Low Pass Filter: Math vs Practice

[bmx]

Thats interesting. I always assume the smaller the resistor, the lower the noise. I would be interested to know what the final verdict on if large grid stoppers cause noise or not.

That 470k gives the mesa dual rectifier a lot of its characteristic tone- mostly that it darkens it. Additionally, it causes the grid cutoff to be sharp, but I think even 100k gris stopper is enough in this circuit due to the additional impedances. One interesting schematic you might be interested in looking at is the Bigeran Tri-Rec, which is a mesa Dual Rectifier clone with two of the common modifications that people would do to their DR's. The Bugera doesn't have the no feedback of modern mode, but it is still instructive. That 470k grid stopper is changed to 10k and the second stage cathode bypass cap is 470nF instead of 1uF. It sounds wildly different than a DR. Some say better and I might agree. The DR is classic now though and the bugbear is not that sound that you either love or hate or both.

[roberto]

It's a long time that I don't build hi-gain amps (even if one is in its early stages) because during the last years I focused more on hi-fi, but I clearly remember the difference between different pots values and grid stoppers as well.

I've (re)found this:

However, the input grid stopper adds the most amount of Johnson noise (hiss) of any resistor in the amp. (A 68k stopper generates at least four times more hiss than a typical 12AX7!) Can we use a smaller resistor to reduce noise, but still keep the bandwidth the same? Yes, it's easy. We simply add a little extra capacitance to make up for the lower resistance. I recommend a 10k resistor, which will make the amp much less hissy, together with an extra 100pF to 470pF capacitor from grid to ground.

source: http://www.valvewizard.co.uk/gridstopper.html

In five gain stages amp I also felt the grid stopper to the second stage to have importance on the hiss.
Of course not when you play, but when you don't.