OTL Asylum

Study of a circlotron with cathode and current feedback

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cellailuca

In simulation world this monoblock with only two 6SN7 and four 6AS7 looks very good.
It delivers about 1A in a 8 Ohm load with 1V input.
It only needs some more LC filtering of the 150V lines.

Omnes feriunt, ultima necat.

Follow Ups:

Nice circuit! - Ralph 08:38:42 10/16/24 (7)
Have you tried Voltage feedback?

The driver tubes could have their cathodes tied together to improve CMRR, using a CCS to ground.

The Voltage feedback then coming from the output terminals to the grids of the input tube (which IMO/IME is the right way to apply feedback in any tube amp).

Since most speakers are designed to be driven by a Voltage source, this might make for a more practical application.
- Output impedance is 2.8 Ohm - cellailuca 09:08:02 10/16/24 (6)
  If I replace the speaker with a 330 Hz-1V voltage generator the current is 350 mA, that is, 2.8 Ohm output impedance.
  It is quite an acceptable value for all commercial speakers.
  
  Omnes feriunt, ultima necat.
  - It's actually lower, it is 1.8 Ohm - cellailuca 09:15:13 10/16/24 (5)
    As you can see in the LTSpice screenshot I set 1 Ohm of series resistance in the 1V voltage generator.
    
    Omnes feriunt, ultima necat.
    - Current feedback usually raises output impedance FWIW nt - Ralph 09:19:01 10/16/24 (4)
      -
      - RE: Current feedback usually raises output impedance FWIW nt - cellailuca 09:03:53 10/17/24 (3)
        
        In Reply to: RE: Current feedback usually raises output impedance FWIW nt posted by Ralph on October 16, 2024 at 09:19:01
        
        I agree with you but in this case the effect of cathode feedback on output impedance is the strongest.
        
        Omnes feriunt, ultima necat.
        
        In that case I would surmise its actually Voltage feedback nt - Ralph 09:11:59 10/17/24 (2)
        
        In Reply to: RE: Current feedback usually raises output impedance FWIW nt posted by cellailuca on October 17, 2024 at 09:03:53
        
        -
        
        RE: In that case I would surmise its actually Voltage feedback nt - cellailuca 03:12:43 10/18/24 (1)
        
        In Reply to: RE: In that case I would surmise its actually Voltage feedback nt posted by Ralph on October 17, 2024 at 09:11:59
        
        Correct, because cathode feedback can not be eliminated without destroying circuit topology and current feedback is only 5 dB.
        For sake of completeness the input impedance with and without current feedback is 83 Ohm and 104 Ohm respectively.
        A very low value for tube standards.
        Thank you for your comments.
        
        Omnes feriunt, ultima necat.
        
        +1 good work! - Ralph 11:48:12 10/18/24 (0)
        
        In Reply to: RE: In that case I would surmise its actually Voltage feedback nt posted by cellailuca on October 18, 2024 at 03:12:43
        
        -
Output spectrum - cellailuca 01:27:21 10/14/24 (0)
This is the calculated output current spectrum @1A peak/8 Ohm.
Never seen a similar result in my previous simulations.

Omnes feriunt, ultima necat.
Better schematic - cellailuca 10:07:00 10/12/24 (1)
I hope...
Omnes feriunt, ultima necat.
- Detail - cellailuca 10:16:55 10/12/24 (0)
  Detail
  
  Omnes feriunt, ultima necat.

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