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My rebuild 6sn7gt tubes line pre-amp.

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I've rebuilt my 6sn7gt tubes line pre-amp as previous version generate quite a lot of hum. This version has improved a lot, the humming has been lower. But still able to listen it when you put place your ear near to my loudspeaker. Before I finish my pre-amp, I add another ground to the chasis body, and It makes more humming, when I remove it, the humming lower, how can I make the humming inaudible? I only use normal wire from potentiometer connect to the 6sn7gt 1st stage grid, will this the problem that make the humming audible?
 
Any chance we could see a picture of the wireing? mabey theres a ground loop somewhere? you could if you havent already try twisting the signal and ground wires from the potentiometer together or change them for screened cable. Are you sure the hum isnt comeing from the power supply? star ground everythign from one point so that all the ground wires connect up in the same place so that you don't have any ground loops. It's hard to say what's going on without seeing pictures/schematics.
Owen
 
Actually my pre-amp wiring is quite messy, but I can tell you I've connected all the ground together, first, I connected the RCA socket ground to the output board ground(I got 2 output board and 1 power supply board), the potentiometer is also connected to the output board ground,the 2 output board ground didn't connect to the chasis body, then the 2 output board grounds connected to the power supply board ground. From there I connected the output board ground to the chasis body, and also the transformer earth connected to the chasis body. Is there ok connected this way? I'll try all my best to attach a pic .
 
I'didn't connect the resistor from heater to ground, but I've followed the Audionote M7 method as shown below in the picture but I changed all the voltages. Today I've twisted the output wire with another wire and lead to ground, it works . The humming reduces, but still audible, I can feel the speaker driver cone has very weak vibration when I touch the speaker cone. Should I connect the ground from each board to the chasis body? Because right now I only connected the power supply board to the body chasis.
 

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I got 2 schematic diagram and both is very similar, only the 1st stage cathode resistor is different, one is using 750ohm and the other one is using 1.5k, and the input voltage is different. I'm using 750 ohm for the 1st stage cathode resistor, but got humming, don't know whether is the resistor makes my pre-amp humming.
 

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You should lift the filament potential. It may cause it to hum if there is excessive potential difference between cathode and filament.

Of course, the B+ ripple should be very minimal. I would say a CLCLC filtering is extremely good for this kind of line amp.

Johnny
 
So what should I do to lift up the filament potential? Because I'm quite new to this tubes gadget, so sometime I don't know what to do when I'm facing problem. And I also don't understand what is CLCLC filtering, may you please explain more about it to me, so I can understand more. Thanks
 
I used number of 6SN7's in my amp project. I don't have much problem with them except those one made by EH. I had to lift the filament from ground to +80V in order to get rid of the hum. The hum was caused by excessive high potential difference between the cathode and filament.

Anyway, there are some basic rules and precautions when we build an amplifier. It is rather challenging to kill the hum and other noises. More you practice, more you learn.

Johnny
 
hi,

i find it a bit strange though but I have used several brands including NOS and EH, and other new production 6sn7gt in my preamp but never had a hum problem. :rolleyes: i must admit though that my pair of RCA VT231 are grossly microphonic. :D

anyway, back to Matt's problem... since even rebuilding your amp did not remove most of the hum, may I suggest that you look into your power supply, I have tried (successfully) using as simple as a 5Y3 and a CRC (22uf-1K-22uf) for a 6sn7gt preamp in the past without any problem. heaters were DC and was lifted about 1/3 of the B+.
 
Actually this rebuild version is better than the last one. Right now the hum is lower than the last one, you still able to hear it if you place your ear near (very near) the loudspeaker. I sometime don't understand what were you talking about, like lift the filament potential. How can I measure the like you said 1/3 of the B+? Is it change the cathode resistor or what. I really hope that you can tell more about this stuff because I really enjoy to build vacuum tubes amplifier.
 
hi Matt,

so it's hum level is already down if you have to place your ear next to the speakers. anyway, what I meant was a when you series a 100K to a 47K resistor and put their ends to b+ and ground, the junction of the resistors would end up somewhere around 1/3 of the B+. Bypassing the junction to ground also helps in reducing hum if it is still audible. This junction is where you will connect one of the filament rail.

JojoD
 
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