After losing many EL34’s in my K4040 I am going to convert it to run on KT88’s. There are several differences that have to be taken into account:
R97-104 need to be changed to 6.8ohm from 10ohm
G3 and K bias need to be set to 3.6v. EL34 bias for G3,K run at 0.4v
C39,40,41,42 220uF -> 450uF 450v
C37,38 100uF -> 220uF 450v
I am a bit concerned about how to set the screen bias to 3.6v and retain the LED indication for setting this bias. I have been advised to change the values of R5, R10 to 10Kohm from 22Kohm. Changing the 10ohm resistor to a 6.8ohm resistor would adjust the value of the bias voltage anyway.
Can this be confirmed please? I can use a volt meter to adjust the values on the tubes themselves rather than rely on the LED’s if necessary. Another alternative would be to modify the K4040 LED driver circuit to match that of the K8010 LED driver circuit. What is the best solution for this?
I am currently still using a G3/K bias voltage of 0.4v on the JJ KT88’s but I do not want to under run them.
Cheers,
Martin