What is the purpose of an input cap in a phase inverter

[hesamadman]

Hello all. Im tuning an amp build and was curious about something. What is the purpose of the input capacitor on the grid of a phase inverter? I understand the necessity and functions of coupling caps, but most phase inverters have a cap on the input even beyond a coupling cap. I know the same frequency attenuation applies to this cap, but why is it necessary?

[sluckey]

Talking about a LTP PI? The cap blocks the dc voltage present on the grid from appearing on the previous circuit. More important, it prevents any previous resistance from interfering with the dc bias voltage on the grid of the PI. The cap is very important.

[PRR]

There's at least eight more-common "phase inverters".

Do you want a write-up on all of them??

Or do you have a specific one in mind?

[tubeswell]

A standard LTP inverter (from a classic Fender/Marshall-type amp that uses such inverters) has 2 grids - one for each stage. One stage is inverting; the other is non-inverting.


The grid on the inverting stage needs a coupling cap to prevent DC on the grid - assuming the stage is AC-coupled to the previous stage(s).


The grid on the non-inverting stage has a 'de-coupling' cap - to shunt any AC that might otherwise appear on this grid, to ground.

[jjasilli]

. . . What is the purpose of the input capacitor on the grid of a phase inverter? I understand the necessity and functions of coupling caps, but most phase inverters have a cap on the input even beyond a coupling cap. I know the same frequency attenuation applies to this cap, but why is it necessary?

Good question.  BTW both grids in the PI have caps.  The answer seems to be stated, rather cryptically, in "The Fender Bassman 5F6-A", 2nd Ed. Richard Kuehnel, p.95, as further "explained" in "Designing Tube Preamps for Guitar and Bass", 1st Ed. (2009), Merlin Blencowe, p. 184.


From what I can decipher, these caps are needed because of the inherent nature of the LTPI, to "decouple the grids at audio frequencies and filter out HF noise".  I think this dovetails with what sluckey already said.

[hesamadman]

Talking about a LTP PI? The cap blocks the dc voltage present on the grid from appearing on the previous circuit. More important, it prevents any previous resistance from interfering with the dc bias voltage on the grid of the PI. The cap is very important.

   palm in head moment. Thanks Steve. I didnt think this one through before posting.