Tube DIY Asylum

Makes perfect sense

24.27.41.140

Triode_Kingdom

In Reply to: RE: 2 reasons posted by Chip647 on October 27, 2024 at 14:40:27

And no question each of the separate chassis has a nice balanced look. I'm surprised the Fender iron would have sufficient bandwidth for hi-fi. Are you using GNFB?

Follow Ups:

RE: Makes perfect sense - Chip647 17:04:01 10/27/24 (5)
It is sort of global, the feedback goes to the cathode of the 6V6GT. It has a bit too much gain though so I am in the process of changing the 6SL7 to a 6SN7 to get the full output to around 1v AC input.
The 6SL7 was being run a little light, will change the 4.5k resistors to suit the 6SN7. Feedback resistor is 5K vs 1.5K and no 220K ground reference resistors to the KT88 Grids.

Edits: 10/27/24
- Feedback alternative - Ralph 13:07:12 11/07/24 (2)
  If you were to invert the phase of the driver transformer or output transformer (not both) you could apply the feedback to the grid of the driver tube rather than its cathode.
  
  When you apply feedback to the cathode of any tube, the tube distorts the feedback while mixing with the incoming signal. This creates additional harmonics and can cause IMD as well.
  
  If you mix the feedback passively at the grid of the tube this doesn't happen since the resistor divider network is far more linear than the best tube around.
  - RE: Feedback alternative - Triode_Kingdom 14:55:41 11/08/24 (1)
    I did this in the preamp I'm building. Output is a cathode follower, feedback from the cathode to the grid of the preamp tube ahead of it. I did this mostly for purposes of gain reduction, as both stages are very low distortion as-is.
    - +1 nt - Ralph 11:37:52 11/11/24 (0)
      -
- GNFB - Triode_Kingdom 21:59:56 10/27/24 (1)
  The reason I asked about feedback is that GNFB creates a more critical situation for the IT and OPT performance. High frequency phase shift through those components will be cumulative, possibly creating problems stabilizing the amp. This is a common problem when builders attempt to use OPTs from products like Hammond organs to build hi-fi amps. The step network and other frequency-limiting techniques needed to stabilize the amp cut into the upper audio bandwidth because the transformers exhibit excessive phase shift. It's almost a non-issue though if the inherent distortion (and damping) of triode-mode outputs is sufficient to dispense with feedback.
  - good points - Chip647 07:49:11 10/28/24 (0)
    I used very little feedback but it wasn't quite right. I removed the feedback and it does sound better!.
    These are not the run of the mill bandwidth limited guitar amp transformers. I have a pair of big ElectroPrint OPTs sitting on a shelf if they become tiresome, though.
    I am running another PP amp with trioded 6V6GT with an Electraprint phase splitting interstage with no feedback for that reason. I tried it in tetrode without feedback and discovered very quickly how terrible that sounds!
    Thanks for your input.
    
    Edits: 10/28/24

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