A hypothetical grid stopper question...

[pinkphiloyd]

Stuck at work for another hour and just doing some reading/thinking/wondering.

In Merlin's book, he states that the grid stopper on your first preamp tube is the noisiest resistor in the amp. To combat this, he recommends adding a little more capacitance to what's already inherently present in the tube(about 200 pF in a 12AX7) by putting 400 someodd pF of capacitance between grid and ground and reducing your grid stopper to ~10K. I'm guessing with this approach he figured the tubes inherent capacitance into the equation, which, it seems to me, would still necessitate getting the grid stopper right up next to the grid of the tube.

My "just curious" question of the night is, could you use even a little more capacitance and less resistance (my math says a 7.5K and 1 nF would give a cutoff frequency of ~21kHz) and move this new RC filter off the pin of the tube, to your board or maybe even the input jack? Would this just not work or introduce some other negative effect that I'm missing? My understanding is most interference is picked up by your guitar cable or a bad joint (which a grid stopper isn't going to help anyway), so it seems to me if you arranged a similar filter at your input jack, the rest of the path to the grid of V1 is inside your earthed chassis and should be relatively safe from interference...what am I overlooking?

[Reeltarded]

There is theory and then there is practice

I have 1000pf from the input to ground and 68k on the tube socket in my dirtiest amp.

If you are making something less dirty than me, you can probably ignore that whole idea and move to anything important. I would say it's all but a waste of time but it's a complete waste of time.

All carbon comps here. noise supression for guitar amps, noise exclusion is for hifi.

[Tone Lover]

In theory still Merlin says best to have the grid stopper on pin of the tube because the wire from input to tube acts as a radio antena.
Bill

[gui_tarzan]

Doesn't shielded cable from the input jack pretty much negate the "radio antenna" problem?

[Colossal]

Doesn't shielded cable from the input jack pretty much negate the "radio antenna" problem?

Nope!

[jelle]

+1

[martin manning]

Doesn't shielded cable from the input jack pretty much negate the "radio antenna" problem?

Nope!

+1

Why not?

[Colossal]

Doesn't shielded cable from the input jack pretty much negate the "radio antenna" problem?

Nope!

+1

Why not?

As enamored as I am with the idea of going sans resistancé on the input stage, my unscientific answer to your question is MEXICAN RADIO and my scientific answer is MEXICAN RADIO. Muy giganté. Even shielding is not up to the task. Add a grid stopper and the fiesta ends.

[martin manning]

Mariachi static? I'm not thinking zero resistance, though... The question is, is the series resistance at the tube pin or at the jacks (Fender BF style)? If the chassis is fully enclosed, or the grid lead is shielded, it shouldn't make much difference as far as keeping RF out, or the Miller capacitance effect.

[pinkphiloyd]

Mariachi static? I'm not thinking zero resistance, though... The question is, is the series resistance at the tube pin or at the jacks (Fender BF style)? If the chassis is fully enclosed, or the grid lead is shielded, it shouldn't make much difference as far as keeping RF out, or the Miller capacitance effect.

This is what I was thinking in my original post. If you're adding your own capacitance, it seems to me you could put your stopper and cap at the jack. Shielded wire from the jack to V1...earthed chassis...seems like this would be sufficient to me. But this is strictly instinct. I have nothing to back it up.

Also, props for the Zevon reference. That's my favorite WZ song.

[didit]

Wall of Vodoo, no?

[Reeltarded]

My channels are japanese shipping and radio free jerkganistan

lmao

Fuzzface seriously does pick 50 year old Japanese SW. no idea.

[Structo]

You can run a much smaller cap from the input to ground if you are just trying to quell RF.

If you are looking for a tone change, buy a real cheap guitar cord that has high capacitance.

[strelok]

Nope!

+1

Why not?

As enamored as I am with the idea of going sans resistancé on the input stage, my unscientific answer to your question is MEXICAN RADIO and my scientific answer is MEXICAN RADIO. Muy giganté. Even shielding is not up to the task. Add a grid stopper and the fiesta ends.

I think the shield acts as a Faraday cage of sorts. If I remember right there is two things that determine the effectiveness at various frequencies.

One is the gaps in the cage, any that is not significantly smaller than the wavelength of a particular frequency will let that frequency through.

The other is the 'skin effect'. Which basically states the lower the frequency the thicker the shielding needs to be in order to be effective. This is why shielding doesn't do a lot of good against 60 cycle hum.

I like your explanation better though

[Garthhog]

Why not?

As enamored as I am with the idea of going sans resistancé on the input stage, my unscientific answer to your question is MEXICAN RADIO and my scientific answer is MEXICAN RADIO. Muy giganté. Even shielding is not up to the task. Add a grid stopper and the fiesta ends.

I think the shield acts as a Faraday cage of sorts. If I remember right there is two things that determine the effectiveness at various frequencies.

One is the gaps in the cage, any that is not significantly smaller than the wavelength of a particular frequency will let that frequency through.

The other is the 'skin effect'. Which basically states the lower the frequency the thicker the shielding needs to be in order to be effective. This is why shielding doesn't do a lot of good against 60 cycle hum.

I like your explanation better though

The shield also acts like a distributed capacitor. This shunts unwanted high frequency signals to ground (assuming you attached the shield to ground). Unfortunately, cheap RG174 as well as instrument cable with high per-unit-length capacitance will shunt some of you high frequency audio to ground as well unless you keep it very short.