Channel mix at the PI ..... a bit confused about Paraphase

[kagliostro]

Few days ago I've draw this (from an idea of a second High Gain channel on the Ampeg Jet J-12-B - based on Sluckey scheme)



I've draw it with mix resistors because was the first thing that come to my mind, but now I'm wondering about a different approach, the mix at the PI

like in VOX AC30 or other amp with similar architecture



but here there isn't a Long Tail PI, the Ampeg Jet uses a Paraphase, so ...... is it feasible and ...... how ?

Thanks

Franco

[sluckey]

It's not feasible. The most logical choice is the mixing resistors . But you could replace the paraphase with a LTP and do as you suggest.

[kagliostro]

Ciao Steve

I think that the paraphase is an important brick on the Jet, so it will be definitively mixing resistors

Grazie

Franco

[terminalgs]

but here there isn't a Long Tail PI, the Ampeg Jet uses a Paraphase, so ...... is it feasible and ...... how ?

With a paraphase, it isn't really doable.  look at the first  triode in the paraphase.  the grid is already used as input for one channel.  On a triode, sometimes the cathode can be used as an input, but not in this case.  there are two problems (1) it is already be used for the NFB input  and (2) even if you decided to not use NFB, the paraphase's cathode resistor value is low (because its a gain stage), so any input you bring to it will be subject to whatever voltage-division is set up with that cathode resistor.  Even if you coupled a 2nd input to the cathode with a cap,  you'll still have a sucked down signal at that point.


The ampeg design almost looks like a LTPI with only a 220ohm resistor between the cathodes and shared 1K to ground, but that 100uf cap ensures no audio AC signals live on that 2nd triode's cathode.


And, of course, you can't use the 2nd PI triode's grid as an input for a second channel because that 2nd channel's input would never get back to the "top 6V6"'s grid.  not exactly.  it would sorta, the 470K/510K voltage divider would send some of it in that direction, but it would be unbalanced.  That divider is oriented as a one direction divider.  it reduces the "top" signal just enough so that the "bottom" signal will match (post 2nd triode gain stage) in terms of signal "size", aka voltage.  that divider in the other direction wouldn't work.

[kagliostro]

Thanks also to you Terminalgs

Franco