Here is the math:

http://www.drtube.com/datasheets/kt88-mov74.pdf

Gm@g2=250V = 11,5 mA/V

rp = 12k

mu = Gm*rp = 138

I use 5k primary on a SE transformer so

Av = mu*Rl/(Rl+rp) = 138*5k/(5k+12k)=v 40,6

and the Miller capacitance is Cg1(all less anode) + Cg1a*(Av+1)=

= 16pF+1.2pF*(40,6+1) =

=66pF

so the so 20kHz the R in this internal RC filter should be

R= 1 / 2*pi*f*C

= 1 / 2*pi*20kHz*66pF=

=

**121k**

Where did I go wrong?

Thanks,

Miha