Long Tail Phase Inverter

[MattT]

I'll probably just muddy the water but here goes:

IMO, thinking in terms of "the grid and cathode are in-phase with each other and out-of-phase with the plate" can make your head spin regarding LTPIs. For me, the key was taking the following as truth regarding triodes (and it is the truth):
A signal input at the grid is amplified out-of-phase at the plate.
A signal input at the cathode is amplified in-phase at the plate.

In a LTPI, there is no cathode bypass cap so there is signal on the cathode and both cathodes are tied together. So the input for the second (bottom) triode is at the cathode...therefore the amplified signal at the second plate is in-phase with it (which is the opposite phase of the first plate).

Amps with no NFB usually just have the tail directly grounded...that 47ohm ground resistor in a BF Fender is just part of the NFB voltage divider (820 and 47 ohm). No 47ohm resistor and there would be no NFB since it would then be directly grounded. If you'll notice, BF Fenders that take an 8ohm speaker have a 47ohm ground resistor and ones that take a 4ohm speaker have a 100ohm...because there is more NFB voltage coming from the 8ohm tap.

Hope that helped...but I have a feeling I completely missed the point.

[Steve Conner]

I'm thoroughly confused by now, but I think I agree with Loudthud. A LTPI is a differential amp, and the two grids are the 0' and 180' inputs. Traditionally, the signal is applied to one grid and the NFB to the other. Connecting the tail resistor to the NFB node instead of ground basically bootstraps it and makes it look more like a current source.

I know it looks like you're feeding the same signal to both the cathode and the grid of the "bottom" tube, so the tube should get confused and stop amplifying, or something. But the cathodes are a low impedance point and a few volts of swing on the end of a 22k resistor won't affect them much. The bottom tube's cathode is still mainly driven by the top tube's cathode (which is how the differential action is achieved, like a previous poster pointed out) and its grid is still mainly driven by the NFB node.

The long-tailed pair is still a fundamental building-block, even if it is made with transistors now.

BTW, I wish they wouldn't draw some of the tubes upside down. I'm used to schematics with more positive voltages towards the top of the page, so I can't help imagining the upside-down ones as PNP tubes running off a B- rail

[MattT]

Here is a very old Ampage (I think) thread.

In the AB165 BF Bassman, the NFB is connected to the main PI input and the OT secondary wires are reversed...and there is no extra resistor to ground.

[Jag]

This thread is fantastic for me. Can we do one on understanding OT primary impedance?

BTW, I'm not being facetious!

[Earl]

All ya have to do is post the question, Jag!

BTW, I'm going to post a webpage that will have a number of PSpice plots with this LTPI. Hope it will be of some interest to the readers here, and a picture is always worth a thousand words! :-)

[Earl]

Here's that PSpice simulation - http://home.texoma.net/~flhh/ltpi/ltpi.htm

[Jag]

Been doing lots of reading on the OT Primary Impedance aspects. It's making a lot more sense to me now. As soon as I'm confident of my understandings, I'll post some questions.

[Old Tele man]

...the 47-om "tail" resistor is used as an injection point for NFB when the signal from the OT-secondary is taken from the "opposite-polarity" leg of the OT...that is, the OT-NFB voltage is "IN-of-phase" with the input signal to the PI.

...why? because injecting the "IN-phase" NFB signal into the cathode is OPPOSITE of injecting it into either one of the control grids...cathode injection results in the NFB being "out-of-phase" with the incoming signal...because:

A) there is control-grid to cathode is phase inversion

...so, conversely...

B) there is cathode to control grid phase inversion (hence the current-fed NFB with un-bypassed cathode resistors).