Caps to ground from preamp plates

[EETStudent]

Not sure if that title made any sense, but has anyone had any experience or seen a small value capacitor sent to ground coming off the plates of preamp tubes? I was fiddling today, and out of complete curiosity I sent a .022 cap to ground just before the coupling cap after the first stage. It really 'dimmed' the tone, and it was something I found fairly useful. Thoughts?

[Satamax]

Well, i would call that snubbing capacitor http://tone-lizard.com/Mods_and_Odds.htm

You're just ditching any frequencies that are going through the cap to ground. with a .022µ you're pretty much ditching everything. With Pf values you're just touching the high end. You can also make a capacitive voltage divider with caps after the plate. http://www.aikenamps.com/VoltageDividerRule.htm

[cbarrow7625]

No harm in doing it but it is not my favorite way to reduce level or high frequencies. When you run a cap to ground right off of the plate (or around the plate resistor back to the power supply - same effect for AC) you are basically making the tube work into zero impedance (and/or extremely low impedance) at the frequencies that are being shunted. You basically "load down" the tube at AC frequencies and keep it from working to make it's glorious "tube sound" correctly.

Caps aroung the plate resistor are fairly common to tame high frequencies but my personal tastes cause me to avoid them like the plague. I prefer to let the tube run "freely" into a nice high series resistance (like 100k-470k) and put a cap to ground following that series resistance. You still dump the high frequencies but you allow the tube to work into a "normal" load so that it is allowed to produce the nice rich harmonics & overtones that you wanted it for in the first place.

That being said, if you want to try some otehr "intersting" things to experiment and see what you like, also try connecting the a cap from the plate to the grid and from the plate to the cathode (try the cathode bypassed & unbypassed) and see what you get. I do agree with Satamax though that .022uF is a little large for this type of thing usually. Usually you will see something from pF up to about .0047uF, try playing around in those ranges. However, I think one of the Fender Hot Rod Devilles has a .022uF cap in series with a 220K resistor to ground directly off of the plate (or right after the coupling cap) so it is done with these large values but the series resistor really helps keep from loading down the tube quite so much.

Chris

[EETStudent]

Yea the .02 cap did have a dramatic effect, but it was all out of experimentation. I tell ya, I did find something that I really like. Going with the same concept, i sent a couple to ground from the wiper of my treble control. This worked really well, and kinda like you said cborrow, the "tube" sound was definitely still retained and I had a nice, high frequency rolloff. Switchable, of course.

[Satamax]

Have you tried local nfb?