (especially the 6as7) it comes through the speakers, not to mention the cranking sound of the volume pot.

I am going to guess that the AC on the heater is part of the problem -- but that can't be all of it. So, can anyone help me get rid of the noise? is the 6080 just a bad tube to use? am I going to have to find a particularly good one? I attached a schematic in case that is useful.

-d
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Old 10th February 2006, 08:08 AM   #2
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How have you referenced the heater AC to earth?

And how much current have you measured going through the 6AS7 cathode resistor?
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Old 10th February 2006, 08:26 AM   #3
arnoldc is offline arnoldc  Philippines
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microphonic...
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Old 10th February 2006, 08:29 AM   #4
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The center tap of the filament transformer is grounded. The rectifier center taps (EZ81) are floating.

I've not measured the current, but based on the voltages on the CCS there should be about 40mA.

i'll post some pics of the inside as soon as I get a chance. Other than the noise it sounds quite good, so I think I am most of the way there.
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Old 10th February 2006, 08:31 AM   #5
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Quote:
Originally posted by arnoldc
microphonic...
And how, but there is also buzzing ... I suppose that could be microphonics as well?
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Old 10th February 2006, 09:04 AM   #6
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Buzzing as in 60 or 120Hz? Sharp and raspy or mellow?
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Old 10th February 2006, 09:43 AM   #7
arnoldc is offline arnoldc  Philippines
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try tube dampers... my 407B tubes are microphonic compared to 407A

ditto with your volume control experience
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Old 10th February 2006, 09:50 AM   #8
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I would not suspect microphonics to be a problem. The hum is probably because of a less than ideal earthing scheme.

That stage should be very quiet with a CCS especially.
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Old 10th February 2006, 10:27 AM   #9
EC8010 is offline EC8010  United Kingdom
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What's the hum like with the volume control turned all the way down? What DC voltages do you have on the anodes and cathodes of the 6080? Can you post a diagram of the power supply as well? Not just to show that you used an EZ80 rectifier, but to show how all of your earthing etc was done. Photos would be very useful.
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Old 10th February 2006, 04:12 PM   #10
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Maybe, there is some oscillation going on? This wouldnt cause the hum maybe, but could cause other strange noises....

Have you used 'grid-stoppers' on the valves grids? Normally 150ohm is a good value......
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