Tube DIY Asylum

You might consider an EF86 in ultra linear mode

75.168.141.235

Ralph

M

In Reply to: RE: feedback applied to grid posted by Triode_Kingdom on December 27, 2024 at 14:36:43

UL for a pentode like the EF86 is done with a divider network for the screen grid between the plate of the tube and ground. So the screen thus is providing feedback.

This technique can be done with a couple of triodes in cascode too. But the more gain the better as far is the feedback is concerned- more linear, lower output impedance. No idea if a 6EB6 or similar would do the job but it might be fin to try it out.

Follow Ups:

RE: You might consider an EF86 in ultra linear mode - Triode_Kingdom 23:30:45 01/01/25 (1)
I believe nested feedback that includes both stages is key to overall performance. I also suspect the best approach won't be the use of two grids, although I do intend to try the pentagrids when time permits. It just appears likely to me that in the end, the most effective method will see the two signals applied to the exact same node (grid). This seems to be supported by SPICE, as well as the circuit I'm using now. 1kHz distortion is under .002% at the medium output levels I would use most of the time.
- One more comment... - Triode_Kingdom 09:55:05 01/04/25 (0)
  Ralph, I completed the difficult switched input wiring in my preamp project yesterday. Still need to finish a few cosmetic items, but it's in my system today, driving a Citation V. I understand now why you're so adamant about this method. Compared to the SS backup preamp I've been using (which is no slouch), the sound of this unit is remarkable. In fact, it's so good, I believe for the first time ever that my system is limited only by the speakers.
  
  I'm thinking now about building a few separate line stages using this technique. Perhaps grid feedback could be used at the input to the phase inverter in a power amp, in order to avoid issues regarding input isolation.
  
  Many thanks for bringing this matter to the forefront. I can confirm that it has an audibly significant advantage over traditional topology.

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