Understanding the OD entrance

[Tdale]

I'm attaching a picture of two quite common OD entrances. One is a resistor into a trim pot, the other is the one with a resistor across two resistors that are grounded through a trim pot.

I'm having some problems with completely understanding them.

First the lower picture (odentrance)

The first one is ok, you can regulate the total resistance with the trim pot. If the pot is all the way in one direction, it has no resistance, and the whole entrance is 200K.

But if you turn the pot all the way in the other direction, won't the signal go directely to ground? There will be less resistance to ground than through the grid resistor in the first half of the OD tube? Is it as simple as simply not turning the trim pot that far, and it'll be ok?

The next circuit is more confusing.

If the trim pot is turned one way, The 100K and 22K will be a combined 122K resistor. But the signal also sees a 100K resistance to ground, through the trim pot, and that is less resistance than through the 470K.. So the signal divied between the 470K and the 122K..

But as you turn the pot, the 22K resistor will be grounded, sending the signal to ground? Again, is it as simple as just not turning the trim pot that far?

But then again, there is a third way of doing it, as shown in the upper attachment (odentrance2). Here, there is no trim pot, the two resistors simple go directely to ground. Why won't the signal choose that path, in stead of going to the OD tube?

[Icetech]

Thanks tdale.. i have been wondeing how this works myself. i use the top one, and it works fine, but i would like a bit more gain:)

[kiwiandy]

both of these schemes do approx the same thing, the major difference is the maximum signal that can pass through them. The max level for the top one (with the 200k resistor in series with the trimmer) is about -9.5dB, while the lower one attenuates the signal by about -4dB minimum.

The 470k on the lower one also skews the trimmer's rotation law a bit and make it slightly anti-log, but this shouldn't matter if it is a preset anyway.

The third one (your design?) cuts the signal by about 15dB, while the 250p boosts the signal above 1Khz or so. I imagine that the 250p would give a quite bright overdrive sound.

I personally like a 150k into a 100k trimmer, with a 68k in series with OD1's grid (similar to the top one).

Hope this helps a bit.

[Tdale]

Yes, it helps a bit..

But am I correct in assuming that if the trim pot is turned to max resistance, the signal will go to ground, and not through the resistor in series with the od1 grid?

As for the third way (the one with the small cap included) there is something I don't get.

When the signal meets the 470K and 100K/250p network, why doesn't it choose the path with lowest resistance (100K/250p) and go directely to ground? I can't see how the signal would get through the circuit, and get to the od1 tube, when the path to ground is less resistive than the path to the tube...

Tommy

[kiwiandy]

Hi Tommy,

yes you are correct about the top scheme- when the trimmer is at max resistance (so the centre pin is touching ground) the signal goes to ground and nothing passes on to the OD stages.

However, in the lower scheme, the signal flows down both the 100k and trimmer path to ground AND also the 470k resistor. When this is set to its minimum, no signal passes through the trimmer, but some signal goes through the 470k to the grid of OD1. But the grid of OD1 is also connected to the 22k, and these 2 resistors form a second attenuator, with the 470k forming the upper part and the 22k the lower. The amount of signal that get through can be calculated by dividing the lower leg by the sum of both-

22k/(470k+22k)= 0.044 (i.e. 4.4%)

this is about -27dB

Also, I made a mistake in the third scheme (I mis-identified the ground connection). In this circuit, the 250p cuts the highs a bit before OD1, and the signal passes through the 470k and the 22k very much like the one I just described, so you will also get about 27 dB of attenuation.

You could increase the 22k to get more signal going into OD1 (or decrease the 470k), but I would suggest the first scheme with the 200k and the trimmer, as it's easier to implement. I'd also suggest getting rid of the 250p, because the caps accross the OD stages are more effective.

Maybe this helps a bit more, and I'm sorry about the mistake in the third scheme. I'll try to read more carefully before answering next time. Let me know if you need to know more.

Andy

[Tdale]

Thanks.

I think I need to read more theory.

I thought that since the lower part (100K and trimmer) has less resistance than the 470 K, no signal would go through the 470K, since the signal wants to choose the "easiest path"...

Also, if the center leg of the trim pot is grounded, the total resistance to ground would be 200K, and the total resistance to ground from the OD1 side, would be only 22K. I assumed that this would make all the signal go to ground.

Tommy

[kiwiandy]

Hi again Tommy,

Electrical signals don't just choose the path with the least resistance and ignore others. They flow through any available path, and the resistance determines how much can get through.

This might be a bit philosophical (yuck!) for a guitar amp forum, but the idea helped me when I was learning. If you think of the signal as water flowing in a river- when there is a fork in the river, the continues flowing through both paths.

Andy

[Tdale]

That's a good way of describing it.. I've used that myself somtimes, when trying to understand the current flow.

One question though.. No matter what OD entrance you use, you'll end up with a certain amount of resistance before the signal meets the grid resistor of OD1..

Obviously, there are several ways of creating this total resistance.. The easiest one is to just add a resistor. The next way is to use a resistor and a trim pot, so that you can adjust easily. And then there is the way it's done on the lower part of my attachment..One resistor across and two that are grounded through a pot, or just grounded directely.

Is there any way that these different ways of doing it, can sound different? If 20% of the signal gets through to the grid resistor, does it matter how it got there?

I guess it does, since people do it....but is there any way of explaining why?

Tommy

[oldmacman]

If 20% of the signal gets through to the grid resistor, does it matter how it got there?

What matters is the total resistance to ground. For example, a voltage divider with 80K/20K gives 1/5 of the voltage at its output, as does 160K/40K, as does 800K/200K.

But they produce different frequency responses in the high-pass filter with the coupling capacitor. Higher resistance to ground, more bass gets passed. Lower resistance to ground, more bass gets cut.

[Tdale]

Intersting point..

What would happen if I didn't ground it at all, but simply ran a 200K resistor from the clean output, directely to the grid resistor of OD1, without any path to ground?

Tommy

[mlp-mx6]

A phenomenon called "grid blocking distortion" is what would happen. Sounds UNBELIEVABLY BAD.

Basically what happens is the grid "fills up" with electrons. Hard for any signal to pass at all - but *some* does. Did I mention it sounds UNBELIEVABLY BAD?

[Icetech]

basically clogs its self with electrons like too much water into a pipe?

[Tdale]

Good analogy!

Tommy

[oldmacman]

It wouldn't work at all.

The grid needs a reference to ground so that the tube biases properly. The resistance to ground keeps the grid at 0 V, and the current through the cathode resistor puts the cathode at some positive voltage, so the grid is negative with respect to the cathode.

If you don't reference the grid to ground at all, the grid builds up a negative charge from the electrons that hit it - since there is no connection to ground, the electrons have nowhere to go. Eventually, it goes negative enough that the bias goes way cold (into cutoff for part of the waveform), and the sound is terrible.

[Bob-I]

A phenomenon called "grid blocking distortion" is what would happen. Sounds UNBELIEVABLY BAD.

Basically what happens is the grid "fills up" with electrons. Hard for any signal to pass at all - but *some* does. Did I mention it sounds UNBELIEVABLY BAD?

I've done that bey accident and I can honestly say, UNBELIEVABLY BAD is the understatement of the year. IF you get any signal at all, it's terrible and brief.