Posted by Thorsten on April 03, 2000 at 13:22:25:

In Reply to: Thanks! was: The Sound of Cathode Followers.... posted by Paul Joppa on April 03, 2000 at 12:51:10:

Hi all,

Now, I can't upload pictures and at that time of the night I sure ain't gonna ASCII Draw this circuit, but no Paul, it's a little more complex. The Circuits is like this:

Starting from Bottom to Top (The Bottom Up approach):

At the bottom is a 5842/417A or similar, operated with unbypassed Cathode Resistor and some Positive Feedback from the whole output of the Circuit to Cathode (Remember negative Feedback = bad; Positive Feedback = good) usually only 2, perhaps 3db PF though.

The Anode Leads to the Grid of the first 5687 and also to either our Choke or to the Cathode of our J-Fet CCS (2SK147 works great BTW). The Choke is for Purists, sounds slightly more natural, the FET has a bit more Detail and a taughter bass (more like being hit by a hardwood 2X4 rather than a Spruce one), overall, I like both.

Anyway, either we have a Resistor in series with our Audio-Choke to set the Current of the 5687 or we have a solid State CCS taking care of it, both work well. In either case the free end connects to the Cathode and no, C4S need not apply, FET's please.

The output is indeed taken from the Cathode of this 5687, the whole stack runs at around 160V per Valve and around 12 - 15mA (sounds better than higher currents, tried it).

So far so good and so boring and done so often. It sure sounds decent, but there is one more step to go. We still have another 5687 Halve looking for a Job. And here is waht we make it do:

We CASCODE it with the other 5687, using a apir of 1M Resistors and couple one of the Resistors between +B and the Grid of the second 5687, the other between Grid of the second 5687 and Cathode of the First. This Resistor is also bypassed by any decent old Foil Film or Oit Cap, 0.1uF or larger.

Now we need around 500V +B for the Whole stack, the Current through our first 5687 is being kept pretty constant for AC by our Inductor, the Inductor is "amplified" by the Mu of the 5687 (around 17), so the 5842 will see an anode Load Inductor of around 500H, well out of the Way even with an unbypassed Cathode Resistor.

The Cathode Impedance of the 5687 (<< 200 Ohm) Sees a 30H Choke in parallel with the load, again, well outside of any area of concern. Best of all, the Modulation of the Anode/Cathode Voltage of the upper Valve is also being significantly reduced, instead of being equal to the Output Voltage, we now have it equal to around 0.06 of the Output Volatage, significantly reducing the remaining non-linearities.

BTW, the Problem Allen Wright had in the original Version of this Circuit with the ECC88 was the Capacitance of the Hexfet (several nF) loading the Cathode Follower and (obviously) being severely modulated by the Signal, hence the unpleasant agressive sound.

Anyway, I hope this little gem Helps.... It's a brilliant Driverstage for a 300B. It's almost as good as a simple EL84 operated as Penthode, but it has more ooompf than the EL84 truth be told. The Super-Mu Stage will drive any Grid not only into Class A2, but probably into Class A3, if there was such a thing.... ;-) The EL84 is good for Class A1 only.

Later Thorsten