EF86 - 6550/KT88
Symmetrical Feedback Amplifier

In this circuit the input pentodes run in a sort of triode mode. The screen is not connected to the plate but the screen voltage will follow the plate voltage due to feedback from the primary side of the output transformers. If the fraction fed back is equal to 1/A, where A is the amplification of the output stage, then the screen voltage will follow the plate, just as in triode operation.

So, when extending the screen-anode loop to include the output stage there will be local feedback, in this case about 4:1 or 12dB which will reduce the output resistance of the output tubes by a factor of rp/(1+(B*mu)).
This indicates that the global feedback could be kept low: 4-8dB gNFB will make a decent output stage for most speakers. The global feedback is symmetrical fed from each of the two secondary windings.

Symmetrical Feedback amplifier
Symmetrical Feedback amplifier

This project is in progres..
Symmetrical Feedback amplifier

04-April-08
Chassis ready for painting, power supply is separate:
Symmetrical Feedback amplifier

  • Transformers by Sowter UK
  • Tubes and other stuff from Demostenes NO, ELFA NO, Partsconnexion CA
  • Overall design by: Jan Erling Veiset.
    (c) Jan Erling Veiset, 10-JAN-2008 - E-mail: valhalla[at]mikke-mus.com
    Akersora 2
    N-6600 Sunndalsora
    Norway