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Post by ourclarioncall on Oct 12, 2020 22:48:50 GMT -5
robrobinette.com/AB763_Modifications.htm#Frondelli_Master_VolumeNoticing some mods for master volumes and find it intriguing that they can go in different places I thought master volume came right at the end of the signal chain once it had gone through the preamp and power amp tubes but these seem to be before the power amp tubes
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Post by ashcatlt on Oct 13, 2020 10:54:34 GMT -5
I’ve never heard of a master volume coming after the power tubes. That sounds dangerous to me.
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Post by ourclarioncall on Oct 13, 2020 11:08:04 GMT -5
I’ve never heard of a master volume coming after the power tubes. That sounds dangerous to me. Sorry , that’s probably my mistake ! Apologies, that’s just one of the many funny ideas I must have picked up through making assumptions Good job you guys are here to straighten me out |
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Post by ourclarioncall on Oct 13, 2020 16:19:49 GMT -5
One an amp with with one volume control
What is it that is being amplified ? The pre amp tubes or the power amp tubes ?
Maybe I should give an example of an amp but I can’t think of any ! I’m assuming it doesn’t matter but that may well another bad assumption!
Say an early fender or Marshall
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Post by ourclarioncall on Oct 13, 2020 16:23:14 GMT -5
My next question will be about master volumes
Which I thought were controlling the power amp section?
I’m just trying to get a foundation so I can go back to the who what when why of the variety of different master volumes
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Post by thetragichero on Oct 13, 2020 19:06:36 GMT -5
on non-master volume amps the volume (gain) generally comes after the first preamp stage
master volume amps generally label that control "gain" with the master volume coming either before or after the phase inverter
placing some sort of pot after the tubes or transformer would create a number of issues with impedance (not to mention requiring a mondo pot due to the current through it). you also wouldn't be able to turn it all the way down because the power transformer primary provides the high voltage to the power tube plates. shorting that to ground would be disastrous
now there are fancy vvr (variable voltage reduction) that reduce the power to the output stage to attenuate volume but i believe they require semiconductors
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Post by ourclarioncall on Oct 13, 2020 19:35:35 GMT -5
on non-master volume amps the volume (gain) generally comes after the first preamp stage master volume amps generally label that control "gain" with the master volume coming either before or after the phase inverter placing some sort of pot after the tubes or transformer would create a number of issues with impedance (not to mention requiring a mondo pot due to the current through it). you also wouldn't be able to turn it all the way down because the power transformer primary provides the high voltage to the power tube plates. shorting that to ground would be disastrous now there are fancy vvr (variable voltage reduction) that reduce the power to the output stage to attenuate volume but i believe they require semiconductors Thanks. Makes sense. So is the master volume also amplifying the signal ? Also, just out of interest, here is a vid of how to make a master volume that goes In the effects loop , I think. Which I’m guessing is just a volume pot that is not amplifying the signal but bleeds it off to ground (like a volume pot on a guitar?) I notice on this amp which I think has two channels and has a gain and master volume for each channel , I think , I’m wondering where in the signal chain this diy master volume is. If it’s in the effects loop then I think it’s after the preamp and before the power amp section But is it before the master volume on the amp ? Sorry I’m Asking a lot of questions here , il take a guess and say I think what’s happening in The signal chain in the vid is there is 1. Preamp volume knob on amp aka “gain” which comes after preamp tubes 2. DIY master volume box that plugs in fx loop 3. Master volume knob on amp which comes before or after phase inverter |
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Post by thetragichero on Oct 13, 2020 20:23:21 GMT -5
well no volume controls (and almost all tone controls you'll come across in tube amps) don't amplify. the tubes do that. they actually load down the signal and REDUCE the amplitude of the signal, even when full up (even if it's just ever-so-slightly)
capacitors, resistors (including potentiometers), inductors, diodes are called passive components as (to grossly simplify it) they can not amplify a signal themselves
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Post by ourclarioncall on Oct 13, 2020 20:33:15 GMT -5
