I got a Merren 560V PT that came with no dedicated bias tap. He suggested feeding from the lowest voltage secondary, which is 360VAC. I tried drop resistors up to 1M and I still get very high negative voltage on one end of the trimpot (around -260v). The bias circuit is the 1959 type. Like I said, I changed the original 27K drop resistor before the diode up to 1M to no avail. What can I try next? Thanks guys.