Sonic effects of upping grid stopper R value

[greiswig]

Hi, all,

With the stock D'Lite value of 1.8k versus the more common value of 5.1k, and some people recommending a minimum of 6k or more for running tubes like KT-66's, I'm wondering what (if any) effect on sound boosting the power tube screen resistors has? I've read elsewhere that there is really no audible effect on highs until you get into many tens of kilohms, but is there no other effect than that?

[Structo]

I changed my screens to 470R and 3K3 on the grids and can't say I noticed any change. (6L6)

[Fischerman]

Are you talking about the screen (pin4) resistor or the grid stopper (pin5)? I'm not aware of any schemos that have a 5.1k screen grid resistor. IMO, you'd definitely hear a difference going that high on the screen grid resistors but probably not (or not nearly as much) if you went to 5.1K grid stoppers.

I've read elsewhere that there is really no audible effect on highs until you get into many tens of kilohms, but is there no other effect than that?

This sounds like it's referring to grid stoppers...IME you'd hear a difference in screen grid resistors way before you got as high as 10K.

[greiswig]

Are you talking about the screen (pin4) resistor or the grid stopper (pin5)? I'm not aware of any schemos that have a 5.1k screen grid resistor. IMO, you'd definitely hear a difference going that high on the screen grid resistors but probably not (or not nearly as much) if you went to 5.1K grid stoppers.

I've read elsewhere that there is really no audible effect on highs until you get into many tens of kilohms, but is there no other effect than that?

This sounds like it's referring to grid stoppers...IME you'd hear a difference in screen grid resistors way before you got as high as 10K.

DOH! I meant the grid stoppers. What kind of differences do you hear, Fischerman?

(PS - Aiken Amps has a writeup on grid stoppers, which is where I read that there is little effect except at very high values)

[Fischerman]

I put 3k3 in my amp and haven't changed them or seen a need to. I have read (on the Trainwreck forum here) that the 1k5 value that KF used was borderline too low and that it is often good to increase them to the 5k6 range with no tonal losses but yet a more stable amp. The grid stopper won't have near the effect on highs on a pentode than it will a triode.

IMO, use a value high enough to 'buy' you some stability but not high enough to affect tone. Also remember that the grid stopper is in series with the 220k bias splitters and there is a max grid-to-ground resistance for power tubes (which we sometimes are already exceeding...I think it's like 100k for fixed-bias 6L6s or something). Going too high on the grid stopper (like tens of Ks) might lead to thermal runaway/meltdown unless you lower the bias splitters which will definitely affect tone. I really don't think the power tube grid stoppers is a good area to tweek tone...IMO.

[heisthl]

The 5K1, 1k8 and 3k3 all sound the same until you try to get maximum clean volume then the 5k1 is the winner by a nosehair. As far as the 1k5 with a 12AX7 spltter - don't go there unless you want low headroom.

[Structo]

As far as the 1k5 with a 12AX7 spltter - don't go there unless you want low headroom.

I'm not following that?

[heisthl]

As far as the 1k5 with a 12AX7 spltter - don't go there unless you want low headroom.

I'm not following that?

I'm trying to say that the 1k5 value was common in 12AT7 splitter stages where it works well, but it does not work as well as a higher value does in 12AX7 splitter amps. That low r value (1K5) is an invitation for the Output tubes to draw grid current the 12AX7 cannot deliver causing PI distortion to come on quicker (less headroom at higher volumes).

FYI - this is a common mistake in Fender mods where the modder forgets to change the grid resistors when switching to a 12AX7 PI.

[Bob-I]

As far as the 1k5 with a 12AX7 spltter - don't go there unless you want low headroom.

I'm not following that?

I'm trying to say that the 1k5 value was common in 12AT7 splitter stages where it works well, but it does not work as well as a higher value does in 12AX7 splitter amps. That low r value (1K5) is an invitation for the Output tubes to draw grid current the 12AX7 cannot deliver causing PI distortion to come on quicker (less headroom at higher volumes).

FYI - this is a common mistake in Fender mods where the modder forgets to change the grid resistors when switching to a 12AX7 PI.

Interesting. Guilty as charged. I'll have to try upping the grid stoppers on my Super clone as I'm simply not getting the headroom I'd like with the 12AX7.

[phalanges]

I'm surprised that there's a noticable difference in headroom performance between a 1k5 and a 1k8 or am I misunderstanding entirely?
I admit to mostly ignorance.