Mikael Abdellah's KT88, Loadlines

[soundbadger]

can anyone help me,
i am trying to work out this scehmatic:
diyAudio

ive heard people in this forum saying that the kt88s are operating at 75mA but when i draw the load line for a HT voltage of 400v and a primary impedance of 5Kohms (like on the schematics) the anode voltage at 75mA would only be about 60v,
can anyone help me
cheers

 

[Svein_B]

The DC conditions define your operating point.

The load impedance define your dynamic (AC) loadline, centered on the operating point.

SveinB.

 

[soundbadger]

so should the operating point be in the middle of this ac loadline, because if 75mA is the operating current then it is not anywhere near the center of the line.

also could you explain what you mean by DC and AC conditions, how should i go about calculating my operating point and current when i know my opt primary impedance and my HT voltage.

i thought i understood this, but working through this amp i dont seem to get the same figures as everyone else

thanks

ed

 

[soundbadger]

after a bit more reading of morgan jones book i think i have it.

if i do ht over rl 400/5000 i get 80mA

so if i wanted a cathode bias voltage of 37.5v i would do 37.5/0.080 and get about 470 ohm

or about 440ohm for a bias voltage of 35v


could anyone how has built this amp let me know if this sound right

cheers

ed

 

[porkchop61]

I have some voltage charts for this amp on my web page.
I have them for kt88's as well as el34's, running in all three modes (UL, Triode, Pentode). This might help you see where this amp operates.
Glenn

KT88 SE full schematic w/voltages

 

[arold19]

It is better with 470 or 560 Ohm cathode resistor??? Maybe 500 Ohm is ¨perfect¨?

 

[arold19]

Why there a 100 Ohm resistor on the UL transformer tap on the (porkchop61) schematic and there no resistor on the Mikael schematic??
Also, can enyone answer me about the cathode risistor?

Thank.

 

[porkchop61]

The 100 ohm resistor is common to prevent parasitic oscillations.