Any reason not to use smaller grid stopper at input?

[Gaz]

Is there a practical limit to how small a input grid stopper can be, say with a 12AX7 and still prevent spurious oscillations? I know that the HF roll off can be re-created with a cap to ground with a smaller stopper.

Is there any reason not to use say a 1K resistor and .0047uf cap to get rid of RF? I've found reducing the stopper greatly reduces hiss in a high gain preamp.

[pdf64]

This at a guitar (or other high impedance source) input?
If so, consider the effect that the cap will have on frequency response.
It isn't just the 1k resistor, the source impedance needs adding on to it.
Assuming the pickup impedance is somewhere between 10k and 100k, and may rise to >250k with a 500k volume pot set halfway, there will be significant treble roll off; which may go toward explaining the reduction in hiss.
Pete

[jpfamps]

Grid stoppers are a significant source of noise and reducing their value reduces noise.

An RC filter can give as good RF immunity with reduced noise.

There may be some effects of any peaking response due to the interaction between pickup, cable capacitance and input capacitance, but these are probably minor considerations.

[pdf64]

'There may be some effects of any peaking response due to the interaction between pickup, cable capacitance and input capacitance, but these are probably minor considerations.'
Even with a 4n7 cap?

[Alan0354]

What is a grid stopper? I don't understand the term.

[Jazz P Bass]

What is a grid stopper? I don't understand the term.

Grid Resistors

[Alan0354]

Grid Resistors

Thanks

Do you mean the one from grid to ground or sometimes in series between the input and the grid?

[imaradiostar]

Grid stoppers roll off RF by putting a resistance in series with the grid capacitance of the tube. There is no reason why you can't artificially increase that capacitance and decrease the resistance if the end result is less hiss in a high gain pre.

I've never tried it- maybe I should!

JT

[J M Fahey]

A 4n7 cap to ground at the input of a guitar amp will turn sound into mud.
I've experimented with many values and the basic result was:
100pF : a classic value, practically no audible effect.
220pF: the largest "inaudible" one.
470pF: starts to be heard. Acceptable for bass or Heavy guitar; too much for, say, Country.
1000pF: almost everybody complains about losing sparkle/attack/punch. Max value not to be surpassed.
Typical of, say, MXR Dist+ (which is not true bypass originally).
4700pF: Mud Kingdom.

[Alan0354]

Why do you put a cap from input to the ground at the input. My biggest cap to the ground in the interstage of even the high gain channel is about 400pF and I have no oscillation.

[Enzo]

Bleed off RF.

[Alan0354]

Bleed off RF.

What RF? Any oscillation I even seen is at the upper range of audio frequency or slightly beyond.

[J M Fahey]

He means *external* RF.
Most don't notice it because it's invisible but we are *swimming* in a pool full of RF.
All radios (AM and FM), TV, ***cellphones*** , all kind of communicators (Nextel/Handys/Police), WiFi, Bluetooth, etc.
One particular case I had (I live in a Port area, less than 80 meters from the riverside) : Marine Radar (drove me crazy).
Guitar cables make excellent antennas (even the shield), and tube grids/transistor bases/Mos gates make excellent RF detectors.
And once it's been demodulated and turned into audio/noise/chirps no filtering kills it without killing legitimate audio.
Edit: I was forgetting SMPS, monitors, PCs, TVs, etc.

[Gaz]

A 4n7 cap to ground at the input of a guitar amp will turn sound into mud.
I've experimented with many values and the basic result was:
100pF : a classic value, practically no audible effect.
220pF: the largest "inaudible" one.
470pF: starts to be heard. Acceptable for bass or Heavy guitar; too much for, say, Country.
1000pF: almost everybody complains about losing sparkle/attack/punch. Max value not to be surpassed.
Typical of, say, MXR Dist+ (which is not true bypass originally).
4700pF: Mud Kingdom.

Juan , this is with what value grid stop?

[Alan0354]

He means *external* RF.
Most don't notice it because it's invisible but we are *swimming* in a pool full of RF.
All radios (AM and FM), TV, ***cellphones*** , all kind of communicators (Nextel/Handys/Police), WiFi, Bluetooth, etc.
One particular case I had (I live in a Port area, less than 80 meters from the riverside) : Marine Radar (drove me crazy).
Guitar cables make excellent antennas (even the shield), and tube grids/transistor bases/Mos gates make excellent RF detectors.
And once it's been demodulated and turned into audio/noise/chirps no filtering kills it without killing legitimate audio.
Edit: I was forgetting SMPS, monitors, PCs, TVs, etc.

I only worked on a few amps, I never seen RF problem. All I've seen is in cascade stages, the tube start talking with the guitar and squeal. It is not from guitar to speaker, it is from the amp to the guitar direct.

I do do very careful layout. I am an RF and signal integrity engineer working with all sort of high speed circuits, I do pay attention to the layout to prevent EM pickup though.

If you look at some of the non high gain amp, I don't recall seeing any shunt capacitor as I never see any issue with noise pickup.