well no volume controls (and almost all tone controls you'll come across in tube amps) don't amplify. the tubes do that. they actually load down the signal and REDUCE the amplitude of the signal, even when full up (even if it's just ever-so-slightly) capacitors, resistors (including potentiometers), inductors, diodes are called passive components as (to grossly simplify it) they can not amplify a signal themselves Aaaahh ! Mind blown 🙂 Ok, now that makes sense to my brain , hooray I understand. Thankyou 👍 👍 |
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Post by thetragichero on Oct 13, 2020 20:50:24 GMT -5
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Post by reTrEaD on Oct 13, 2020 21:09:23 GMT -5
I’ve never heard of a master volume coming after the power tubes. That sounds dangerous to me. Well, there are such things but they aren't very popular. L-pads can be used for low power (a few watts) applications. Attenuators and 'power sinks' are available so you can crank a tube amp but not be heard in the next county. But those don't have infinitely adjustable controls. They are step-wise with a few switch settings. And they tend to sound a bit different than having the speaker connected directly to the amp. |
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Post by ourclarioncall on Oct 13, 2020 21:13:42 GMT -5
Thanks. I had a look at some of the Robinette stuff previously. It’s great stuff , just a bit to deep for me. My Brain doesn’t seem to be able to retain info. I read and getting a basic understanding but because it’s not a complete understanding I lose it and have to relearn it again. I am learning lol, but it’s very slow. I watched tons of amp vids on YouTube , I found that very helpful. I will probably keep revisiting sites like Robinette and the other one you linked and I’m sure it will start to click and make more sense . |
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Post by ashcatlt on Oct 14, 2020 11:18:08 GMT -5
Yeah for the most part the actual gain stages in a guitar amp - whether preamp or power amp - are fixed gain. The volume controls just attenuate things in between. I’ve always kind of wondered why that is. There are ways to change the gain of a tube stage either by changing the one resistance to ground or via negative feedback, but nobody ever does that. (  ) Anyway, a volume pot (in a guitar, amp or otherwise) doesn’t “dump” anything to ground. It’s a voltage divider. There’s a little bit of Ohms Law involved in describing exactly how it works, but basically the full input voltage drops across the whole resistance of the pot, and when the wiper is somewhere in the middle, some of the voltage is dropped across the top part of the pot and the rest is dropped across the bottom and that bottom part is our output. |
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Post by reTrEaD on Oct 14, 2020 13:46:24 GMT -5
The volume controls just attenuate things in between. I’ve always kind of wondered why that is. There are ways to change the gain of a tube stage either by changing the one resistance to ground or via negative feedback, but nobody ever does that. ( ) Ash, I think the reason other methods aren't employed has to do with creation of noise and/or the inability to reduce the signal to zero. For instance: since a common-cathode amplifier is essentially a voltage-controlled current source, you can change the gain of the stage from zero to some rather high number by using a rheostat as the plate resistor. The problem arises if the wiper contact ever becomes intermittent. Even with no signal present at the input, the sharp changes in DC voltage at the plate will result in a HUGE amount of noise. In and inverting amplifier (like a common-cathode tube stage) you can use negative feedback reduce the gain to unity but not less than unity. That's less than ideal. But the huge benefit is that the more NFB used, the more the 'warts' (distortion) is cleaned up. That's pretty cool. Another problem with varying the amount of NFB by use of a pot or rheostat is that if the wiper becomes intermittent, you can experience huge changes in gain. If there's no signal applied, that isn't a problem. But if the rheostat is essentially the feeback resistor, going from a low value (lots of NFB and low gain) to open circuit (no NFB and full gain of the stage) means you'll be blasting a lot of signal through even though the pot setting was meant to be low-volume. An attenuator volume control is inherently less problematic if the wiper becomes intermittent. Worst case is if the pot is dimed. The signal will intermittently drop out when you wanted a loud volume but that's far less ugly than having the signal intermittently blast when you wanted low volume. I suppose you could use a LOT of NFB and get unity gain, then use a rheostat to shunt the NFB to get more gain. That way if the wiper went open you'd just drop to unity gain. Not such a big problem. But again, the minimum gain would be unity, not zero. *shrug* |
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Post by ashcatlt on Oct 14, 2020 15:35:06 GMT -5
I guess that makes some sense, but we do these things in pedals all the time. Seems like if the amp was loud enough, we’d have similar issues with the gain pot on a fuzz face. shrug.
