long tail phase inverter miller capacitance?

[yunger]

how do you calculate?

[Jazz P Bass]

Miller Capacitance

Must read:Miller Capacitance

[Jazz P Bass]

12AX7 Datasheet

Read this datasheet. It lists the specific interelectrode capacitances.

[yunger]

thanks.

so the rc lowpass filter effect is calculated the same as a typical triode gain stage would be? i mean, if i had say a 1M resistor in series with the input of the ltpi, would there be a huge high frequency roll off like there would be on a typical stage?

[Jazz P Bass]

Miller Capacitance

The rolloff is usually attributed to audio (ie: HiFi) & radio frequencies.
Should not apply to guitar frequencies.
(I am not even going to ask why a 1 Meg R on the LTP input)

[Merlinb]

thanks.

so the rc lowpass filter effect is calculated the same as a typical triode gain stage would be? i mean, if i had say a 1M resistor in series with the input of the ltpi, would there be a huge high frequency roll off like there would be on a typical stage?

Yes. However, the Miller capacitance of an LTP is only half that of a regular gain stage, so about 50pF.

[yunger]

jazz,
i just used 1M as a hypothetical example. although a 1M pre-pi master turned way down would be awful close to that. speaking of which, you can definately hear the rolloff of a 1M pot before a typical preamp stage.

[yunger]

Yes. However, the Miller capacitance of an LTP is only half that of a regular gain stage, so about 50pF.

cool. i think i get it now. the ltpi has half the gain so......Cin = Cgk + Cgp*(A/2+1)

[tubeswell]

cool. i think i get it now. the ltpi has half the gain so......Cin = Cgk + Cgp*(A/2+1)

I think what Merlin was saying is the miller capacitance of an LTP is half what it would be in a single triode - not that the LTP has 1/2 the gain.

The LTP is two triodes fed off a single grid with the other grid tied to ground at AC. Therefore the signal entering the stage via the grid has two ways of getting through the stage, and the tube current has two paths between the ground return and the HT. So the overall inter-electrode capacitance as it affects the signal will be lower. That's my (simplistic) take on it anyhow.