2 ways to wire a volume pot, what's the difference?

[Willabe]

How do these work as far as loading the 1'st stage feeding the volume pot and the 2'nd stage it feeds?

Will the 2 sound or act different?


               Brad     

[SILVERGUN]

I don't know BUT it belongs on an IQ test somewhere

My brain hurts just looking at it

[tubeswell]

'A' leaves a constant AC load on the preceding stage (and the wiper is 'tapping-off' a part of that load for the grid of the following stage)

Whereas with 'B' the AC load on the preceding stage varies depending on where the wiper is.

The difference in sound on a geetar amp would be subtle methinks.

[jeff]

Good question, I'd like to see what the experts have to say.

 I was just skimming last night and found a model on how to calculate gain from a gain stage. I don't have the formula on hand right now but both the plate resistor and the following resistor to ground(IE the next stages grid resistor) play a part in how much gain you get from a stage. The plate resistor's and the following resistor's values, in parallel, determine the load and thus determine how much gain you get.

 So, in the first one[Constant 1M to ground] you get a constant amount of gain from the first stage and the volume pot allows you to send a certian percentage of that gain to the second stage.

 In the second[Variable value to ground] you are actually changing the load of the first stage therefore changing the amount of gain the first stage produces.

Like I said I hope the experts have more to say about this.

The tweed Deluxe used the second option and some say that's part of it's sound. If you have two channels doing it that way saves two isolating resistors($$$) by seperating the two channels using the vol pots.(IE some les pauls also use this so when in middle position turning one vol down won't kill signal)

....I wonder what having a volume pot and a variable resistor across that pots outer lugs would do for you. The VR would control the amount of gain the first stage produced and the pot would control how much of that gain gets to the second stage?...

Good question I really like to know more too!

[kagliostro]

....I wonder what having a volume pot and a variable resistor across that pots outer lugs would do for you. The VR would control the amount of gain the first stage produced and the pot would control how much of that gain gets to the second stage?...

If I remember well some time ago (not few time) there was a tread about the use of such configuration

May be RicharD and/or DummyLoad were talking about

but my memory is every day worse

K

[Willabe]

So, in the first one[Constant 1M to ground] you get a constant amount of gain from the first stage and the volume pot allows you to send a certian percentage of that gain to the second stage.

OK, but doesn't changing the grid return R's on the 2'nd value change it's gain?

In the second[Variable value to ground] you are actually changing the load of the first stage therefore changing the amount of gain the first stage produces.

So either way, in A it changes the gain of the 2'nd stage and in B it changes the gain of the 1'st stage?


                    
                            Brad        

[sluckey]

in A it changes the gain of the 2'nd stage

No. The gain of the second stage is fixed by the tube and other components, not the pot on the grid. The pot is simply a voltage divider that feeds a variable portion of the signal to the grid input. For example, say the tubes gain is 20. Set the pot to feed 1 volt into the grid and you will get 1 x 20 = 20 volts out. Or set the pot to feed 2 volts to the grid and you will get 2 x 20 = 40 volts out. The gain remains constant and always the output = input x gain, unless you hit saturation or cutoff.

[jeff]

What I think is happening is the first setup changes the level of the signal the 2nd tube recieves
so it outputs a bigger or smaller "copy" of what the first tube is doing.
The second set up actually changes how the first tube operates/responds by changing the load line.

I've always considered the 2nd schematic to be wrong but this gives me a great idea for a "flavor" control. I'll need some time to experiment, but this could work.

[HotBluePlates]

How do these work as far as loading the 1'st stage feeding the volume pot and the 2'nd stage it feeds?

Will the 2 sound or act different?

Everyone has danced around the differences. So let's wrap them up in one shot.

Volume A:
Standard volume control, sends a bigger/smaller signal to the following stage by acting as a voltage divider for the a.c. signal output of the stage before the volume control. Note that the preceding stage always sees the total resistance of the pot to ground regardless of the control setting.

Volume B:
Quirky volume control, a la tweed Deluxe. Reduces the volume by loading down the signal. As the control is turned down, the wiper is closer to ground, tending to shunt some of the signal to ground, while an increasing resistance is present between the wiper and grid of the next stage. Depending on the value of the pot and the Miller capacitance of the following stage, this might shave extreme highs in the signal (but probably won't unless the pot is a very large value and/or input capacitance of the following stage is very high; look at it something like a grid stopper).

But it also has a second effect: as the control is turned down, the resistance seen by the preceding stage gets smaller (this will be the resistance from wiper to ground). This places a heavier load on the preceding stage, which will have a tendency to reduce its gain, with more gain reduction the further the control is turned down. Depending on the design of the preceding stage, this may have little/large effect, and in extreme cases could change how/when the preceding stage distorts. However, in properly-designed circuits (read: most amps implemented as factory-standard), this will likely be a small effect.

