Tube DIY Asylum

RE: SE amp noise of 200mV

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VoltSecond

A

In Reply to: RE: SE amp noise of 200mV posted by 6dj8triode@gmail.com on September 08, 2024 at 22:20:25

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.

Follow Ups:

RE: SE amp noise of 200mV - 6dj8triode@gmail.com 20:06:18 09/09/24 (4)
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
- RE: SE amp noise of 200mV - VoltSecond 14:17:02 09/10/24 (2)
  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.
  - RE: SE amp noise of 200mV - 6dj8triode@gmail.com 07:02:18 09/11/24 (1)
    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
    - Woo Hoo! - VoltSecond 12:49:51 09/11/24 (0)
      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.
- RE: SE amp noise of 200mV - Tre' 09:19:46 09/10/24 (0)
  "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"

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