Vox AC30/ 6 (TBX). Add an MV ?

[Helmholtz]

To be precise, the distortion of an original AC30 solely depends on the channel volume setting (and a bit on the tone controls).
A MV will not change that.

Having some (around 10 years of) experience with playing an AC30 I can state that this amp doesn't provide much distortion, even when dimed.
Reason is that the amp only has 2 preamp gain stages, while a typical MV amp may have 3 to 5 gain stages. Also supply voltages are comparatively low.
Many if not most famous AC30 players (including Rory Gallagher and Brian May) use(d) a booster in front of the amp to increase total gain and get sufficient distortion and sustain.

I rarely used the crossline MV in my AC30. Didn't give me what I had expected.

[g1]

Turning a MV knob has no effect on preamp distortion whatsoever.
Seems that misconception or misinformation keeps you from accepting how a MV really works.

Further to this point:
Master Volume can allow preamp distortion by reducing the signal into the power amp.
Again, you need to understand the concept that a MV set on 10 is the equivalent of a non-master amp. You can't get preamp distortion because it's just too loud (when master on 10).
Dial down the master and you can boost up the preamp vol. creating preamp distortion.
The increased preamp volume is what is responsible for the distortion, but the master (at lower settings) allows you to dial the preamp vol up without going deaf.

[Chuck H]

Well... In a way an MV could be considered preamp distortion I think. Only because preamp distortion is what you're limited to when a MV is used. This is often distortion that wouldn't be heard the same or even at all for many amps because the power tubes are clipped by the preamp signal before these distortions happen. But indeed a MV does not (and can not) add preamp distortion. It will always reduce an amplifiers gain and sensitivity to input signal.

NOTE: I'm not trying to correct Helmholtz. I'm trying to explain any misconceptions regarding master volumes and preamp distortion. Misconceptions that happen because of internet gear jargon and a generally poor understanding of guitar amp circuits in this genre.

[Helmholtz]

I consistently read that introducing an MV is to to do with -preamp- stage. MV is often referred to as PREAMP distortion, no??

Turning a MV knob has no effect on preamp distortion whatsoever.
Seems that misconception or misinformation keeps you from accepting how a MV really works.

To those who can read schematics it's all very obvious.

[Helmholtz]

Actually re-reading your "Master Treble Reducer" has just made my head explode. I assume a joke- good for you if so!!

Not a joke.
The involved terms are carefully chosen:
- Master, because the cut control sits in the power amp and controls both channels. As it sits after the preamp, it also affects distortion harmonics produced in the preamp.
- Treble Reducer, because it can only reduce treble as opposed to a normal treble control which can do both: boost and attenuate treble.

[Chuck H]

Great clarification of the box diagram justin. Whatever comes out of box 1 is then controlled by the master volume. In amps designed for maximum clean tone box 1 is not designed for desireable distortion. So whatever distortion you do get with the master turned down will not usually be desireable. Whereas if box 1 is designed for distortion (usually detectable because there will be one or more "gain" controls) it is then the master volumes function to strictly control the level from box 1 to the power amp. So, again, if box 1 is designed for an amp that is supposed to achieve maximum clean volume then it most likely does not create desireable clipping or distortions. And that is what you hear when a master volume is used with such an amp. And on that note...

From the early 70's into the mid 80's there were quite a few vintage type amps that simply added a master volume to a design intended for clean tone at full volume. This was a ham handed attempt by manufacturers to keep up with genre trends. And it didn't usually work very well even on amps from respected manufacturers. In other words, just because Fender added a master volume to their Twin Reverb design in the early 70's DOESN'T mean it was a good idea or that it sounded good. The Fender moniker DOES NOT automatically equal "nothing but good ideas". In fact a popular modification for many early master volume amplifiers is to remove the master volume circuit.

[Justin Thomas]

If the input is next to the first stage bit, & that has Vol 1 written next to it... & I KNOW that the input is the guitar input.. then your Vol 1 in the diagram, is the preamp section: & I consistently read that introducing an MV is to to do with -preamp- stage. MV is often referred to as PREAMP distortion, no??