[Willabe]

No. The gain of the second stage is fixed by the tube and other components, not the pot on the grid. The pot is simply a voltage divider that feeds a variable portion of the signal to the grid input.

The gain remains constant and always the output = input x gain, unless you hit saturation or cutoff.

OK, so it's the KR, AR, tube and the B+dcv that set the gain, not the grid return R.

The difference in sound on a geetar amp would be subtle methinks.

Depending on the design of the preceding stage, this may have little/large effect, and in extreme cases could change how/when the preceding stage distorts. However, in properly-designed circuits (read: most amps implemented as factory-standard), this will likely be a small effect.

The tweed Deluxe used the second option and some say that's part of it's sound.

This is why I asked. An author (Texas) wrote years ago that it made a big differance in the sound in a tweed Deluxe. Listed a name for wiring up a volume control that way. Said it was a mistake on Fenders part that turned out to have a good effect on the tone of the circuit/amp. Always wondered about that.

Thanks for all the answers guys!


                   Brad       

[Willabe]

If you have two channels doing it that way saves two isolating resistors($$$) by separating the two channels using the vol pots.

As many times as I've stared at a 5E3 schemo through the years , even built one, I never noticed that or saw it that way before      and I should have.    

That could come in handy some day.

(Thanks Jeff, your growing fast!)



                Brad      

[sluckey]

That could come in handy some day.

Tubenit wires his reverb pots that way. His stuff sounds good.

[Willabe]

Tubenit wires his reverb pots that way. His stuff sounds good.

Good point. That could be another good place to wire a volume pot up that way.

KOC shows the verb pot wired that way in 1 of his TUT books. He likes having the grid return R staying at a constant value.

His stuff sounds good.

Yes and then some.    


                  Brad      


Edit; Fixed quote.

[12AX7]

I'm the stupid guy here. Don't know much theory and all that, but i DO know what sounds good. And i have experimented a lot with using set voltage dividers vs a simple ground shunt to reduce gain when going for a set amount of gain. (actual gain pot further upstream. Over the course of a few years i have played around with both on a particular cascaded mid/hi gain marshal style preamp, and it has become more and more apparent that i always much go back to the simple ground shunt resistor. It just somehow works better tonally no matter what i do. This preamp has seen many different changes, but that one thing always remains consistently better sounding than a voltage divider. Like i said, i'm electronically a idiot so i don't know the technical reasons, i just know i have spent a long time perfecting this amp and it just works best like that. Come to think of it, maybe i should try replacing the gain pot with A above ! I may have to try that. i suppose the taper would be bizarre tho.

[Willabe]

Come to think of it, maybe i should try replacing the gain pot with A above ! I may have to try that.

A is the normal way to wire it, B is the uncommon way. Did you mean B?

i suppose the taper would be bizarre tho.

Why? Tapper is still the same, as long as you don't wire the O/X ends of the pot backwards. It's just switching the pots wiper from output (A) to input (B).


                   Brad     

[12AX7]

Yes, B not A....sorry.

On the taper, i realize the pot's taper is the same, but it just seems a voltage divider would give a different degree of gain reduction at a given point in the rotation than a shunt setup. Not saying i'm right, just seems like it world. I'm going to surely try this tho. One issue tho....i like a treble bleed carefully chosen, so this would eliminate that possibility and may be a a deal breaker for me. But i gotta try it. If it doesn't change the tone as much as a VD i suppose could just add another R/C filter if the tone is darker than it was with the regular divider pot.

EDIT: tried it...no way....Guess it's all about where you do it.

[HotBluePlates]

... An author (Texas) wrote years ago that it made a big differance ...

They say everything is bigger in Texas...

That particular author has a way of exclaiming that everything makes enormous differences. So maybe, maybe not.

But the important part of what he was saying is that the two channels of the 5E3, both wired with type B volume controls, causes interactions between channels that can make major tonal changes.

To hear the effect, set one volume knob of your 5E3 to full volume, the other to half volume. Aside from distortion, you will hear a very different midrange tone in the two channels.

You would not hear that with only 1 channel or 1 volume wired this way. That's because it's not the volume wiring changing the tone, but how the volume wiring changes isolation between two channels which then changes the tone.

