Adding NFB to an 18W

[Mike Sulzer]

So that says that the tail should be effectively very long. With the feedback in place, the input voltage to the left grid for full power output is higher than with no feedback. Then the voltage across the tail resistor changes more than in the no feedback case. The asymmetry comes about because one tube swings positive with a higher current than the other (and thus does not get as far positive), and vice versa for negative swing. So bootstrapping is the right way to go.

Edit: Also, I guess you could use the bootstrapping with the resistor divider even if you do not want to use the feedback to the grid, as long as you very carefully adjust the resistor ratio. You would be working with the open loop gain.

I'm guessing that the bootstrapped circuit has better common mode rejection because the bootstrapping keeps the tail current more constant. When I replaced the tail resistor with a constant current sink both circuits were equally well balanced.

[robrob]

Silvio55, Since you don't know how much NFB resistance you need I would start with a 100K pot wired as a variable resistor using either pot outside and wiper terminal. Set it to max resistance before powering up. On first power up you have a 50% chance of positive feedback screeching. If that happens you will need to swap the output transformer's two primary OR secondary wires to get negative feedback.

Play around with the pot to find the NFB level you like, then measure the resistance of the pot and replace the pot with that value resistor.

[Jazz P Bass]

Here is a Marshall Artist 2040.

NFB loop included.

Artist 2040.pdf