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Dear All,
I built a SE amp now having 200mV noise of 50Hz when turning the volumepot to max, how I can minimize the noise
6sn7 →6sn7 →300B →845 The amp has 4 stage, when I short the first Stage 6sn7 to ground, the noise reduce to 2mV , but when I remove the short wire, the noise , as shown, is 200mV
Follow Ups:
1. Are you using any local or global feedback. I would assume no feedback, but I'll ask.
2. With all the tubes in, if you short the input RCA plug does low frequency issue go away or reduce? I'd consider running twisted pairs from the RCAs to the Pot and from the Pot to the tube to reduce radiated pick up.
3. How far down do you have to turn the volume pot to get to 4mV of noise (twice the shorted input noise.)
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
Hello Voltsecond,
1. No feedback
2. I will check as per your indication, but just found the 6SN7 filament DC cap not grounded, what would be the issue if not grounded ？
3. 3 inches from RCA to pot, and 5 inches from pot to 1st 6SN7
If filament cap is not grounded, noise will be at 60 or 120 Hz in the US.
Besides the capacitor on the filament that makes the filament DC, you will want to bypass the filaments to signal ground at the HV power supply. I prefer to DC bias the filaments so they are above the cathode voltage. A good spot is 1/2 the rated filament to cathode voltage.
I'll check later for the answer to these questions
2. With all the tubes in, if you short the input RCA plug does low frequency issue go away or reduce?
3. How far down do you have to turn the volume pot to get to 4mV of noise (twice the shorted input noise.)
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
2. With all the tubes in, if you short the input RCA plug does low frequency issue go away or reduce?
Ans:short the RCA, odd spike below 1mV appear
3. How far down do you have to turn the volume pot to get to 4mV of noise (twice the shorted input noise.)
Ans:not understand the question, sorry
But I seem to have fixed the problem. As soon as I connectd the ground to filament cap negative, the spike is gone and the 50Hz odd shape noise has minimized to 10mV as I turn up the volume know to maximum
Can't argue with success.
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.
. . .Oh!. . .Remember: Modifying things voids their warranty.
"the 6SN7 filament DC cap not grounded"
Does that mean there is no (properly connected) cap in the heater supply after the rectifier?
If so then the voltage is going to go way up when you connect that cap.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
You appear to be in the US so it's odd that you're getting 50Hz noise. Usually it would be 60Hz or 120Hz as the mains frequency in North America is 60Hz and that's where most noise of that freq. originates.
Are you using a DC filament or heater supply?
I am from Asia, using simple bridge rectifiers then 10000uF cap
what is the value of the grid resistor on the first 6SN7 stage?
Where is that resistor grounded? (It should be grounded at the same spot the cathode bias resistor is grounded)
Grid stopper all 1K, I just heard from my friend that the 6SN7 filament DC(negative of 10000uF E cap)has not connected to ground
What impact would it be?
Hi
put the scheme, please and a photo of connections.
The - (minus) of filaments goes ot amp ground ( and chassis) ?
The pin jack in input has the body isolated from chassis?
Noise in oscilloscope attached
That looks like some kind of oscillation to me.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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