Effect of cross-coupled "local" FB on plate load of previous stage

[ray_moth]

My only remaining problem with his circuit, now that I understand it better, is the low level of bias on the first stage. I measured only 0.5 v between each 6SL7 cathode and ground. This would not allow a big enough input signal, as I understand it.

 

[ThorstenL]

Konnichiwa,

My only remaining problem with his circuit, now that I understand it better, is the low level of bias on the first stage. I measured only 0.5 v between each 6SL7 cathode and ground. This would not allow a big enough input signal, as I understand it.

Remember, there is local feedback and a negative supply....

Sayonara

PS, trust me, this circuit works as drawn and very well indeed.

 

[ray_moth]

Remember, there is local feedback and a negative supply....

I'm sorry, I don't see how that helps ... unless you're implying that the unbypassed individual resistors in the 6SL7 cathodes allow the cathode potential to "follow" the grid potential, thereby "opening up" the bias, so as to make it effectively much greater than it appears to be? Or does the "long tail" do that anyway? (I'm rather uncertain about this, I must admit.)

 

[ThorstenL]

Konnichiwa,

I'm sorry, I don't see how that helps ... unless you're implying that the unbypassed individual resistors in the 6SL7 cathodes allow the cathode potential to "follow" the grid potential, thereby "opening up" the bias, so as to make it effectively much greater than it appears to be? Or does the "long tail" do that anyway? (I'm rather uncertain about this, I must admit.)

Consider the cathode resistors to the negative supply as imperfect current sources. Consider the cathode coupled through a modertae value resistor value (1...2K in my case). Now consider what happens if the respective valve starts to conduct (both balanced and SE Drive).

Sayonara