One thing I don’t think most people really realize is that with a lot of classic amps, where the volume (whether channel or master or whatever) comes after the fixed gain input stage, the amount of actual preamp overdrive is completely dependent on the level of the input. If you turn the amp’s V pot up and get more distortion, it’s either the phase inverter or power amp (or both) that’s doing it. The preamp tube just does what it does, and if you want to hit it harder, you need to change something before the amp.
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Post by ourclarioncall on Oct 17, 2020 2:59:42 GMT -5
One thing I don’t think most people really realize is that with a lot of classic amps, where the volume (whether channel or master or whatever) comes after the fixed gain input stage, the amount of actual preamp overdrive is completely dependent on the level of the input. If you turn the amp’s V pot up and get more distortion, it’s either the phase inverter or power amp (or both) that’s doing it. The preamp tube just does what it does, and if you want to hit it harder, you need to change something before the amp. Ah! Lightbulb moment . Makes sense. So why have a master volume pot if it’s just doing the same thing as the channel volume pot ? I always thought the channel volume (pre amp ) controlled the amount of overdrive coming from the preamp tubes . Then that signal , whether full or finished slightly by turning down the volume , then fed into the power amp output tubes which are controlled by the master volume pot. |
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Post by ourclarioncall on Oct 17, 2020 3:13:13 GMT -5
Yeah for the most part the actual gain stages in a guitar amp - whether preamp or power amp - are fixed gain. The volume controls just attenuate things in between. Okay, so let’s say the volume control which comes after the preamp tube and before the output tube Is FULL UP. Then we are hearing the Fixed full volume of the Preamp tube being amplified by the output tube with all of its might . But if you start turning down the volume , then what’s happening ? Are you lowering the signal coming from the preamp tube and therefore the output tube has less to amplify so the volume coming out the amp gets quieter? |
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Post by ourclarioncall on Oct 17, 2020 3:16:49 GMT -5
Anyway, a volume pot (in a guitar, amp or otherwise) doesn’t “dump” anything to ground. It’s a voltage divider. There’s a little bit of Ohms Law involved in describing exactly how it works, but basically the full input voltage drops across the whole resistance of the pot, and when the wiper is somewhere in the middle, some of the voltage is dropped across the top part of the pot and the rest is dropped across the bottom and that bottom part is our output. So the voltage that is dropped across the top part of the pot Where does that go ? I thought that went to ground ? |
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Post by Yogi B on Oct 17, 2020 7:37:09 GMT -5
So the voltage that is dropped across the top part of the pot Where does that go ? I thought that went to ground ? I was going to reply to your capacitor discharging thread, but I reckoned the essence of what I had to say had either already been said or would only muddy the waters. However, as only voltage & ground are in question here, that should help to keep my answer fairly succinct. The first thing I'm going to do is encourage you to is either replace the word "voltage" with "(electric) potential difference" or to strongly equate the two in your mind, because the word "difference" here is important and I think a key part in what you are missing with your "where does the voltage go?" questions. Voltage (or, as I say, potential difference) can change, but it does not (or rather cannot) move. Voltage is a relative measure, by which I mean it is measured between two points, and describes how much electrical charge per time (current) will flow through those points depending on the resistance/impedance that is connected between them. (This is essentially a restatement of Ohm's law.) Despite the fact that people often label or state that part of a circuit is at a certain number of volts, you can't have some isolated 'lump' of voltage at a single point. What they mean is that the voltage relative to a common reference point, usually ground or another terminal of the power supply / battery, is a certain number of volts -- this is somewhat similar to how we often measure altitude from sea level. A component "dropping" voltage, simply means that there is a voltage difference across the component, thus one end of the component has a lower voltage difference to ground than the other end. Continuing the analogy "dropping" voltage is similar to dropping altitude, if I start at the top of mount Everest and drop 1000m I'd then be at 7848m above sea level, but the altitude of the top of the mountain hasn't changed relative to sea level. (Nor has any altitude "gone" anywhere.)
