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previous: kg For those who are interested in add... -- 10/1/1999 6:45 PM View Thread

Re: More info on cascodes and diff stages

10/1/1999 11:52 PM
Randall Aiken
Re: More info on cascodes and diff stages
For what it is worth, I am a big fan of the tube cascode configuration in my guitar amp circuits. The cascode has gain equal to or higher than a pentode, with less noise, and, most importantly, less microphonics than a typical EF86.  
The output impedance of a cascode, for all practical purposes, is equal to the plate resistor of the upper stage. This is no worse than a standard pentode circuit output impedance (actually, it is a bit lower because the pentode usually has higher internal plate resistance), and pentodes are commonly used in guitar amp designs. However, the impedance depends greatly on which tube you use and how much gain you want out of the circuit.  
The typical gain you can get with an EF86 pentode is around 150 or so, with a plate reistor of 220K, giving an effective output impedance of 220K. A 12AX7 in cascode will give a gain of around 270, with a plate resistor and output impedance of 220K, while a 6DJ8 (designed for cascode use) will give a gain of around 210 with a plate resistor and output impedance of 100K. A 6N1P is also suitable for cascode use with results similar to the 6DJ8. By contrast, these gains are higher than a pentode, but lower than two cascaded triode stages.  
The overdriven harmonic content of the cascode is similar to a pentode, but with a higher percentage of even-order (mainly 2nd) harmonics. It sounds better, to my ears.  
The important thing to watch out for in a cascode is to not use the "standard" fixed-bias cascode, which biases the grid of the upper tube with a bypassed voltage divider from the B+. This adds a supply voltage dependency to the gain stage, and, in addition, doesn't sound very "tubey". The better approach is to use a self-bias cascode (similar to an SRPP circuit, but with the output taken off the junction of the plate of the upper tube and a plate resistor), which sounds much fuller.  
The other thing to be careful of is to not exceed the heater to cathode voltage rating of the upper tube section, which is typically 100V for a 12AX7. The 6DJ8 is better in this respect, being rated at 150V for the second section of the tube, as it is designed specifically for cascode use.  
Randall Aiken

Ken Gilbert Thanks Randall.. especially for the... -- 10/3/1999 12:32 AM