Gain stage with g2-feedback.

[jane]

A common way to do a gain stage is to combine a common cathode circuit with a cathode follower. The common cathode block may be a triode strapped pentode. Something like this:




Since the cathode of the CF is close to the plate voltage of the pentode it is temping to connect the screen grid to the CF’s cathode. This will result in feedback from the CF to the input stage.




This will at least do two things: reduce the output impedance and linearize the CF.

Here are some measurements of the two circuits. Blue curve is without and red curve with g2-feedback. The output impedance is 690 ohm for the plain CF and 280 ohm with g2-feedback.
Most of the distortion belongs to the common cathode block, so it might be worthwhile to play with the working point for that stage.




Jan E Veiset

[Tubes4e4]

Nice idea and interesting results, Jan!

Would be even nicer with some empirical additional data about the change in stage amplification factor, although this probably could be computed backwards from the changes in Zout and THD, together with the coefficients of the tube being used in the CF position, and whatelse.

So hey, please also add the A figures, if the circuit still is on the bench and it doesn´t take you too much effort to measure four AC peak-peak voltages

Regards,

Tom

[jane]

The gain is quite identical in the two circuits, in the first circuit the feedback is local from plate to g2 and in the other the feedback loop is extended to include the CF. With the same plate resistor (56k) A=11 when 6SJ7 runs as a triode and A=10.5 with the extended feedback loop.
I'm not sure why the gain is a little bit lower in the second case. Since the gain of the CF is less than unity there will be less feedback to g2 versus triode mode and that should have given me more gain, not less.

Jan E Veiset

[Shoog]

I was thinking of trying this with my FVP5 clone which has a super linear cathode follower second stage. I got into discussion with Joe Rasmussen about it and he was strongly of the opinion that the Common Cathode stage as a triode with a high R load it would sound better than a UL configuration.
He put me off trying the idea, but I may just give it a go when I do my next major rebuild.

One thing that might be an issue is that having the anode and the screen at the same voltage will increase the screen current which puts an additional none linear load on the common cathode. I thought about using a resistive voltage divider to avoid this and reduce the screen current to insignificant levels.

Shoog

[jane]

Originally posted by jane
I'm not sure why the gain is a little bit lower in the second case. Since the gain of the CF is less than unity there will be less feedback to g2 versus triode mode and that should have given me more gain, not less.

I think it finally sunk in. When the screen is not connected to the anode I will lose some transconductance and the gain will decrease.

Jan E

[korneluk]

Hei Jan,

I visited your website - what program do you use to draw those professional-looking schematics? They look publication ready!

Takk,

-- josé k.

[Gordy]

Originally posted by korneluk
Hei Jan,

I visited your website - what program do you use to draw those professional-looking schematics? They look publication ready!

Takk,

-- josé k.

Yes, my comments exactly! What program?

Good work Jan.

[jane]

Originally posted by Gordy
Yes, my comments exactly! What program?

MS Visio...

Jan E.

[Gordy]

Thank you.

[pedroskova]

Hi Jan,

I see one flaw in this whole scheme...you haven't come up with a fancy name. Please try to correct this.

...by taking the cathode feed from the junction of two resistors, you could control the amount of feedback that the screen sees - kind of like using different taps in a UL output.