coupling cap question

[12AX7]

I have a question regarding a detail in the soldono SLO schematic in the direct link below. Look at V1A and coming off the plate theres the typical .02 coupler, but then a 120pf over a 2.2M. I understand how that resistor and cap work, i've use lots of those. But heres what i don't get. Why is the .02 even there? heres why i ask that. If i go into a electronic calculator app i have and add together a .02uf and a 120 pf cap in series, it comes out as about 119 pf. So what does the .02 even do? is it just there because they can't get a 120pf that handles plate voltage or what?

[birt]

the link is not there and the cap is there to block DC from interfering with the next stage.

[12AX7]

Oops, sorry, forgot to paste it in. http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/soldano_slo100.pdf 

I see what you mean now. With the 2.2M in series with the PF cap, DC can get past. So the .02uf is not for tonal reasons at all where as usually it's chosen not just to block DC but to trim the signals a bit to reduce flab. So i assume that if not for DC blocking they wouldn't even have used that .02 there at all.

[HotBluePlates]

So the .02uf is not for tonal reasons at all where as usually it's chosen not just to block DC but to trim the signals a bit to reduce flab.

Sure it is. Just like the 0.02uF cap for the OD cap is to shape the low end for that channel.

The 2.2M for the clean channel is there to form a voltage divider with the 330k resistor to ground to lower the voltage output of the clean channel. It also helps provide a resistance for the optoisolator (VR2) to work against when switching channels. Basically, the whole network of resistances are reducing channel interaction and bleed-through.

But such a large-value series resistance will react with the wiring capacitance and input capacitance of the next stage to noticeably reduces highs. So you need the small 120pF cap to restore the highs that would otherwise be lost.

[12AX7]

Ahhh, you're right, i never thought about that being a VD and i do know how they attenuate highs. But the reason i asked was in my amp i have a .022 coming from the first stage  plate to the gain pot and after the .022 i had a 470k in parallel with a .0047 to add some high mid sweetness. But then it hit me that having that RK network in series with the .022 gives a summed capacitance of almost the same as the .0047 alone. I think my calculator said .0043. Therefore i was thinking why even have the .022 there till someone mentioned DC blocking and i realized yes, with the 470k bypassing the .0047 DC had a path to get thru. Anyways, i went back to the original setup there (470k/500pf) and added a .0047/470k network at a different point and that worked quite well to get me what i was after, which is to shape the gain stages to get that certain marshall mid i have had in some of my favorite marshalls. It's really quite amazing how much tone shaping towards the high end you can do before the tone gets too thin and trebly. I always had issues with too much and too fuzzy lows and low mids when i first started building but I have since learned that in a cascaded preamp you really have to knock down a heck of a lot of the lower end of the signal or it will never have clarity and articulation.