Driver tube for Power Tubes

[XgamerGt03]

So I'm working on a bass amp design for a friend and have been trying to figure out exactly how to go about designing a driver stage for the power tubes after the PI.

So obviously I'll have to have a negative voltage somewhere so that I can set the bias voltage of the power tubes.

I've been looking at the set up of the ampeg SVT and the Hiwatt DR405 to try to figure these things out. I see that the "ground" voltage of the driver is the negative voltage, and they have a voltage divider from the negative voltage to ground that sets the grid voltage of the driver. It is pretty clear to me that you have to use the negative voltage here due to the fact that you need a negative bias voltage.

What I don't really get is how exactly I would analyze a stage like this from a load line perspective. Would I treat the negative voltage as ground and just add its absolute value to the anode voltage?

Thanks guys.

[martin manning]

Yes, it's just the difference between the negative and positive voltages. In the 6146B SVT the working voltage for the load line is supply D to supply E, 220 - (-150) = 270V. The 12BH7 is a cathode follower like any other, and it's grid voltage (set by the bias control) determines its bias point, and therefore the (still negative) voltage at it's cathode, which becomes the bias voltage for the power tubes.

[diagrammatiks]

the use of the driver tube doesn't dictate how the negative voltage and bias are derived.

you can use fixed bias with a driver and not worry about the interaction of the negative bias on the driver.

http://image72.webshots.com/172/5/4/96/ ... gSO_fs.jpg

it'd only be an issue if the driver was dc coupled to the power tubes.

if the driver output was taken from the plate you'd analyze it like any other stage.

if it's taken from the cathode you'd simulate current drive instead of voltage drive.