Some “seriously beginner grade” questions regarding resistors

[Gtr0]

Like the kind someone is afraid to ask in fear of being thought as a dumbass.

I’ve been building amps on and off for almost 20 years (yikes!). Over the last few years I have been learning a lot of the way things actually work (before that it was all based on layout files), but I am severely plagued by a few tidbits of basic info that I think I might understand, but something tells me there's more to the whole picture than meets the eye.

This is in regards to resistors. Any reference to circuit in amplifier, I will use the JTM45 as my reference of choice.

When first learning of resistors, some offer the analogy of a “hole in the dam” or “the size of a water pipe” etc. Others talk of resistors as “slowing down the movement”. So going with these analogies, how, or why, does changing the channel mixer resistors (again in the JTM45) from 270k to 470k supposedly increase gain to the second stage?

Another spot is the plate resistor… increasing the resistance increases the gain also - I just can’t grasp that… because… I fully assume that resistors lessen the flow of current and voltage. So obviously there are some super duper basics that I just can’t seem to grasp.

One resistor that makes sense to me is a resistor to drop voltage off of the secondary for a bias supply.

Thanks you, as always, for your time!

[glebert]

Another spot is the plate resistor… increasing the resistance increases the gain also - I just can’t grasp that… because… I fully assume that resistors lessen the flow of current and voltage. So obviously there are some super duper basics that I just can’t seem to grasp.

I don't think this part is a basic resistor question, you are talking about the interaction of a resistor with a tube. Here's how I think of it, right or wrong, happy to be corrected if wrong: Tubes act like a current valve (hey, some people call them valves!) where the current is proportional to the input signal (assuming we are in the linear region). So we get a varying current through the tube with an AC input. The current is coming through the plate resistor. The plate resistor converts the change in current to a change in voltage (more current equals more voltage drop). Increasing the plate resistor creates a larger change in voltage for a given change in current, so there is more gain.

[Dave H]

how, or why, does changing the channel mixer resistors (again in the JTM45) from 270k to 470k supposedly increase gain to the second stage?

It doesn't increase the gain of the second stage. It reduces the loading losses between the first and second stage.

[eschertron]

It doesn't increase the gain of the second stage. It reduces the loading losses between the first and second stage.

So to use the analogies, the original (smaller value) resistor lets more current leak out from stage one's plate to ground. The larger resistor, on the other hand, lets less current "leak out" and that current makes it to the second stage grid to effectively get a bigger signal out of the second stage (gain increase).

[gui_tarzan]

I'm looking at a schematic for an old Harmony that I'm refreshing right now. There is a 470k resistor going to ground from the first preamp tube plate that's going to the second preamp tube grid. That's after the coupling cap of course and the tone pot is also connected there as well. So if I understand (I'm still amazed at the theory side of things) correctly, the 470k would be a route for the guitar signal to go through to ground to control how much of that signal gets to the second tube grid? Being 470k it wouldn't let as much of the signal go to ground as a 100k?

[Helmholtz]

... the original (smaller value) resistor lets more current leak out from stage one's plate to ground. The larger resistor, on the other hand, lets less current "leak out" and that current makes it to the second stage grid to effectively get a bigger signal out of the second stage (gain increase).

There is no grid current and the second stage grid is driven by the signal voltage at the grid side of the mixing resistor. The mixing resistor can be seen as the upper/series part of a voltage divider. THe lower part being composed of the other "channel's"mixing resistor, vol pot and input tube. As the ground path of each mixing resistor runs through the other channel's vol pot, volumes are somewhat interacting.
(Signal and overall peamp gain would be maxed if the other mixing resistor was disconnected.)

[Dave H]

I'm looking at a schematic for an old Harmony that I'm refreshing right now. There is a 470k resistor going to ground from the first preamp tube plate that's going to the second preamp tube grid. That's after the coupling cap of course and the tone pot is also connected there as well. So if I understand (I'm still amazed at the theory side of things) correctly, the 470k would be a route for the guitar signal to go through to ground to control how much of that signal gets to the second tube grid? Being 470k it wouldn't let as much of the signal go to ground as a 100k?

It's a potential divider (as Helmholtz said) only in this case the bottom resistor of the divider is the 470k and the top resistor is the output impedance of the first preamp tube which is about 38k for a typical 12AX7 stage.

[Enzo]

It is all about the circuits, not the parts. REsistors most often work with other parts, other resistors for example. Learn to spot voltage dividers, they are everywhere.

That and Ohm's LAw.

[gui_tarzan]

Ok so the signal goes from the instrument into the volume control in this case, which isn't on my pic, then across the grid in tube #1 to the plate, through the .05 cap to remove DC, then the signal voltage is divided by the tone pot (C) and 470k resistor (D) and then down to the grid of tube #2? I'm having trouble envisioning the path of the signal and specifically why those values.

[Helmholtz]

Ok so the signal goes from the instrument into the volume control in this case, which isn't on my pic, then across the grid in tube #1 to the plate, through the .05 cap to remove DC, then the signal voltage is divided by the tone pot (C) and 470k resistor (D) and then down to the grid of tube #2? I'm having trouble envisioning the path of the signal and specifically why those values.

Ok, a schematic always helps.
The 470k is a grid leak leak resistor necessary to ensure that the grid sits on 0VDC. As there is no DC current there is no DCV drop across it.
I don't think it is intended as part of a signal voltage divider but nevertheless it also acts as such. The ~10% (around -0.9dB) signal loss should be negligible, though.

[gui_tarzan]

Doesn't the .05 cap stop any DC from going through?

[Helmholtz]

Doesn't the .05 cap stop any DC from going through?

Yes, the caps block DC. So there would be no DC path to ground without the grid stopper. Meaning the grid would be floating, making tube behaviour unpredictable. A tube grid needs to be tied to a defined DC potential, being 0V in this case.

[gui_tarzan]

So what would be the consequence of raising or lowering the value of that resistor, or is it simply based on the impedance of the previous tube?

[Helmholtz]

So what would be the consequence of raising or lowering the value of that resistor, or is it simply based on the impedance of the previous tube?

Increasing to 1M (a typical value with ECC83 circuits) would increase signal level by maybe half a decibel. But check the tube data for recommended/max grid resistance value. Often 2.2M is a limit for safe operation.

[gui_tarzan]

That's why theory stuff is so confusing. If the purpose of the cap is to eliminate DC, why would the grid resistor need to exist if there's no DC to destabilize it?