Tube DIY Asylum

RE: fixed bias ????????????

76.90.100.230

Tre'

P

In Reply to: RE: fixed bias ???????????? posted by xaudiomanx on October 08, 2023 at 18:47:17

Those are the grid resistors and they should not be going directly to pin 5 of the output tubes. There should be a 1k grid stop resistor connected to the grid and the 470k grid resistor go from the other end of the grid stop resistor to the wiper of the bias pot. Just like the 270k grid resistors do on the st70.
Can you post a schematic?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

Follow Ups:

RE: fixed bias ???????????? - xaudiomanx 07:32:56 10/09/23 (5)
So it goes from the wiper through a 470K and ties into pin 6 which has a 1K resistor on it going to pin 5. Pin six is used as a soldering connection.
- RE: fixed bias ???????????? - Tre' 09:41:16 10/09/23 (4)
  That makes perfect sense but I still wonder why such a high value?
  For fixed bias the data sheet says 50K max value for the grid resistor on a 6550.
  Tre'
  Have Fun and Enjoy the Music
  "Still Working the Problem"
  
  Edits: 10/09/23
  - RE: fixed bias ???????????? - xaudiomanx 09:59:10 10/09/23 (3)
    Tre'
    
    Does it matter that I am using a 100K pot?
    - RE: fixed bias ???????????? - Tre' 17:33:05 10/09/23 (2)
      If there is a cap at the wiper then the tubes sees 470k as the grid resistor. If there isn't a cap at the wiper then the tubes sees 570k (or 470k plus the portion of the pot depending on where it's set) as the grid resistor.
      
      Tre'
      Have Fun and Enjoy the Music
      "Still Working the Problem"
      - RE: fixed bias ???????????? - xaudiomanx 08:57:02 10/10/23 (0)
        
        In Reply to: RE: fixed bias ???????????? posted by Tre' on October 09, 2023 at 17:33:05
        
        I did try emailing you personally but I guess you changed your email.
      - RE: fixed bias ???????????? - xaudiomanx 08:56:20 10/10/23 (0)
        
        In Reply to: RE: fixed bias ???????????? posted by Tre' on October 09, 2023 at 17:33:05
        
        Tre'
        
        Should I put a cap on the wiper? I'll leave that up to you. Whatever is the best way to go! My friend who is helping me with this said it's not necessary. But since I say a 50UF cap on the Dynaco schematic I got a little skeptical about his decision. He is a great tech but there are so many of you out there all with different opinions.
        
        I asked my friend who is helping me with this project about such a high grid resistor and he said and he said that it is 470K paralleled with the adjoining tube making it half(235K). There is 4 tubes per channel.
        
        Parallel P/P
        
        These are technically Dynaco M125's but with a pot for each tube and a different front end designed by my friend.
RE: fixed bias ???????????? - xaudiomanx 07:23:55 10/09/23 (0)
Hi Tre,

It is a Dynaco MK III schematic.

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