Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ]

---

[ Alert Moderator ]

---

direct coupled partial feedback

Posted by Perseo (A) on December 6, 2006 at 09:37:07

In Reply to: Re: Partial feedback (T-Rex) with grid choke posted by fatbottle on December 6, 2006 at 06:53:12:

Thanks for the schematic link -- yes, T-Rex has cap coupling.

Partial feedback can be direct coupled, too. Looking backward from the plate of the output tube (e.g., the 300B), you have a feedback resistor. Typical values seem to be between 50k and 120k (frequency rolls off as this resistor value gets higher). The other end of the resistor is attached to the plate of a driver tube and to the grid of the output tube. Voltage drop over this resistor is equal to the output tube's Vp-g. The cathode sits at a voltage potential up above the grid voltage/driver tube plate voltage, which of course give the output tube its bias. In other respects the design is just a direct coupled SE amp.

Gary Pimm's push pull DHP amps, the well documented one using 47s and the later Tabor using 1624s, is direct coupled partial feedback. Broskie had a really excellent article about partial feedback. It is hard to research the subject because "partial feedback" is hardly a well accepted name (nor is "plate-to-grid feedback").

I have a single ended version using 6SL7 as the voltage amplifier and 6EM7 as the output tube. For a feedback resistor I use the unused triode in the 6EM7 (Gary Pimm points out that the quality of the feedback resistor is important).

Steve Root

This post is made possible by the generous support of people like you and our sponsors:

The Cable Cooker

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---