100 pF PI Cap?

[Kramer]

What can I expect from a 100 pF Phase Inverter cap? I heard a 2204 with a 100 pF PI cap that sounded good, and I was wondering what this might do in my plexi.

Many thanks in advance!

[xbearxau]

It just tames a little high end fizz. Its supposed to be what EVH had in his Plexi, but who really knows?
I tend to use a 100p on all my marshall builds rather than 47p because I like the smoother top end it seem to give when you push the PI harder.
Its subtle, but it makes a difference.

[Kramer]

Thanks! Guess I'll go with 100pF.

[Cabbagebreeze]

You might also want to try a 100K R in series between the master volume wiper and the PI input cap.

[axeman]

What would that do.

[darkbluemurder]

That would keep the tone a bit more consistent through the travel of the master volume pot. I read that the reason for that is that the pot's resistance to ground forms a high pass with the PI input cap. At low master volume settings there is a low resistance to ground, thereby shifting the frequency knee of the high pass upward (e.g. kills bass frequencies). By adding the 100k resistor in series with the pot wiper the input cap "sees" 100k more to ground, thus lessening the bass kill.

Cheers Stephan

[Cabbagebreeze]

re: 100k R in series with the master volume and PI input cap

What sounds nasty to me, and not in a good way, is a stage that starts drawing grid current, and the previous stage not being able to provide it. In this case the PI, when hit hard, will start drawing grid current. The 100k R will offer a little compression or rounding instead of the untasty distortion that can occur without it.

[xbearxau]

Remember its a plexi were talking about, not a JCM. The 100k trick works on 2204's because of the pre PI master. I dont think its relevant here.

[Kramer]

Can someone explain a little more or get a diagram and draw over it? I'm not sure exactly what your talking about with the MV wiper (the pot?) and the PI input cap. Would it look like this?

[eddie25]

Yeah that's right.

I used a 100pf on my most recent build and it was the first thing I took out. Made the amp a little dull and compressed sounding. Still, I could see it sounding great on some amps.

[Kramer]

Cool, thanks for the feedback.

[HTH]

I too find that 100pF can sometimes be a little too much in terms of rolling off the top end, but it depends on the spec.

I'm currently using a 82pF PI cap, but the preamp is a JCM800 type of affair and therefore quite bright. This is tempered by using 0.1uF PI caps.