This is one reason why I'm confused g1.

Ah, okay. In g1's diagram, and incorporating Chuck's point about vintage vs. modern designs:

In a vintage design such as your Vox or your Twin Reverb, that first & other early stages were designed to get as much CLEAN headroom as possible to pass on to the volume control. In a modern design with a distortion channel, those early stages are designed to DISTORT a LOT more.
That difference of intent & hpw a designer goes about accomplishing their intent is what Chuck meant by "gain structure."

So in g1's diagram, that first "box" represents either a preamp that is designed to make as clean a signal as possible OR a preamp that has just ridiculously high amounts of "gain" (in the sense that I was using it in my long post). So in g1's diagram that first box could represent a single or 2 gain stage or five cascading ones.

So "whatever" happens in that first box, whether clean sound as much as possible or mega-crunch distortotron, is then sent to Vol. 1. That Vol. 1 is what sets the level of signal from Box 1 i to Box 2, which would be another "gain" stage, which would mainly he designed for maximum headroom - this stage is probably desingned intentionally to NOT distort, but to provide maximum signal for the power tubes for maximum loudness if desired. Which is why this one is called Master.

So maybe a good way to think of it is that, a Gain control deals with what KIND of signal comes out of the early preamp stages; a Master control deals with how MUCH of that early preamp signal gets sent TO the power tubes. So in G1's diagram, Vol 1 "deals with" the preamp and "gain structure" & Vol. 2 "deals with" the power amp.

But regardless of an amp's design or whatever they call the knobs, the Master is pretty much whatever knob ULTIMATELY controls the loudness of whatever else comes out of the speaker. In some ancient designs that would be the Volume knob on your guitar.

Jusrin

[Chuck H]

Chuck HPara 2: When you say "Once the preamp does start clipping (after the power tubes are already clipping) you now have plenty of signal to drive the power tubes into clipping comig from the preamp".. I seem to find a contradiction: the preamp tubes clipping -after- the power tubes are in the opening section of this sentence, & this seems counterintuitive as I thought the preamp xyz always preceded the power tubes xyz. And to back this up, then '..power tubes into clipping coming -from- the preamp' seems back to logic/ intuitive/ as I expect.

So again, like the Vol 1&2 reversed logic g1 gets me in a pickle over, the exact same reversed logic is here: & furthermore you seem to even be stating revered logic, within one sentence.

I may have a bead on where some of the confusion is. Your perception seems to be that the preamp should set the tone characteristics (distortion level and equalization) and then the master volume should set the volume. To your credit this is exactly the case with modern high gain amplifier channels.

But even these amps usually include a "clean" channel which would be designed to send a big enough clean signal to the power tubes so that the power tubes distort before the preamp. This is also how all vintage designs were made. Without this arrangement you could not get a clean tone at the amplifiers full volume. Now...

If you add a master volume to a "clean channel" on a modern amp (as some do) or if you add a master volume to a vintage amp THEN what the master volume does (when turned down) is to defeat the circuits ability to send enough clean signal to the power tubes to drive them into distortion. So now, since the power tubes will not be distorting, what you have left is what the preamp actually sounds like. Even to the point where the preamp distorts. This would never be heard if the master volume were not turned down. And strictly preamp distortion for "clean channel" modern amps and vintage design amps does not generally sound as musical as power tube clipping. That said...

Already stated above, a modern amp distortion channel (or "drive" channel or "gain" channel, etc.) is designed to create clipped and/or otherwise distorted tone that is then amplified by the power tubes. In this case a master volume only controls loudness. With the design goal being that a distorded tone must now be amplified to full volume. So this is indeed inverse of how clean channels and vintage designs are set up. And this is why master volumes used on clean channels and vintage designs often negatively affect the sound from the amp.

Even though master volumes used on high gain designes generally do not negatively affect the sound. only the volume. This is because moder high gain preamps are designed to generate "desireable" sounding distortions before being amplified by the power tubes. Where most clean channels and vintage design preamplifiers are NOT designed for desireable sounding distortions. Rather, they are designed to drive the power tubes into desireable sounding distortions. This is a hugely different job and adding a master volume to this type of amp actually defeats it's ability to sound as good as intended in it's design.

