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possible ground loop?
possible ground loop?
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Old 16th July 2017, 06:44 PM   #11
lexx21 is offline lexx21  United States
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possible ground loop?
Nothing....it was still noisy.

The thing is though, after replacing the mesh plates with solid plates it started behaving much better. I will work on it more tonight after work.
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Old 16th July 2017, 07:33 PM   #12
jan.didden is offline jan.didden  Europe
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Originally Posted by lexx21 View Post
I did connect the two chassis grounds together but that was no help.

Tone change or level change: Both.

I did discover that grounding the frame of the grid chokes (pre-amp) helped a bit. I touched the top of the grid choke and it is vibrating. I am going to go back to the original schematic and replace the grid chokes with resistors. The #27 tubes that I am using are microphonic and I think that the vibrations of the grid chokes are making the noise worse. The chokes will get a soak in transformer varnish.

Going to pull the chokes now..... I will report back when I have that done.
Well hum is always the line frequency. If you say the tone changes (if that is what it does) it looks more like an oscillation. Are you sure the tone changes, in the sense that the pitch or frequency changes?

Ja
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Old 16th July 2017, 07:34 PM   #13
jan.didden is offline jan.didden  Europe
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Originally Posted by scottjoplin View Post
What happened when you disconnected one channel?
Yes, important diagnostic.

Jan
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Old 17th July 2017, 02:48 AM   #14
lexx21 is offline lexx21  United States
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possible ground loop?
I removed the source of the tonal changes ...the mesh tubes. Now it is just going through the preamp and isolating things.
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Old 17th July 2017, 11:27 AM   #15
lexx21 is offline lexx21  United States
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possible ground loop?
Tonight I did the following:

1) wrapped all filament wires in copper foil and grounded the foil
2) grounded any input/output panel (rca jacks and pot are mounted on aluminum)
3) tried shielding the tube with grounded tin foil (ala #26 tubes)

These didn't do anything to eliminate the hum. It dawned on me that the source of the hum could be coming from the stw11nm80 heat sinks. I didn't put a mica insulator between the to-247 device and the heat sink and the back of the part is not isolated. So, it would pick up general junk and amplify it, or at least put it in the output signal.

I ordered some islet insulators and some silicone pads. We will see. If this doesn't work I will lay out the plate ccs on a pcb and etch it so that I can shield it. I kind of need to do that anyway before I build this into a permanent case.
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Old 17th July 2017, 11:59 AM   #16
lexx21 is offline lexx21  United States
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possible ground loop?
I found a bunch of to220 ceramic insulators and I installed those. Now you have to turn the volume up more than half way to hear the noise over the 120hz that is coming from the 2a3 amp. Next up, build them on pc boards and shield them. I'm also thinking about better filtering of HV going into the boards.... common mode chokes, etc... I will have to play around with that.

I did the same things to the 2a3 amp that I did to the pre..... wrapped all of the filament wires in copper foil tape and grounded it, grounded all mounting plates, etc... I have the hum from there nulled out pretty well, but I want to do better. Working on a better power supply for the filaments.....


ps... my grounds are star grounds, so those are ok. I really can't improve on those very much.
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