plate to cathode cap

[rdrgtr]

I noticed something the other day when I was looking at a preamp circuit (dumble I think).  The circuit called for a 100pf cap from the plate resistor to the cathod.  Basically the cap was between pins 1 & 3 on the first tube stage.  High freq. roll off?  Just curious.

[tubenit]

Yeah, it rolls off some of the highs to "smooth" out the tone.

With respect,Tubenit

[Fresh_Start]

Any benefit of that application over a small cap across the plate resistor?  (sometimes called a "snubber" IIRC)

Chip

[RicharD]

Snubbers are usually used to control high frequency oscillations.  The down side is it may snub into the audio band.  Often high gain amps need a snubber and can fortunately afford some hi frequency attenuation.

[tubenit]

Well, I'll tell you what I've done. Such as on the TOS build, I tried BOTH ways on different gain stages. Sometimes I like the plate to cathode ......... sometimes I preferred across the plate resistor.

I have a cap substitution box that makes quick work of experimenting that way.

I could not find a particular guideline for me other than simply listening to it.

With respect, Tubenit

[PRR]

In principle it makes little difference. Plate is "hot", either B+ or cathode is "pretty near AC ground", so it is a simple plate-to-ground high-roll-off.

You might hear a difference one way or the other.

I suspect in many cases you can just go wherever is easiest.

[Merlin]

Plate-to-cathode (or plate to ground) is better from the point of view of PSRR, but in practice it makes little difference.

[Tiny_Daddy]

Or tie a cap across the plate resistor as in some Fender silver-face amps.