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Old 21st June 2005, 01:56 PM   #1
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Default Help! 6sn7GT

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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Old 21st June 2005, 02:29 PM   #2
ilimzn is offline ilimzn  Croatia
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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.
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Old 21st June 2005, 05:11 PM   #3
SY is offline SY  United States
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Default Re: Help! 6sn7GT

Quote:
Originally posted by Matthewong
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?
Put the raw supply on a separate chassis. Transformers are far too often prodigious radiators of hum and noise.
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Old 22nd June 2005, 01:21 PM   #4
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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.
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Old 22nd June 2005, 01:29 PM   #5
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did you use AC or DC for the heaters? some designs reference their heaters via a voltage divider on the B+ to ground an this helps a lot IME.

does the transformer vibrate or make a humming sound on it's own? i had a tranny that does that and it hums too.

make sure you have no ground loops too.
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Old 22nd June 2005, 01:47 PM   #6
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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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Old 22nd June 2005, 02:06 PM   #7
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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?
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Old 22nd June 2005, 02:46 PM   #8
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Yup, I've changed all the voltages. I only followed the power supply but not the voltages.
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Old 22nd June 2005, 03:01 PM   #9
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I would be wise to double check the psu section of your preamp. Did you ground your chassis?
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Old 22nd June 2005, 03:27 PM   #10
amperex is offline amperex  United States
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Default No PS schematic........

Nice to be able to look at the schematic. Anyways, place a .1uF @ 200 volt cap on each filament lead to ground.
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