Teemumm,
The graph you showed in your post #8 is certainly not normal, it does in fact show a sharp tendency toward l.f. instability. There seems to be several poles all around 3 Hz. Apart from post #10 by DF96 the decoupling/power supply capacitors C4, C5, C12, C15 will also enter the picture at sub-10 Hz. Their values are too low for the high NFB of the 5-20; these days there is no reason not to increase those to some 32 - 50µF each. (Unfortunately my Spice program is down at present so I cannot investigate so as to give practical advice.)
A general design procedure is to have one pole high enough so as to diminish loop gain to near unity before others cause significant phase shift - this of course with due regard to having a maintained loop gain down to about 30 - 40Hz. I normally use C2 for this purpose, as it also has 'returning-to-zero' phase shift at very low frequency, after it has done its job.
The separate cathode capacitors C13, C14 can in fact be significantly increased to some 2200µF each. This gives a semblance of fixed bias under music conditions with decrease in EL34 distortion at high output. (Such values being high enough not to appreciably charge/discharge under the mark-space ratio of normal music.) Further as DF96 said.
The graph you showed in your post #8 is certainly not normal, it does in fact show a sharp tendency toward l.f. instability. There seems to be several poles all around 3 Hz. Apart from post #10 by DF96 the decoupling/power supply capacitors C4, C5, C12, C15 will also enter the picture at sub-10 Hz. Their values are too low for the high NFB of the 5-20; these days there is no reason not to increase those to some 32 - 50µF each. (Unfortunately my Spice program is down at present so I cannot investigate so as to give practical advice.)
A general design procedure is to have one pole high enough so as to diminish loop gain to near unity before others cause significant phase shift - this of course with due regard to having a maintained loop gain down to about 30 - 40Hz. I normally use C2 for this purpose, as it also has 'returning-to-zero' phase shift at very low frequency, after it has done its job.
The separate cathode capacitors C13, C14 can in fact be significantly increased to some 2200µF each. This gives a semblance of fixed bias under music conditions with decrease in EL34 distortion at high output. (Such values being high enough not to appreciably charge/discharge under the mark-space ratio of normal music.) Further as DF96 said.
It would be nice if you also provide tubelibrary.inc and 5ar4.inc
The sim is not working without deze files
Next time it maybe better to place al the files in a zip file
Thanks
The tubelib is attached. 5ar4.inc is not actually needed because the retifier tube is not in the schematics.
View attachment tubelibrary.inc.zip
I think I have to try to do some simulations and see what is the open loop frequency response of the amplifier to better understand what is happening here.
Do you think there would be really need to increase C4 and C5 from 8u? The current drawn by EF86 and ECC83 is quite small. Another thing I have been thinking is that I'll replace C12, C15 and choke with just one 50uF capacitor. Would there be some benefit using choke supply instead of capacitor with this amplifier? I think 50-60uF would be max value for GZ34. Of course replacing choke with capacitor would mean another mains transformer, but this is not a problem.
Yes but i do have to delete the 5ar4.inc from the ASC file to simulate
Thanks
If you change C4 you also have to make C3 the same size
U3 U4 is a part of your fase inverter.
So both tubes U1 U2 must be driven the same way to each other.
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