Wanted to share this circuit I’m working on. Cascode output is somewhat unusual and it’s an easy way to get into A2. I just finished the breadboarding and testing/tweaking. I spent a few days designing PCBs and I’ve just ordered some. I’ll post some updates here once I get the PCBs populated. Then I’ll be designing the chassis. Let me know what you think.
Attachments
Wow!
I need to get somebody local who can do the building part of my projects, and make them look as good as your amplifier.
I am interested in how well your Cascode circuit comes out.
Generally, cascode circuits cause the top device's output impedance to increase (in this case, the EL34's Triode Wired plate impedance, rp, will become high impedance).
But with the two negative feedback loops, at least that will cause the EL34's plate impedance to be reduced again.
The question is, with the cascode circuit plus the negative feedback . . . Will the triode wired EL34's plate impedance end up higher than, or end up lower than, the approximate plate impedance of 1,250 Ohms that you get with a non-cascode triode wired EL34 with no other negative feedback.
I need to get somebody local who can do the building part of my projects, and make them look as good as your amplifier.
I am interested in how well your Cascode circuit comes out.
Generally, cascode circuits cause the top device's output impedance to increase (in this case, the EL34's Triode Wired plate impedance, rp, will become high impedance).
But with the two negative feedback loops, at least that will cause the EL34's plate impedance to be reduced again.
The question is, with the cascode circuit plus the negative feedback . . . Will the triode wired EL34's plate impedance end up higher than, or end up lower than, the approximate plate impedance of 1,250 Ohms that you get with a non-cascode triode wired EL34 with no other negative feedback.
All true, the cascode takes the already very large Z of the MOSFET drain and multiplies that by the gain of the output tube then adds that to the plate Z. So you do end up with a naturally pentode like Z driving the output transformer. Luckily the MOSFET also has gain that can then be used to add a bit of feedback to allow the plate to respond to the voltage drop that would typically result from a non infinite output transformer inductance.
How it exactly compares to a simple triode plate Z I’m not sure but it’s in the ballpark. How can I tell? Because the measured output Z at the output transformer secondary is in a very typical range.