[Sea Chief]

Justin Thomas I'm going through your previous comprehensive post, hugely appreciated.

I am though, stuck fairly early on, even with this: you see sinewaves & squared off bits of it (I vaguely know this = clipping) to consider ontop, is too much for me when I haven't got the basics beforehand. I'm also confused as to why I'm told my AC30 isn't very good for an MV addition, when I know it has additional preamp tubes to other amps (I do know that the Top Boost, for eg, adds one 12ax7 itself, & the very idea of this WAS to provide an additional gain stage).. so another black & white counterintuitive situation, yet another, it seems to me lands on my plate.

Tbh I honestly just don't think I'm ever going to understand it.

I'm just going to have to make something, my Pro even kindly said he'd make me a VVR thingy for me to put in (this idea is my 3rd choice), without understanding the basics of Gain/ MV etc. Heck, I've built a Twin Reverb without knowing what a tube actually does! (again: tried & tried but nothing clicks really- they just seem like little magical mystery bottles!!).

Very kind, SC

[Sea Chief]

But the goal of an attenuator <IS> to keep the critical tone elements of power tube clipping in the equation, which is why I and others so heavily advocate for them.

But! Like all things in life, trying to tame a genuine experience is going to water down SOMEthing. No attenuator is perfect but some will definitely perform better than others at preserving the full "cranked amo experience." And in <my> opinion, any attenuator is preferable to adding a Master Volume to an amp design that was never intended to have a Master in the first place.

Jusrin

Hi Justin,

I have always known this: an attenuator is far easier to comprehend as it's a physical box -after- the whole amp. I've even built one, but it seemed lacking/ I may resort to rebuilding it entirely, just to satisfy myself that any mistake can be corrected like so. This attenuator idea is my 4th choice, well, becuase it strains the tubes: I do not want (or can afford) to fork out £80 on 4x el84's every 2 years. So I may make one for occasional use, only. Which kinda defeats the point a bit.

Thanks alot, SC

[Sea Chief]

Chuck H Thanks again for trying. Ok if I break down your recent post into 6 paragraphs. Some I understand, I've always understood too.

Paragraph 1: always knew this.
Paragraph 5: again I knew this, & so never expect either TR or AC30 in their raw state to OD at low volumes.
Paragraph 6: I've always known that MV additions only ever achieve compromised results.

Para 2: When you say "Once the preamp does start clipping (after the power tubes are already clipping) you now have plenty of signal to drive the power tubes into clipping comig from the preamp".. I seem to find a contradiction: the preamp tubes clipping -after- the power tubes are in the opening section of this sentence, & this seems counterintuitive as I thought the preamp xyz always preceded the power tubes xyz. And to back this up, then '..power tubes into clipping coming -from- the preamp' seems back to logic/ intuitive/ as I expect.

So again, like the Vol 1&2 reversed logic g1 gets me in a pickle over, the exact same reversed logic is here: & furthermore you seem to even be stating revered logic, within one sentence.

Paras 3 & 4: are therefore incomprehensible to me, because I have this 'counterintuitive imbalance' has been in place by the previous paragraph.. so I can't possibly understand both paragraphs.

Apologies, this must be frustrating for you I realise. Infuriating I'm sure. SC

[Sea Chief]

I asked you before if you had even read the description included with the diagram and you claimed you had done so. What part of "Vol2, aka MV" is it that is so far beyond your comprehension?
Read better, or if you can't be bothered, stop griping about people trying to help you.

Because Vol 2 -seems- to me in your diagram to imply it's the power stage.

If the input is next to the first stage bit, & that has Vol 1 written next to it... & I KNOW that the input is the guitar input.. then your Vol 1 in the diagram, is the preamp section: & I consistently read that introducing an MV is to to do with -preamp- stage. MV is often referred to as PREAMP distortion, no??

This is one reason why I'm confused g1.

