Hi All – First time poster here. So, with my COVID-19 down time, I am trying to learn the inner workings of guitar amps … being able to read schematics, layouts, and understand what different components do and how they interact. I’ve read through Blencowe’s and Megantz’s books – they’ve helped quite a bit (and on the topic to follow I’ve read through posts from @joe p @tubeswell @jbfumo @PRR); however, I learn best by doing and getting feedback, so here we go (again) 😊.
So I’d like to design a clean/high-headroom push-pull bass amp from the ground up, with a cascode input stage, DC-coupled cathode follower driven tone stacks, LTPI and either tube or SS rectifier … let’s say E88CC > 12AU7 > 12AU7 > ECF82 > 2x KT77 with GZ34 rectifier tube. I’ve scratched down a few block diagrams – I want to start sorting out the sections by starting out with the cascode input stage. See attached schematic … assume B+ ≈ 300V, gm E88CC ≈ 6mA/V, target Vg2 = 45V, center bias @ -1V, quiescent current @ center bias = 4.5mA, load resistor = 15K, gain factor = 0.006*15K = 90, cathode bias resistor = 1V/4.5mA = 220 ohms.
1. If I want 45V on the upper grid, I suss the calculation for the voltage divider for the fixed bias version. For the grid-leak bias version, how do I calculate the upper grid voltage?
2. Am I calculating gain correctly for this stage, that is transductance * load?
3. Is gm E88CC ≈ 6mA/V an appropriate value, and is 300V about the right B+ voltage?
4. Further on, if I have two 12AU7’s in DC-CCF configuration, the first driving the mids-tone stack and the second driving the treble-bass tone stack, is this too much since I have another gain stage in the phase inverter?
I’d appreciate feedback confirming I have the layouts correct, and that my assumptions / numbers / component values aren’t completely out of whack … then more to follow. Cheers (and very wet behind the ears), Peter
Topic: Cascode input section for bass amp (Read 4184 times)