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

---

[ Follow Ups ] [ Post Followup ] Thread: [ Display  All  Email ] [ Tube DIY Asylum ] [ Return to Search Results ]

---

[ Alert Moderator ]

---

Grounded cathode / no grid bias 6DJ8 ?! Schematic attached

Posted by Jim Doyle (A) on March 27, 2011 at 09:43:15

These are excerpts from a famous, well-spoken of product that shall remain unnamed, but it's a great case study in WTF.

In both the phono stage and the line stage, the 6DJ8 is run with grounded
cathode and zero grid bias. I'd instantly expect runaway oscillation as the ECC88 simple loves to scream in delight... In fact, in a post-factory service bulletin, the manufacture recommends a 3 pF bypass from plate to
grid of the phono stage to eliminate chirping. For the line stage, a
grid stopper resistor seemingly was put in place for the original design.

What's baffling about this is running the 6DJ8's in a condition where the
grid could exceed 0V. Perhaps this is why 4M grid leak is used even when the Amperex datasheets for this tube recommend maximum 1M values. While this might be done in the MM input where the signals are so low, it's probably a problem in the line stage.

It would seem this preamp would benefit from some very easy, non-invasive mods. For one, adding LED cathode bias... Reducing grid leaks... Getting rid of 3pF HF feedback networks and direct coupling the grid to the phono cartridge. :) For the line stage, the gain of a 6DJ8 seems impractically huge - and another tube might sound better - perhaps a 6CG7 properly cathode biased and maybe a CCS plate load.

It's too bad Allen Wright has passed on... This would be the kind of
think he'd chime in on.


-- Jim

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

VH Audio

---

[ 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 ]

---