If the very pg 1 basics seem back to front, then I have no way of understanding form here on it seems. Reminds me of pg1 economics: supply & demand, which also seems counterintuitive to me (& I'm as hopeless understanding it).

SC

[Justin Thomas]

Lower volume is the goal of a "master volume" circuit. But equivalent tonality IS NOT. So what you get with a master volume is strictly what the preamp sounds like. With the master volume turned down the power tubes are no longer part of the distortion generating process and all you get is whatever the preamp offers. for most vintage designs this is usually a disappointment because a critical tonal element has been removed...

But the goal of an attenuator <IS> to keep the critical tone elements of power tube clipping in the equation, which is why I and others so heavily advocate for them.

But! Like all things in life, trying to tame a genuine experience is going to water down SOMEthing. No attenuator is perfect but some will definitely perform better than others at preserving the full "cranked amo experience." And in <my> opinion, any attenuator is preferable to adding a Master Volume to an amp design that was never intended to have a Master in the first place.

Jusrin

[Justin Thomas]

A long bit on why the need for all the different names for "gain" or "volume" controls... Not professing to be an expert, but just trying to put a complete "beginning-to-end" explanation in a single long post. I'm also open to corrections of gross errors, but I'm also trying to keep it simple without lots of math. Basically I'm trying to be Dave Hunter the author here...

Regardless of WHAT they call it, Gain, Volume, Pre, Post, Master, Loudness, they all do the exact same thing. They all adjust the amplitude of a wave.

A "gain stage" (usually a single triode or half of a 12AX7) will amplify a small signal up to a point cleanly; after that point the signal will distort & eventually clip if you keep giving the stage a bigger and bigger signal.

So, you have a guitar signal of say, 0.1V going into a triode gain stage. That stage will amplify that signal say, 20 times. So the Gain is 20. I'm just going to pick numbers based on very simple math here because I don't like math. So when that signal exits the triode, it's now 2V. Then it goes to a "gain" control. So let's say you wind that "gain" control to 10 & so you then pass that 2V signal to another triode gain stage that amplifies another 20 times. Now your signal is at 40V. Feel free to add gain stages like this as many asyou want; you can see that the "gain" quickly gets out of hand, because remember - each stage can only CLEANLY amplify a signal to a certain point. Probably after maybe 3 stages like this, without any tweaking of volume knobs or voltage dividers, you're probably making a square wave or something else just as audibly & visibly as ugly. So consider this an extreme example - this amp would probably be incapable of passing a clean signal of any usable "actual volume" or, loudness.

So, what a knob called "Gain" on a guitar amp is trying to do is, to amplify a signal right into distortion early on & easily. Since in Guitar-Land "Gain" = "Distortion," then the name "Gain" describes the "primary function" of the knob's use - adding distortion. Or you could set the knob low & not add lots of distortion...

After that gain stage distorts the signal good, there might be a voltage divider that will reduce the SIZE of the wave while keeping the SHAPE of the wave. This smaller signal then will feed a second gain stage which can also only amplify a signal to some point before it also starts to distort. After this particular stage, there might be another "gain" control to adjust the size of that wave after that stage but keeping the shape the same. Or you could just keep cranking the knobs until you get total sludge. Etc. Etc. One of my favorite activities on my amps is to just crank all the knobs on 10 except the Master Volume just to see what kind of horror show I can unleash; my Fender 75 is especially horrible!

So amplifier makers will put in various "gain" controls at various points in a circuit to keep the amp under control, and to allow a wide variety of sounds at a variety of loudnesses. Fenders use one of these controls simply labeled Volume, which is located fairly early in the circuit; Peavey & Mesa can have 3 or more. My last amp build has 4 "gain" knobs - I'll label them Gain 1, Gain 2, & Master Left/Master Right. The Master on my amp is a concentric one that allows me to send a different size signal to each side of the phase inverter; most amps will use a dual pot that turns both at the same time with a single knob.

