Asymmetrical Phase Inverter?

[David Root]

Thanx for posting that link Funky, I just ordered it. I have his two other books already, didn't know about this one until now. I have to read them several times to get it all but it's well worth it!

[gearhead]

Guess what it comes down to is -what- mechanism(s) transpire to make a common-grid amp not invert?

Referring to Aikens http://www.aikenamps.com/LongTailPairDesign.htm

What may be confusing you is that the input to common-grid is not at the grid. It's at the cathode. Under DC conditions, the point where Rg1, Rk, Rtail and Rg2 connects is setting the bias. Cin1 and Cin2 are open under DC. Then under AC conditions you can assume Cin1 and Cin2 to be shorted. So the grid of the right triode is grounded and the preamp signal drives the grid of the left triode (via Cin1). So as the voltage rises on the grid of the left triode, the voltage at cathode also rises (call this node Vk). It's in phase like the output of a cathode follower. The resulting AC signal on node Vk is the input to the common-grid. But now the grid of the common-grid (call this Vg) is grounded. It's 0. So Vg-Vk is more negative if the input is going positive. This makes the plate of the common-grid swing positive (in phase). More negative means less current through the tube, so the plate voltage swings positive. The opposite happens when the input swings negative. Vg-Vk becomes more positive, so more current through the tube, so the plate voltage drops.

It's very confusing. It took a while for it to sink in for me and I had the luxury of having a book to read several times over. This book is really good... http://pentodepress.com/power-amp/contents.html He breaks it down really well and he goes thru the process of deriving all the equations.

Whoa, ok. Think I need to get that book too. BINGO, got it. Thanks for the great rundown.

I knew that the signal entered the NIT at the NIT cathode, but what got me confused was the effects of biasing vs signal swings.

Imagine there is a DC voltage (bias) on the common-cathode, elevating it enough so falling signal swings don't allow the NIT cathode to dip anywhere close to zero (grounded grid).

In weird way, you're getting a non-inversion because the signal is being inverted twice. lol.

[FunkyE9th]

Yeah, those Richard Kuehnel books are really good. I have all 3. Have learned a lot (actually still learning) from it.