power tube grid stoppers

[iknowjohnny]

Is there a technical reason that grid stoppers used on the power tubes tend to be on the order of a few k vs preamps that often use up to 470k? And what is the largest you've seen? 10k is the biggest i saw looking thru a few dozen schematics.

[bgrady]

There is no theoretical reason, but for guitar amps, there is a big trade off in sound. Usually the hard upper limit for grid stoppers is determined by input capacitance, but for most power tubes this is no bigger than any of the preamp stages. The corner frequency with 100k stoppers should still be above 10kHz, and so have little effect on the sound.

These high values do however seem to make the onset of distortion really hard (buzzy and nasty), for the same reason they do in the preamp. They limit the amount of grid current, and the smoothing effect that it has on clipping. I would not go higher than 10k, unless you really are getting seriously bad blocking distortion.

Disclaimer: This is from my own modding/building/research. Some people may get fantastic results with large grid stoppers, and all the power to them.

[iknowjohnny]

Thanks. I never noticed buzziness in the pre with big GS's, but I guess it depends on other factors.

[bgrady]

Unless you are driving the tube to the point of a square wave, it is unlikely to sound "bad" for preamp tubes, because there is usually a whole bunch of tricks after the fact to take care of the fizz. In the power amp, the output is THE output, and any stray harmonics are going to be noticeable. Also, even though the input capacitance is the same, the grid current characteristics for a power amp are different. When the grid goes to conduction, the grid stopper effect is much more pronounced simply because the current is so much greater.

Hence most designs stay within 1k-10k, as this is equivalent (in grid current terms) to very large resistors on the preamp tubes.

[Leo_Gnardo]

Is there a technical reason that grid stoppers used on the power tubes tend to be on the order of a few k vs preamps that often use up to 470k? And what is the largest you've seen? 10k is the biggest i saw looking thru a few dozen schematics.

Grid stop 47K on 6550's in SVT. (Not the SVT-CL and other recent issues.)

JCM2000 Marshalls - until they "fixed" 'em - 220K to EL34's - schtoopid - bad design - made to fail - besides those conductive circus boards. What. were. they. thinking?

[Blackburn]

On a pair of EL84s in a Dominator style amp I built a while back I started off with 10K on each and then later switched to the standard 1.5K. The difference was very noticeable. 10Ks made it much darker and even a little bit browner sounding. I was going for clarity so the 1.5Ks stayed, but I can see doing something interesting with high values like that for different kinds of styles. Jazz comes to mind...

[iknowjohnny]

Just tried 15k and i didn't hear any difference ! I was actually hoping for a darker tone but i didn't get anything. Odd !