Explain this please

[iknowjohnny]

I'm not ashamed to admit i am a player, not a tech. And this question will make that obvious. I need an explanation of the 1 meg input resistor. heres why. I started thinking about the fact that the tone seems tighter and just sweeter all around with a pedal in the front even when the pedal is off. So it hit me that maybe if i changed the 1 meg R at the input to the same asthe low output impedance of the pedal maybe that would sound similar. I found that it did seem to do what i'd hoped, tho it is subtle. So i started to experiment going from no R at all to where the tone and gain just died, which i believe was somewhere around 50k or less. (put a pot where the 1 meg input R goes)

so this got me wondering exactly what the 1 meg R is for since it doesn't even seem to need any R at all, and it doesn't change much till you get it down to around 100k which seemed to be the point at which the tone seemed to change enough to matter. What also confuses me is that the input R is basically in parallel with your guitar's volume pot, and using a electronics calculator, a 50k in series with a 250k yield around 40k. So why doesn't using a R around 50k change things drastically? after all, wouldn't putting a 50k pot in your guitar change things dramatically? I'm sure i'm missing something and i'm also sure one of you guys will point it out to my tiny brain.

[iknowjohnny]

I should explain the main thing what i'm trying to do here and what i just found after posting the initial post. Although i said i was trying to replicate the sound i get with a pedal in front, even more than that i was trying I have been trying to reduce gain so turning the guitar down cleans up well w/o hurting the high gain sounds. That said, i just tried a 100k from AFTER the 68k input series R to ground. Or in other words, a 100k from the input grid to ground. This seems to have worked quite well in giving me the dynamics i was looking for, yet my high gain tone remains awesome. Is this a common thing? I don't think i've ever seen a R right from the grid to ground. and if it IS used by some, is this the same reason they use it?

[Jana]

That 1 meg resistor is there to reference the grid of the tube to ground. With the cathode resistor, this creates the bias for the tube. Let's say, for example, that you measure 1 volt on the cathode resistor. This is +1 volt. that means that the grid is 1 volt more negative than the cathode which equates to a bias of -1 volt for that stage.

Why 1 meg? Well, if you look at a schematic for the standard dual input arrangement with the two input jacks, two 68k resistors and the 1 meg, you will see that if you plug into the low input jack you don't have that 1 meg to ground, you have 68k.

Why 1 meg? Why 68k?

You have to go back a step and think about the device (guitar, pedal, etc.) that is driving this input. The goal with properly designed circuits is to not have an input loading down an output. The rule of thumb is 10 to 1. Which means that if the output impedance of your guitar is 6.8k (which is about what a standard single coil pickup is) then the input should have an impedance of at least 10 times that, hence 68k.

Following this rule, the input should not change the tone of whatever is being plugged into it (within reason, of course). If you start lowering that resistor too far then you will find that different guitars, different effects, anything you plug into it will react somewhat differently. If the device has a higher output impedance and sees a low impedance input, then it will probably lose some highs or react in other ways that were not intended.

Generally you shoot for a low impedance output and a high impedance input. There are limits, however. A higher impedance input is more susceptible to noise, whether it be RF, stray signals, or just plain noise.

Either that or the wires are too small, lol. Sorry, couldn't resist.

edited typo

[iknowjohnny]

It works with no R there tho, which i was confused about. But i gotta tell you, with a 100k from grid to ground the tone is much more rounded as far as the dynamic range and overall usefullness. If higher impedence is supposed to be the way to go, this makes little sense, no? I'll have to take a ear rest then try it back to normal and see if i still feel this way.

[Jana]

It worked so far with no resistor there because either your guitar or pedal that was plugged into it had a resistance to ground that created the DC reference to ground. What happens if you plug a pedal in that has a blocking capacitor on the output?

Trace out the standard hi/lo input jack arrangement I mentioned. Draw the equivalent circuits for what it would be if you plugged into the high and then into the low.

I never said higher impedance is the way to go, I explained the reasons for it (legitimate reasons at that).

You might want to wire up the hi/lo jack arrangement. This gives you both options. Maybe wire it up so in the low jack you have your 100k to ground and in the high jack it is the normal way.

Be wary of reducing that resistor too far in an attempt to tailor the sound. What you will do is create a setup that will only work for your particular guitar/setup. Plug a different pedal into it and you might wind up with a muddy mess.

[iknowjohnny]

Thanks, i'll remember that and try different guitars.

[iknowjohnny]

Ok, heres another question. I see a 1M resistor on a bad cat amp from the first grid to ground instead of the traditional input to ground. the only difference of course being that the cat's 1 M is after the grid stopper instead of before. What is the reasoning behind this compared to the normal way?

[Jana]

With the 1 meg after the 68k grid stopper this creates a voltage divider which attenuates the input signal slightly. This way of wiring it up is a common mistake.

[iknowjohnny]

Ahhhh....not much of a bad cat fan, eh?

The rule of thumb is 10 to 1. Which means that if the output impedance of your guitar is 6.8k (which is about what a standard single coil pickup is) then the input should have an impedance of at least 10 times that, hence 68k.

that one got past me at first, but i'm glad i re-read your post. I have both a super hot bridge humbucker and vintage singles in all my starts. So this rule tells me 10 times that of my 14k super distortion, or 140k. I have a 180k so i'll use that. for some reason the high gain sounds better with a small input R. So i'm getting the cleaner tones i wanted and still plenty of high gain tones w/o any sacrifice that i can hear at this point at least.