Ground is just an electric equipotential, a region that is assumed to be at a single shared voltage (relative to anything else) throughout the entire range it encompasses (in other words: it is assumed that the voltage difference between any two points of ground is zero volts). And again, much like the idea that sea level is assumed to be at an equal altitude around the world. Mostly, ground is only really relevant because humans have this rather unfortunate habit of being connected to ground, or at least it's an unfortunate habit if the human in question also happens to be connected to something which isn't ground. Because voltage is a relative measure and ground is such a common reference point people often refer to ground as being 0V (zero volts), but really all that means is that there is no voltage difference between ground and itself. Note that this does not mean that if a voltage difference of 0V is measured between two other arbitrary points then both those points are ground; just that the voltage of each of those two points relative to some other point (e.g. ground) is equal. This shared voltage difference to ground could be 0V, but it could also be several thousand volts. (Going back to the altitude analogy and Everest, at a half to noon on May 29 th 1953, the difference in altitude between the feet of Edmund Hillary and the feet of Tenzing Norgay was 0 metres, but both were most definitely not at sea level.) |
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Post by ourclarioncall on Oct 17, 2020 12:00:45 GMT -5
Thanks Yogi , read this a couple times. Won’t pretend to have understood completely but there is a few rays of light coming in and I can sort of see what I need to study more. 👍
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Post by ashcatlt on Oct 17, 2020 13:21:04 GMT -5
Yogi's answer is really good! I'm gonna attack it from a slightly different direction. I kind of hand-waved the Ohm's Law thing, but lets do it...
So we can think of the pot as two resistors in series with each other between "hot" and "ground". We have a certain voltage difference between the two extreme ends, which means a certain amount of current runs through them. ALL of the current runs through BOTH of them because, well, it has to. Ohm's Law tells us that a given amount of current through a given resistance makes for a given voltage difference across that resistance.
So we have the input V across both R1 and R2 which causes a certain amount of current I to flow:
V = I * (R1 + R2)
I = V / (R1 + R2)
All of I is going through both resistors, so the voltage difference across each one can be figured with Ohm's law again:
VR1 = I * R1 and
VR2 = I * R2
If you add those together, it will equal our original V. The wiper of the pot is between R1 and R2 and connects to whatever our load is, which is generally wired so that its top is the wiper and the bottom is "ground" which is the same place the bottom of R2 is connected (assuming it's R1 over R2, which is not always the way you see it designated). So the voltage that appears across the load will be the same voltage that is across R2. Of course, the load is actually in parallel with R2, and will make it look smaller, which sort of changes everything, but we usually make sure that the load is enough bigger that it doesn't change it enough to worry about, especially in this kind of "napkin scribbling".
Well, if there's more than one channel then they're not actually doing the same thing. The channel volumes can be used to balance one against the other kind of like the individual faders on a mixer. Then the master volume adjusts the overall volume of those. No, we don't normally actually mix the two channels like with a different input on both, but even if you're switching back and forth, the point is about the same - channel volume to adjust the relative level between the channels, and master to make those both louder or quieter at the same time.
But then too in the (many) cases where the "preamp" volume comes before the phase inverter and the master comes after, the preamp volume will adjust how hard you hit that phase inverter tube, which is yet another point where you might get some overdrive.* Then there are some amps where the channel gain does actually either adjust the gain of a stage or in between a couple of stages or after a stage but before some clipping arrangement or whatever, and in those cases, it actually will have an affect on the overdrive/distortion generated in the preamp itself.
Basically, specifics matter and there are lot of different designs out there. Most of the classic Fender, Vox, and Marshall amps are pretty similar, but there are plenty of exceptions and innovations that have cropped up through the years so it's a little tough to generalize.
* And even that is kind of ignoring the fact that some channel volumes (like in Vox Top Boost, the Bright channel on some Fenders...) actually affect the frequency response, usually cutting bass when turned down from full.
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