I have used the 6DJ8 and 6922 in the standard Ultra linear cascode. I have found that what tube sounds best is determined by the plate load resistor and the grid leak resistor or volume control. Plate resistors between 10k and 35k work best to my ear with the 6DJ8 family while Plate resistors between 40K and 100k work better with med Mu tubes like the 12AU7,6SN7 and 6FQ7.krash wrote:FWIW, I am a huge fan of this cascode 6DJ8 design, and I am putting it in many of my production amps. I have been meaning to try it with a 12AU7. I think this is an unfortunate neglected gain stage design that has a lot to offer.
Plate loads above 100k work best with High Mu tubes like 12AX7 and the 12AT7.
the Circuit I posted is intended for Hi-Fi applications however with suitable Coupling capacitor and proper selection of the Compensation capacitor the circuit should be great as a Pre amp booster stage in guitar amp applications.
My Cascode circuit goes beyound the Ultralinear cascode by adding more feedback loops. notice in my circuit that the lower voltage divider is returned to the cathode as opposed to ground and when combined with the other end connected to the plate there is no longer any fixed reference point for Bias and thus allows he grid voltage of the upper Triode to more freely follow the waveform of the bottom triode since both ends of the bias voltage divider network is now floating and providing local Negative feed back where in the standard ultra-linear cascode the lower end of the Voltage divider is grounded providing a fixed voltage reference and no feedback from this node. In addition the Grid Leak or volume control resistance is also returned to the cathode and not ground.
Some of the Advantages besides trading gain for distortion is substantially improved Power supply rejection ratio and this is important with a cascode circuit since this topology has rather poor power supply rejection ratio until you go with the Ultra-linear cascode,this is because the bias voltage is sourced from the clean DC on the Plate not the more polluted raw supply rail. By virtue of practically every node of the gain stage under local feedback aging of tubes is less noticeable since as the tube ages and its parameters change the feedback corrects that, i have used an almost completely dead 12au7 to test this and while there was hiss and more noise the tube worked with enough gain so as to not cause an imbalance between two stereo channels with a fresh 12AU7 in the other ch. this tube was totally inoperative in a conventional grounded cathode configuration if any of you try this circuit as a preamp booster i would like your thoughts on the sound vs other cascode topologies...