Cathodyne biasing

[lowell]

I have a cathodyne in my amp and I've set it up per Merlin's book and am using a 12au7. It's being fed from my screen supply at 460vdc. I have a .01 coupling cap, 1 meg grid stopper, 100k grid leak, 1k bias resistor, 56k cathode resistor and 56k plate resistor. The voltage on the grid reads some 50v off. Shouldn't the grid be only -2 to -4 to the cathode voltage? Or should this be what the voltage reading is at the BOTTOM of the grid leak resistor?

[ThermionicScott]

I have a cathodyne in my amp and I've set it up per Merlin's book and am using a 12au7. It's being fed from my screen supply at 460vdc. I have a .01 coupling cap, 1 meg grid stopper, 100k grid leak, 1k bias resistor, 56k cathode resistor and 56k plate resistor. The voltage on the grid reads some 50v off. Shouldn't the grid be only -2 to -4 to the cathode voltage? Or should this be what the voltage reading is at the BOTTOM of the grid leak resistor?

Had to edit out my previous response after rereading Merlin's page... the cathode should be a couple volts over whatever the grid reads.

- Scott

[Merlinb]

The voltage on the grid reads some 50v off. ?

That's probably because you're reading the voltage between grid and ground and your multimeter has a lower impedance than the grid, so it drags the grid voltage down and gives a false reading. Same thing happens on an LTP. Measure the voltage between grid and cathode instead.

[lowell]

oh ok I'll try it... thanks!

[lowell]

yep right on. I also realized through this that my bias is way off. My supply is so large that I need a Rb of 10k to get it center biased with the grid at -14v. Thanks Merlin.