How does a PI circuit work?

[BashCoder]

I've been studying the schematics of the JTM45 and JTM45/100, and I'm still trying to understand just how the phase inverting circuit works. Can someone tell me if I'm on the right track? After PI circuit, I see how only one of the two outputs will actually have an inverted phase, which sets up the push-pull in the final amplifier stage.

It seems that the grid of V3a is in phase relative to B+, while the grid of V3b is in phase relative to ground. [ Actually I think, since the audio has been through a preamp stage and a cathode follower stage, that V3b is actually IN phase relative to the guitar. ] I see how that sets up the push-pull in the power amp stage.

I think I get that the .1u between the presence pot and ground works with the pot to create an RC filter circuit, allowing only some frequencies to feed back into the PI, while everything else finds an easy path to ground.

But I'm a bit confused about how the Presence circuit and the negative feedback works here. What role does the .1u cap between the grid of V3b and the presence pot play? Why doesn't that cap go directly to ground? Doesn't NFB feed the anodes? Doesn't NFB affect each of the triodes equally? I don't get why the ground reference for V3B is via the presence circuit, while V3a feeds directly from the tone stack.

[Guest]

The other phase input is obtained from the same tube's cathode, since the current thru the cathode is opposite polarity (out of phase) with the plate current.

[Eoin]

Hmm... but how does that work if both cathodes are joined?

[BashCoder]

The other phase input is obtained from the same tube's cathode, since the current thru the cathode is opposite polarity (out of phase) with the plate current.

I've seen that in some Marshall schematics, yes, like this: http://www.metroamp.com/forum/download.php?id=1543

[ Edit: Duh... I read that schematic wrong when I originally posted this. ]

But the JTM45's is like this:

[novosibir]

I've been studying the schematics of the JTM45 and JTM45/100, and I'm still trying to understand just how the phase inverting circuit works.

If I'd try to explain this with my German's English and with the technical dictionary in the left hand, I'd sit about 2 hours on the computer - too much for a very busy Larry...

... therefore I'd recommend you, to take a look into Dave Funk's Tube Amp Workbook, where the different kinds of PI's are very good explained.

Or you take a look at Randy's website: http://www.aikenamps.com./

and then go to --> Tech Info --> Advanced (Aiken) --> The long-tail pair

There he'd explained it better, than I ever could do it anyway

Larry

[BashCoder]

Thanks, Larry -- I will get that book and dig in.

[BashCoder]

Anyone ever try a pot in place of the NFB series resistor? Or alternate resistor values on a DPDT switch? Hoffman sells a very good three position rotary switch that could be used to change which OT tap provides the feedback. That could be cool to have on the back panel...

[Eoin]

I've heard of both mods having been done.