Tubes Asylum

I have biased tubes in the past above true ground but. .

104.28.85.159

John PA

A

what is a basic formula for this? It has been a long time and I don't remember the values I used for this. I want to bias a filament 20 volts above true ground. This is DC. The filament DC voltage is 1.25 volts, a miniature tube with a current draw of .01 amp for the filament. Or, can it be done for this tube?
Thank you.

iBasso DX300MAX Ti. Focal Utopia and Stellia. iBasso SR2. Mr. Speakers, Ether II, Voce stats. Manley, Absolute headphone amp. LTA MZ3, Z10e electrostatic amp. Many other headphones, amps, cables etc.

Edits: 10/22/24 10/22/24 10/22/24 10/22/24 10/22/24

Follow Ups:

RE: I have biased tubes in the past above true ground but. . - Tre' 07:42:26 10/26/24 (3)
Do not otherwise ground the DC filament supply. It will be grounded by this resistor network.

The combined resistor values of R2, P1 and R3 (in series) is in parallel with the filament so that will be an extra load on the filament supply.

Make R5 whatever value it needs to be, vs. the cathode current, to give 20 volts of cathode bias for the tube.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
- RE: I have biased tubes in the past above true ground but. . - PakProtector 08:39:46 10/29/24 (2)
  I think he is wanting to use filament bias. So to say, applying .01A at 20V and use a smaller cathode resistor. 20V/.01A=2000Ohm biasing resistance. Instead of something huge to give that same voltage but with only cathode current through it( and thus likely requiring a bypass cap ).
  
  And a 2kOhm cathode R would likely find it useful to bypass in any case... :)
  
  Douglas
  
  Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
  - I wondered about that. That's a different conversation. - Tre' 12:08:01 10/30/24 (1)
    What tube operates on 1.25volts filament at only 10ma.? That seems insanely low.
    
    Tre'
    Have Fun and Enjoy the Music
    "Still Working the Problem"
    - beats me - PakProtector 02:30:34 11/02/24 (0)
      seems low, so I imagine that plate current would have to be small too...
      
      Douglas
      
      Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
what tube is that? nt - Tre' 20:55:28 10/25/24 (0)
.
Have Fun and Enjoy the Music
"Still Working the Problem"
RE: I have biased tubes in the past above true ground but. . - Chip647 15:23:45 10/25/24 (0)
I assume you are connecting pin 2 to pin 1 to make a DHT? Your tube bias voltage is going to be measured as the difference between Pin 3 and Pin 4. I don't know what tube this is, but the bias of these types of tubes is generally done from a grounded grid. If you "bias up" the filament to 20 volts over ground, you will have to adjust the grid voltage up to get the desired operating point.
You might have better luck in Tube/DIY -nt - E-Stat 08:29:47 10/25/24 (0)

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