I just calculated the value for the KT88(tetrode connected) grid stopper for 20kHz roll off, and it seems very high (usually its from 5k-10k in guitar amps).
Here is the math:
http://www.drtube.com/datasheets/kt88-mov74.pdf
[email protected]=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
Here is the math:
http://www.drtube.com/datasheets/kt88-mov74.pdf
[email protected]=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