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noisy noisy noisy noisy noisy tube

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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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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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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.
 
Here are some pictures. I have perhaps the worst digital camera ever, so hopefully the quality is enough that people can see what is going on.

Anyhow, there seem to be several noises at any given time, and depending upon the tube they are at different levels.

I think the hum/buzz noise is at 120Hz as it doesn't sound low enough to be at 60. It is pretty constant even with the volume all the way down. Could be 60, though?

Some of the other noise is clearly banging in the tubes due to heat. With the volume down, the tube itself is hissing and sounds like springs being tapped. With the volume up this all comes through the speakers. One of the tubes (a sylvania) makes a loud screak -- not high pitched exactly, but higher than the hum -- maybe at 240Hz or 480Hz?

Anyhow, some pics.
 

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Here's the PS schematic. There are actually 2 of these. When I started, I thought I was going to run this at 80-100mA which was going to require the 2 power supplies. I subsequently backed off on that, but since I'd already drilled the holes and bought the parts, I left them both thinking I might need to adjust the curent up at some point.
 

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One thing that immediately jumps out at me is the separation of the power supply grounds. The loop around the transformers, diodes and filter caps carries a boatload of ripple current. You don't want anything grounded within this loop. C3, C4 and the transformer CT should all meet at one point, and that point be the far end of the ground bus. The output stage should be returned directly to that junction point.
 
Here's how I did the grounding in case there is something obviously wrong:

Each PS has a star ground where the ground side of the PS caps connect. The center tap of the high voltage winding also connects to the same point.

each cathode connects to its own ground (through a resistor and cap). These are connected together, and both of the PS ground positions connect to one of these spots. The volume pot and the input and output ground also connect to this spot. The CCS grounds and the filament transformer center tap also connect to this spot. This spot is connected to the chassis plate at one leg of the filament transformer. The other leg of the filament transformer connects the chassis plate to mains earth.
 
Ex-Moderator
Joined 2003
You have audio travelling on a pair of twisted wires. That might work if one was earth, but you appear to have twisted left and right together. At the very least, that will cause crosstalk. You also appear to have some of these wires passing near the mains transformer. That will cause hum. I hate screened lead and normally make my audio wiring sufficently short not to need it, but I think you're going to have to resort to it to get rid of the hum.
 
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