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

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Eli Duttman

In Reply to: RE: Grounded cathode / no grid bias 6DJ8 ?! Schematic attached posted by Jim Doyle on March 28, 2011 at 16:49:49

Jim,

I did not closely at the circuit you posted. If you "smell" overpriced, it probably is. The tweaked RCA circuit I posted employs cathode bias in the 1st gain block and grid leak bias in the 2nd gain block, specifically as a load lightener of the passive EQ network. I also put stopper resistors on both triodes, even though they're low gm. No prisoners taken here. Several "inmates" have built the tweaked RCA setup and the reported result is "highly satisfactory". A possible refinement is CCS loading of the 2nd gain block.

FWIW, the 6SC7 based design GE provided for use with their early mag. carts. uses grid leak bias in both gain blocks and an active RIAA network. That design also relies on the inductance of the cart. for HF roll off.

Eli D.

Follow Ups:

RE: Grounded cathode / no grid bias 6DJ8 ?! Schematic attached - Gingertube 20:07:21 03/29/11 (1)
The problem which can occur with grid leak bias is the necessity for a VERY large Rg1 (grid to 0V) resistor often called (un-surprisingly)the "grid leak" resistor.

This resistor will have high noise contribution which really needs it to be driven from a lowish AC impedance which shunts both resistor noise and tube grid noise back to 0V. The coupling cap is essential to allow both AC low impedance to 0V and DC high resistance from the grid to 0V for the "grid leak" bias to establish itself across.

So whether "grid leak" bias sounds OK (or even good) or is just really noisy is determined by the output impedance of whatever drives it.

Usually (in normal cathode biased stages)tube grid noise will swamp Rg1 resistor thermal noise, but with the high Rg1 values for grid leak bias contributions to noise from the 2 sources will be about equal. The shunting effect of the drive impedance will determine the residual noise level. This is something which High Gain Guitar Preamp designers know all about (and curse profusely).

Cheers,
Ian
- RE: Grounded cathode / no grid bias 6DJ8 ?! Schematic attached - Eli Duttman 17:28:19 03/30/11 (0)
  Ian,
  
  The fundamental physical facts of thermal and quantum shift noise (Nyquist and Johnson) can't be ignored. A large resistance is comparatively noisy. However, how the resistance is fabricated can exacerbate the negative implications of fundamental Physics considerably. That's why, when asked, I state damn the cost and use Caddocks in those 20 MOhm positions.
  
  BTW, that early GE 6SC7 based design used 22 MOhm Carbon comp. parts in both gain block grid leak positions. OUCH!
  
  Eli D.

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