The reason they all have different names is because various amps are different circuit structures & ALSO, because in Guitar-Land, the term "gain" has come to mean "DiStOrTiOn!" But "gain" technically is about voltage gain, or, how much bigger a signal is coming out than going in. On MOST (MANY?) amps, the idea of a "Gain" control is to indeed amplify a signal to absolutely ridiculous proportions in order to intentionally distort the wave shape. So to keep that distorted wave shape the same SHAPE to pass on to a following stage, there will be a "Volume" control to adjust the signal size. Now, the amp maker might call that knob "Volume." But Peavey used to call their "gain" (distortion) knobs "Pre" & then a later "volume" control "Post" followed by a final "Master."

Finally, you will have a "Master Volume" which will be last in the signal chain and which will set the loudness of the signal. You can either have "quiet but distorted" or "loud & distorted" or maybe you can set your Gain & Volume knobs to not drive any stages into distortion & you can then have "clean & quiet" or "clean & loud" depending on where you set the Master Volume.

The thing is, every amp design is going to be different & every manufacturer will call their various "gain knobs" different things. Each knob SHOULD be called something different, because if you have 3 or 4 "gain" controls & you want to change the sound or loudness, you have to know which knob to turn. If they were all labeled "Gain," you wouldn't know which one did what & it would just be more confusing than it already is. It's kind of like when I'm at work & some customer says "can you pass me a jug of milk?" & I hafta say "can you be a little more specific, please?" because I'm looking at half gallon, gallon, skim, low fat, whole, chocolate, & about 6 different brands. So yes, even though all of the various knobs are all "gain" knobs, they get different names depending on the usual purpose in the circuit. Some mad scientist could theoretically string up 20 gain stages with a "gain control" before & after each stage, having 40 different knobs labeled Gain 1 through Gain 39 with a final Master Volume. It'd be massive overkill & likely practically useless, but it could be done...

The only real "constant" in the Knob Naming Game seems to be "Master Volume," since they're pretty much always the last control in the signal chain & they all basically set the "Loudness" of the amp. A Master Volume should affect the tonality of an amp little to none at all, depending on the design & where the Master Volume is in the circuit. That said, "should" is the operative word. Nothing is perfect, and some designs work better than others at certain goals. I myself don't like a "Crossline Master Volume" because at extreme settings (meaning, trying to play very quietly) it sounds crackly, thin, & just bad. A Crossline is good to "take a little off the top" but for playing sludge metal in my apartment I much prefer the post-phase-inverter Master Volume.

Hopefully this can help a little bit; I know it can all get kinda crazy...

Jusrin

[Chuck H]

Ok... Vintage design amps were not intended as distortion generators. But they were used that way by players anyway. So the familiar distortion sounds you hear are the result of these vintage designs being driven and cranked up into generating distortion. This is a LOUD business! The clean signal on vintage design amps is intentionally designed to NOT clip before the power tubes do so that a full power clean tone can be delivered. Now...

Once the preamp does start clipping (after the power tubes are already clipping) you now have plenty of signal to drive the power tubes into clipping comig from the preamp. But now the power tubes are clipping an already clipped signal. This generates additional harmonics in cranked vintage amps. Still very loud. So...

When you add a master volume to these vintage designs ALL OF THAT is completely lost and gone BECAUSE THE POWER TUBES AREN'T CLIPPING!!! So all you have left is whatever the preamp was doing before the power tubes clipped that signal. In vintage amp designs this is NOT what you should expect from the amps tone strictly at reduced volume. I have outlined clearly that the master volume removes a critical tonal aspect. It's to be expected. So what's left???

Lower volume is the goal of a "master volume" circuit. But equivalent tonality IS NOT. So what you get with a master volume is strictly what the preamp sounds like. With the master volume turned down the power tubes are no longer part of the distortion generating process and all you get is whatever the preamp offers. for most vintage designs this is usually a disappointment because a critical tonal element has been removed.

Preamps designed for good clipped and overdrive tones are a more recent concept than anything existing when the Twin Reverb and A30 were designed and sold.

There's A LOT more to it than this. I only hope the help you understand why any "master volume" added to these vintage designs can never give you the magic of their tone at lower volumes,.

Sigh.