help with 2 stage cathode follower

[dimitrij]

Hi everybody

i need some help with this circuit! it's a good build in my opinion, but i have
a couple of questions to which only tube experts as you can answer :
- do i need to adjust the bias if changing the supply voltage (in the range 250-350 VDC) ?
- how can i reduce the voltage gain? now it's supposed to be 8, but 2-4 would be enough for me

thanks a lot!

[Arnulf]

Originally Posted by dimitrij

- do i need to adjust the bias if changing the supply voltage (in the range 250-350 VDC) ?

Given your subsequent remark (about only wanting 2-4x gain) I don't think you're getting anywhere near clipping so I wouldn't bother until you have everything set up. Once that is done, short the input, measure cathode to anode voltage, compare it to 1/2 of B+ minus a couple of volts dropped in cathode of the left half (over R3) and adjust R3 accordingly to bring Va closer to the middle if it's really far away.

- how can i reduce the voltage gain? now it's supposed to be 8, but 2-4 would be enough for me

Either by reducing input signal (= bad way) or by applying negative feedback (= better way of doing it that has useful side effects such as improved linearity), but first of all: remove C3 from the circuit. It increases gain in the circuit.

I would apply NFB back to first stage to linearize it a bit.

[gingertube]

I would redesign it for a different tube or at least to run the 12AU7 at a different operating point. If you read a lot of posts here you will find that the 12AU7 gets a lot a bad comments. That in my opinion is because most of the 12AU7 circuits under discussion run them at far too low a current and is not because the 12AU7 itself is a bad tube. 12AU7 is best with 9 to 10mA idle current. That input stage is running at less than 1mA and the cathode follower at about 5mA.

You can of-course build it as per the schematic and you may be happy with the result. I wouldn't worry about the gain, that is the gain with the volume control at max. You can simply delete capacitor C3 to get less gain (and better linearity).
Cheers,
Ian