grid resistors on preamp stages

[daz]

I read the article on aiken's site on this but i wasn't able to find an answer. Probably there but over my head, or more likely can't be answered w/o doing some math which just always eludes e. Anyways, i have removed any grid resistors in the preamp except for the input one. The tone like this to my ear is far far better. They were killing the highs i like. Aiken says a 470k rolls off down to 2.2k which makes sense given how it sounded then and now. Question is, should i have grid resistors in my 3 gain stage preamp to avoid potential blocking or other issues and if so what is the smallest value i can benefit from w/o affecting the tone noticeably?

[Helmholtz]

Any grid stopper larger than 50k at a gain stage is likely to kill some treble.
Typically something between 1k and 10k is enough to avoid HF oscillation.
Blocking distortion is most likely with power tubes but doesn't always sound bad.

If you like the sound better without large grid stoppers, don't use them.

[daz]

Thats exactly what i was hoping to hear, thanks Helmholtz. I'd add 10k in front of the grids.

[Chuck H]

I also notice that larger grid stops can reduce phase shifts under clipping conditions. I interpret this as making operational conditions for the preamp less variable as it responds to attack and decay. Flatter and less springy/bouncy feeling. Not at all good technical information but true just the same

[daz]

Flatter and less springy/bouncy feeling.

Yes, thats a big part of what i try and avoid and removing them went a long ways to that end. I did as Helmholtz suggested add small ones tho for the sake of tonal reliability. 10k...didn't seem to change the tone at all so thats good. I thought about a trim pot to see if i could find a point where the tone might actually be better by finding the perfect knee but it sounds so good now i'm afraid to mess with it. I've made a LOT of changes so it's quite different and the improvements seem substantial. Of course i felt similarly till i brought it to that jam so i take nothing i do at home at face value. In 3 weeks i'll get to try it again and see if it's as good as it now sounds here. Plus by them i hope to have a louder speaker in it.

[Helmholtz]

Phase shift between what?
Do you rather mean duty cycle shift by grid conduction?

To clarify, when I said gain stage I meant a typical preamp tube having a gain of 50 to 60 and an effective input capacitance of maybe 150pF.
Power pentodes or tetrodes have much lower gain (maybe 10 to 15) and an effective input capacitance of 15pF to 20pF.
So power tubes can take up to 10 times larger grid stoppers than preamp tubes without much change of frequency response and phase.
Of course, frequency reponse only has a meaning with sine signals, i.e. without distortion.

[Chuck H]

Phase shift between what?
Do you rather mean duty cycle shift by grid conduction?

Yes. That is what I meant.

[mhuss]

Merlin Blencowe has a good discussion of grid stoppers in his book "Designing Tube Preamps for Guitar and Bass" (an abbreviated version is at https://www.valvewizard.co.uk/gridstopper.html,) covering both frequency response effects and blocking distortion reduction in higher gain situations.