[jeff]

 I really wonder though, in the Deluxe, if most that interaction has to do with the way the tone knob is wired not necessarily how the volume pots are wired or if it's one or two channels.
 picture this:(If I'm looking at the right schematic)
You're plugged into the lower input with the volume full up and the tone full up. Then turning the other volume down is actually a treble bleed- shorting the highs through the .0005 cap. Know what I mean? Ignore the top triode and it's .1 cap, just picture the tone and upper volume pot not connected to a second channel. That volume pot (as far as the lower channel is concerned) is part of the tone circuit. It's a tone knob. I think that volume would be interactive weither the top triode was there or not and doesn't have to do with the fact that two channels are being mixed. Remove the tone knob and I suspect you remove the interaction.

So yes the deluxe's volumes are wired this way, and yes, it's controlls are interactive but that doesn't necessarly mean the controls are interactive because the volumes are wired that way. It's like, the rooster 'cock-a-doodle-doo's, the sun rises, but that doesn't necessarly mean the rooster causes the sun to rise(maybe it does over Texas). I don't have one but if you'd like to do me a favor and try an experiment please try disconnecting the .0005 cap and see if the volumes are still interactive. I'm curious if the effect is caused simply by the asymmetry of the tone circuit with respect to the two channels and not the fact that there are two channels. Or better yet remove the top channel's .1 coupling cap and see if you still get that interaction with only one channel in the circuit(and if you like that interaction you could build a one channel amp with tweed Deluxe controls!)

When I brought up the Deluxe I was thinking more volume knob itself, not even thinking/remembering about the interaction. How people say that it is very touchy turning it up. "A" will be a smoother transition from off to full where as "B" will tend to dump more volume/gain eariler. I never tried a Deluxe but I tried wiring a guitar like B and that's what I found.

[HotBluePlates]

Then you take your 53, disconnect the tone control, and try it. I'd tell you for sure, but my 5E3 is several states away.

[jeff]

I don't have a 5E3, and I don't know if I'm right, that's why I was asking for the favor, but does that seem to make sence?

 I always saw the Deluxe as being wired "wrong" and people have exploited this flaw by playing with all the knobs when using one guitar. But thinking about it the way you pointed out, is it possible that the tone control isn't wired "wrong" but is supposed to be interactive?
This amp was designed for two inputs/guitars to be used at once. Assuming you aren't on 10, if you plug one guitar into 1 and the other into 2 turning the tone down makes the tone of both darker. But does turning the tone up actually make one channel brighter while at the same time making the other darker? Not a flaw(as I first saw it) but a way to balance the two guitars? Is it possible that's what Fender intended?

Glad you're here HBP you always make me see stuff in a new way.

 Much respect
       Jeff

[HotBluePlates]

There's nothing wrong with the tone control, and it's wired exactly like the tweed Princeton. Just the design of a simple 1-knob tone circuit.

I wouldn't say the interaction is the result of bleed to the tone control, based on my experience with my 5E3 (but without doing the definitive experiment I suggested).

If the channel other than the one you're using is mid-way up, it results in a scooped midrange for the channel being used. If the "other channel" is full up, it results in maximum midrange for the used channel.

[thelonious]

Just out of curiosity - in 'A', doesn't the total resistance of the pot form the highpass filter with the coupling cap? So wouldn't 'B' also change that resistance and result in a different corner frequency depending on where the pot is set?

[HotBluePlates]

Just out of curiosity - in 'A', doesn't the total resistance of the pot form the highpass filter with the coupling cap?

Yes, the pot resistance in series with the total effective resistance represented by the prior stage's plate load resistance and tube internal resistance in parallel. That's a mouthful, so let's call the pot Rpot, the previous plate load Rl and the previous stage's internal plate resistance Rp.

F -3dB = 1/[2*pi*(Req+Rpot)*C]

where Req = 1/[(1/Rp)+(1/Rl)]

In other words, the total output impedance of the prior stage is the internal plate resistance in parallel with the plate load resistor, and when calculating the -3dB frequency the cap sees both the resistance in front of it as well as behind it.

But yes, as you turn Volume B down, you will be cutting bass more and more.

[jeff]

Ahh good point, I didn't see it that way.
I think I need to build this ciruit and play with it a little to see what you mean.

[12AX7]

I hate to mark myself as an electronic idiot, but what the heck....i am, so there. I looked at this again, and my tiny brain just does not at times seem to see whats going on in the simplest schematics. Long story short, i misread whats going on there. So i then realize it's nothing more than reversing the in and out of the pot. So i tried it and i was surprised to find it sounded pretty good, possibly better. But it's one of those tweaks that needs time to be sure. Often a tweak like this gets me going then a week or 3 later i switch back and wonder what the heck i was thinking. Anyways, going to play around with it a while and later on switch back to the regular wiring and see what i think then.