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

[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]

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.

[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.

[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.

[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.

[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.

[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.

[Sea Chief]

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.

But this is what I meant by asking *if adding an MV to a non-master-amp, is akin to relabelling the Vol.. to MV, then the Vol just changes function to become the preamp Vol.

If so, then I'm finally getting somewhere. If not... then I'm stuffed. I just can't understand it huge apologies.

I can almost understand -this- post ^^ by g1 IF this* is correct, otherwise I'm back to square 1 & tbh I'm just never going to understand it.

My head aches trying to read the longer posts beforehand, sorry chaps. The notion of 'power amp before the preamp' still resolutely is counterintuitive, with the word PRE involved. This inherrantly means before. It cannot mean otherwise. I can't cope if the preamp is somehow to be considered AFTER the power amp. My head just will not accept it.

Thanks chaps, SC

[Sea Chief]

I'm having trouble understand this 'clean signal' phenomenon, especially with regard to the Vox TB as it is known to have an extra gain stage (or something? Extra tube anyway) to facilitate distortion. So I was under the impression the TR and the Vox are chalk & cheese. Clean vs dirty amps.

I'm having trouble with the notion of what Gain is, the word seems to pop up whenever it wants to, there's a knob with this mystery word on too which I don't know what it is (but all my ears tell me it kinda just sounds like a distortion pedal in the amp- is it??). I specifically chose to exclude this word Gain, as with the Vox, there is no such knob with Gain on. There is on the Randall.. but whether including this 5w amp as an example is helpful, or has been nothing but a hinderance to my grasp of this MV addition idea, I can't tell.

SC

[Helmholtz]

I'm having trouble understand this 'clean signal' phenomenon, especially with regard to the Vox TB as it is known to have an extra gain stage (or something? Extra tube anyway) to facilitate distortion. So I was under the impression the TR and the Vox are chalk & cheese. Clean vs dirty amps.

The "Top" in Top Boost (TB) stands for treble.
The additional stage in the TB AC30s was added to increase treble response.
The earlier non-TB AC30s had only one preamp gain stage allowing even less distortion.

I'm having trouble with the notion of what Gain is..

Gain means signal amplification. A gain stage amplifies the signal, so at its output the signal is (much) larger than at its input.
The larger the signal the more distortion it produces in the following amp stages.
A volume pot is used to lower the gain (and signal strength) and thus lowers volume and distortion when dialed back.

[Chuck H]

But this is what I meant by asking *if adding an MV to a non-master-amp, is akin to relabelling the Vol.. to MV, then the Vol just changes function to become the preamp Vol.

If so, then I'm finally getting somewhere. If not... then I'm stuffed.

This is pretty much correct. Though I might not have described it just like that. But you are getting there. re: not stuffed.

[Chuck H]

Read Justin Thomas (clouds part and music *OOOOooooohhhhh!!!) who sayeth that modern amps like to use the moniker "gain" to mean "distortion" when applied to panel controls. If you see a knob labeled "gain" it's almost certainly a distortion control and the master volume function doesn't even need to be considered for the overall design. It's only WRT clean channels and vintage amp designs that the master volume can foul things up. That said...

Helmholts is ABSOLUTELY correct that gain is signal level. Either signal level in voltage or signal level in watts depending. This is how the word is used in the electronics nerd arena. So watch for that here because we might use the term gain on the forum differently than it is typically applied to guitar amp controls (sorry, that's just the reality). In our defence we were using the term correctly all along and it's the guitar amp manufacturers that allocated it to mean "distortion control" on amp faces. Like I said, Justin outlined it pretty well as to how knobs tend to be labeled for function on guitar amps. Though even then there can be confusing flyers in the group that still don't seem to make sense. So...

If you read us here using the term gain WRT a circuit we are strictly talking about signal level. Either voltage or power. If you read the word gain on an amplifier it's almost surely a knob to control the amount of distortion.

You don't have to become a nerd like us to "get it" but it very often feels that way. You're trying and getting a lot closer because of the tones you like and chase. So just chill and let it settle in. It will make sense some day. Or not? But I trust you'll find your way to happy sounds anyway. But don't stress. It doesn't help and it's counter productive to the artistic mind.