Dull/snubber caps in preamp

[ampnewbie]

I’ve seen various places in the first gain stage where a ‘dull’ (sometimes called snubber) cap can be placed to decrease highs:

- across the plate resistor
- from plate to cathode
- from grid to ground

Generally these are small in value (1000pf or less). Can anyone help me understand these different positions? How do they compare to each other?

Thanks!

[2deaf]

'Across the plate resistor' and 'from plate to cathode' are pretty much the same, especially if the cathode is bypassed by a capacitor.  The B+ end of the plate resistor looks like ground to audio signals as does a bypassed cathode.  A dull cap from the plate straight to ground is also the same.

A capacitor from the grid to ground adds to the input capacitance of the gain stage.  If you have a grid stopper resistor, the high frequency roll off depends on its value and the total input capacitance.  Just the grid stopper alone can also 'dull' the signal because an input capacitance is still there in the absence of an external grid capacitor.

You can also use a grid stopper and a capacitor from plate to grid.  This capacitor will be multiplied by the Miller effect, so its value will be much less than the other dull capacitors. 

[ampnewbie]

Very helpful, thanks.

[jojokeo]

Sometimes a speaker change will work wonders. There's many ways to trim high end and sometimes people forget the obvious - the treble and tone controls on your guitars and amps. I always say, you can always "tone down" an amp and rig but if you don't have it in the first place then you can never get it back. There's a lot of harmonic structure created in the preamp and it gets lost quickly in the "dull" preamp stages. There's also more in the way of snubber and Zoebel circuits put before/after the OT. Lots of ways to skin this cat. But in the name of amp design there's this: tubenit created a sheet and called this effect "smoothing" capacitors. The reason is with high gain you want smoothness and to get it you have to loose the ice pick high end fizz and brittleness. I hope he doesn't mind me posting his worksheet? But he put it here for everyone at one time: