NEW KT66 - 6AU6 single ended

[JORGETronic]

Hi all,
I found this circuit in an old book and I have all the valves to go, so please I am a newvie and I would like your advice as I want to avoid to destroy the valves.....

So please your opinions are all welcome

[JORGETronic]

Can someome help me to understand the feedback loop on the KT66 (R10) and how it can be calculated

[AndreasS]

Originally posted by JORGETronic
Can someome help me to understand the feedback loop on the KT66 (R10) and how it can be calculated

This is voltage feedback over the output tube (parallel feedback). The calculation is difficult, because it depends of the plate resistance of the driver and output tubes and the ampflification factor of the output stage.

This feedback is not so effective as a voltage feedback over the driver and the output tube; more than 6 dB NFB increases the distorsions (more second harmonics, generated by the driver).

Regards Andreas

[JORGETronic]

many THANKS Andreass, any comment about the circuit, I was doing some calculation over the 6AU6 and as the Ig2 current is dificult to calculate I do not know exactely the max input voltAGE, as I want to attact the amp directly from a cd palyer, lets say 2 volts....

[Miles Prower]

Originally posted by JORGETronic
Can someome help me to understand the feedback loop on the KT66 (R10) and how it can be calculated

That calculation isn't particularly difficult since it's just a voltage divider. The feedback signal appears across the parallel impedance of the r(p) and Rp of the 6AU6, and the grid resistor of the KT66. The actual value isn't particularly critical, and O. Schade recommends a 10 : 1 voltage division.

Given this circuit, I wouldn't change it since the coupling is DC, and changing the 330K resistor is likely to upset the DC operation of the 6AU6 stage.

[JORGETronic]

Thanks for the answer, the book also mention that if you change the KT66 for EL34 yuo must change the resistor to 470K so i was traying to do some paralles with the different rp(KT66) and the rp(EL34) with no succes, someone can tell me if the 6AU6 con handje 2 volts in its imput easely...

[JORGETronic]

I have notice that you daid that the amp is DC coupled, the KT66 is coupled to the 6AU6 via a capaitor of 0.5 uF...no DC coupled right?

[Miles Prower]

Originally posted by JORGETronic
I have notice that you daid that the amp is DC coupled, the KT66 is coupled to the 6AU6 via a capaitor of 0.5 uF...no DC coupled right?

The local feedback from the KT66 to the 6AU6 is DC coupled. The stages are AC coupled.

[ray_moth]

I just modeled the EL34 version, using LTSpice, out of curiosity (I don't have a model for KT66). I used 470k for the NFB resistor, as you said. I also used 3.5k plate load and 190 ohm cathode resistor for the EL34, as per Mullard's recommendations with 300v B+. All other values were as per your schematic.

The amp seems to be designed for 250mv rms input signal for maximum output. 2 volts input signal would be far too high.

[tinitus]

How about this one, instead of some old dusty curcuit - should be possible with KT66

http://www.tubelab.com/SimpleSE.htm