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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Help! 6sn7GT

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I've made a pre-amp using 2 6SN7GT tubes. But the problem is the output is very high. How can I lower the output level? And this pre-amp generates humming. When I move away the transformer from the chasis, the humming dissapear. When I put the transformer inside the chasis, the humming come again. What sould I do? Below is the schematic that I've followed. Hope you all can help me.


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To reduce gain, for starters you may want to remove the 330uF in parallel with the cathode resistor of the 1st tube. You can further lower it by reducing the plate resistor for the same tube, but that will also shift the operating point and increase distortion. On the other hand, removing the cathode resistor bypass cap will lower distortion, so you may get an adequate compromise.

Of course, you may want to look into the 'heretical unity gain preamp' thread by SY, in particular he part where he discusses chosing of the appropriate overal gain for the preamp.
Thank you to help me to solve the high output level from my pre-amp. But humming problem become more obviuos after I reduce the resistor and remove the cap. Right now I put the transformer away from the chasis and the humming still there, not like previous, before I modify the pre-amp. What is going wrong right now? Hope you all can help me. :confused:
I've checked all the ground already, there is no ground loop, even I've connected a few more cable to the ground, because some people told to do so. But this method doesn't help. I've followed power supply from below shcematic. Is it ok because if I only change the voltage and use the same power supply as below? One more thing, I've used relay for the input, is it ok?


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you can use psu designer software from duncan amps in designing your psu. the schematic you posted is from an M7, did you increased your tranny's secondary voltage for the HT? it says 200V on the schematic...

btw, your schematic has a heater voltage of 12V, your 6sn7 tube needs 6.3V only. is it reading somewhere near 6.3 volts after you changed the secondary?
Thank you all of you. Today I've checked the pre-amp again and I found out that the HT voltage for my pre-amp is around 244V DC and it drops to around 185V DC after the resistor to the tube's pin number 2. The first stage cathode voltage is around 4.7 to 5.0v DC and the second stage is around 183v DC. Is it ok for the 6sn7 tube?
If it were my preamp I would use a voltage divider to bias the heater at 150v,which is what I did when I built a similar cct.I would use a 1Meg resistor from B+ to ground via another resistor.Aim for 150VDC at the junction between the two then connect the heater centre-tap to the 150v.Got rid of all my hum.I now use it in all my preamps and input stages of power amps.
I would also use both halves of one tube for left and right first stage and another tube for left and right second stage.You only need the above for the second stage.
Matthewong said:
Ic, but I've checked it many times, still can't find out the problem, can you send me your schematic diagram for the pre-amp using 6sn7 and psu? Because maybe I can use it as reference to find out the problem I'm facing.

try here...


the 5692/6SN7 is in post #12

see also post #31, a lot similar to your project.

I have built the one in post #12 with no problems encountered. :up: I did built my own simple CLCRC psu using psud software and raised the heater potential too via voltage divider. Problem is I don't have a schematic of my psu. :smash:

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