Importance of grid-stopper in interstage coupling?

[Rod]

HI everyone!

What is the importance of the grid-stopper in interstage coupling like for exemple the grid-stopper marked in attahed file?

In the input stage the grid-stopper blocks high frequencies, forming a low pass filter, in a interstage couplig will also block high frequencies, and reduce the harsnhess frequencies?

If i increase the value of the grid-stopper resistor what frequency values will i attenuate?

IF the purpose of a grid-stopper in a interstage coupling is to attenuate high frequencies, what will be the diference in tone to a anode bypass capacitor?

[pdf64]

Have a good read of Grid Resistors - Why Are They Used?
As Aiken says, grid stoppers also mitigate blocking distortion, whereas a low pass filter at the anode won't help with that.

A big downside of grid stoppers is that as they are in series with the signal source, they will introduce noise, proportionate to their value.

Input stage grid stoppers may be the most significant noise source in an amp.

[R.G.]

In the input stage the grid-stopper blocks high frequencies, forming a low pass filter, in a interstage couplig will also block high frequencies, and reduce the harsnhess frequencies?

A grid stopper doesn't block high frequencies, it shunts them to ground.

There are no "harshness frequencies", only distortion products which sound harsh with the basic note and with one another. In general, for any distortion of anything other than a pure sine wave, the higher the harmonic number generated, the more discordant and harsh the combination of pure tones and distortion-generated harmonics will sound. These are in general higher frequencies, so some treble roll-off helps attenuate them, but the freuquencies themselves are not harsh.

The "harder" the clipping of a signal, the more high harmonics generated and the more chance of generating harsh sounding cross-products. A grid stopper can help with reducing the signal level, and making for a more softly clipped signal.

If i increase the value of the grid-stopper resistor what frequency values will i attenuate?

For that, you have to know how a grid stopper works. There is a real capacitor between the input grid on a tube and the plate, cathode, and real ground. The capacitance of the wiring and such leading up to the actual grid wires can all be lumped in with the real capacitance of the tube grid to ground. If there is a resistor before the grid, that resistor and the real capacitance to ground form a R-C lowpass shunt filter. The frequency of rolloff of this filter is taken to be the frequency where the capacitor's reactance of Xc = 1/(2*pi*F) equals the resistor, or
F = 1/(2*pi*F*R*C).

If C = 10pF, R =100K, then F = 1/(2*3.14*100k*10pF) = 159,235Hz. If R was 1M, that would make it 15.923khz,
and if C was instead 5pF, it would be 318.4kHz.

But that's not the only capacitance. The capacitance to the cathode is there, but the tube is doing its best to make the cathode move at exactly the same voltage as the grid. So the effect of the grid-cathode cap is between 1x and 0 times the real grid-cathode capacitance. They're generally in the 1-20pF range, depending on the tube.

The big kicker is the grid-plate capacitor. The plate is not only moving in great big signal voltages compared to the grid, it's moving in the opposite polarity. There is a page or two of algebra to show this, but the grid-plate capacitance is multiplied by the voltage gain of the tube when you measure its effect on the tube.

All the effective capacitances add at the grid. So if your tube has Cgrid =5pF, Cgk = 5pF, Cgp = 5pF, and the voltage gain is 30, the effective capacitance to ground is 5pF + 0pF +150pF = 155pF. The 150pF is the capacitance of the grid to plate multiplied by the signal voltage gain from grid to plate, 30x in this example. So the grid-plate capacitance is a BIG DEAL.

The frequency of rolloff of a grid stopper is then the frequency F = 1/(2*pi*R*(Cg+CgK+G*Cgp)). Obviously, the gain-times-Cgp is the big deal in this equation. So the rolloff of the grid stopper depends on the voltage gain you're getting from the tube.

IF the purpose of a grid-stopper in a interstage coupling is to attenuate high frequencies, what will be the diference in tone to a anode bypass capacitor?

An anode ("plate") capacitor causes a frequency rolloff that is dependent on the actual plate resistor Rp and the internal plate resistor rp. The rolloff frequency is the same form, F = 1/(2*pi*R*C), but the R in this case is the parallel combination of Rp and rp.

If there is no difference in rolloff frequency, there is no difference in tone. But the capacitance value and values of plate and grid stopper resistors to get the same rolloff frequency may be quite different. So there is no good answer to your question, at least when stated that way. It's kind of like asking what's the difference between a basketball and a house cat. Um, lots of things. They may weigh the same, occupy the same volume, have similar colors, etc. but in some ways they're quite different.

[Dave H]

A grid stopper doesn't block high frequencies, it shunts them to ground.

How does a grid stopper shunt high frequencies to ground when neither end of it is connected to ground?

[R.G.]

I guess to put it more clearly, the grid stopper acts like a resistor, the various grid capacitors shunt signal to ground.

The grid-ground capacitance is always there - it uses the ambient air for a pathway. The grid-cathode capacitance has the pathway to either real, DC ground through a cathode resistor or through a cathode bypass. The plate capacitance is a "virtual" capacitance, caused by the plate sucking signal current out of the grid by pulling really hard on the grid-plate capacitance. The reference for all of these winds up being signal ground, power ground, or the B+ power supply, all of which are tightly coupled to real ground by capacitors.

[Dave H]

I guess to put it more clearly, the grid stopper acts like a resistor, the various grid capacitors shunt signal to ground.

The grid-ground capacitance is always there - it uses the ambient air for a pathway. The grid-cathode capacitance has the pathway to either real, DC ground through a cathode resistor or through a cathode bypass. The plate capacitance is a "virtual" capacitance, caused by the plate sucking signal current out of the grid by pulling really hard on the grid-plate capacitance. The reference for all of these winds up being signal ground, power ground, or the B+ power supply, all of which are tightly coupled to real ground by capacitors.

Thanks for the explanation. I was pulling your leg because I thought saying that the grid stopper "shunts HF to ground" was just as inaccurate as saying it "blocks HF"

[R.G.]

That makes perfect sense. No wonder my leg felt funny as I wrote that.

[cjenrick]

note that resistors have inductance and capacitance. How much? not enuff to do anything to a guitar amp,

but it is fun to know everything that is going on, or not, depending on how much of a geek you are,

the Ham radio guys know all about it, and they wear pocket protectors, jus sayin,