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

[Sea Chief]

Turn the Crate vol. control up full and leave it there. Now you have the equivalent of a non-master amp and the gain pot acts like the volume pot on something like a non-master Twin.
You won't get any dirt by turning up the gain, it will just get loud. Try it.

Ok I can visualise doing this (the Randall.. not Crate: annoyingly it isn't working!! Haha, something simple I know that much, but I do have todo another QUICK thread, as there's no way I can sort it as it is). So let's say I do both knobs as you suggest in this post.

I can't understand what I am supposed to ascertain, by hearing the clean tone, from doing this. I can't understand the point of why your suggesting turning these two knobs, in this fashion, if the end result, is no distortion, if the whole point of the thread, is to achieve distortion. I can't see any logic in doing it.

I am consistently flummoxed by every facet of this business. Even simple suggestions like your post here, is unfathomable to me (in fact it seems actually counterintuitive).

Thanks again though, SC

[g1]

Sorry I don't understand this post. It's too confusing for me, so I'm left equally as confused as when I asked what this (Randall) amp's gain knob actually was.

Turn the Crate vol. control up full and leave it there. Now you have the equivalent of a non-master amp and the gain pot acts like the volume pot on something like a non-master Twin.
You won't get any dirt by turning up the gain, it will just get loud. Try it.

[Sea Chief]

Well, at least he has the Crate there as an example. If he sets the 'volume' up full, it will act like a non-master amp with the 'gain' control now the only volume control.

Hi g1.

Sorry I don't understand this post. It's too confusing for me, so I'm left equally as confused as when I asked what this (Randall) amp's gain knob actually was.

Thanks for trying to help though, instead of berating me.

SC

[Sea Chief]

Pedro Vecino Hi thanks for adding the two schematic diagrams. And your reply there gave me confidence to maybe try the C1 link mod again.

Can you put up the whole schematic though? I still need to establish if the one I put up in #3.. is correct. There are usual members who have such low confidence in me (even though I can rebuild a Twin Reverb bettering the original, & my hunch that a fault plaguing it for 2 years -wasn't- a conductive board, but a bad component [one loose heater wire is akin to a bad component in my book]) that anything I put up is immediately acredited as being wrong.

I did ask what a master volume was too, but instead of answers I'm just castigated for not knowing what a master volume is (&/ or for not knowing the difference between vol gain etc but at the same time saying it IS confusing that different Co's use different terms, for these very same things!!).

I might not ever know the answers to the mystery of what MV/ vol/ gain is, but that doesn't mean I can't add a link, try your double-pot addition, or even solder in a much more complicated VVR circuit. If my Twin Reverb sounds great (finally) this at least proves that my solder joints ARE up to par.

Thanks, SC

[g1]

Asking any lay player to come on board with what "we" understand about the circuits is seldom gratified. Sea Chief may never understand the intricacies of gain structure. Even though it would help him there's nothing more we can do than we have to help him understand it.

Well, at least he has the Crate there as an example. If he sets the 'volume' up full, it will act like a non-master amp with the 'gain' control now the only volume control.

[Chuck H]

g1, Sea Chief has never come to terms with the nebulous descriptions of gain and volume by manufacturers because he doesn't trouble to understand gain structures. I know that 90% of the population figures these things out intuitively on spot while using products but there's the other 10% that think "gain" and "volume" should be defined in concrete. And I can't argue. Having to define these things on the fly for every circumstance is a real pain in the ass. And it would be nice if the moniker attached to a knob was a known effect. But even now, today in this long lived world of this genre terms like gain, distortion and even "master volume" continue to have variable meaning dependent on manufacturer or design. This said...

You and I can simply look at a schematic and understand what these things mean. Sea Chief's trouble is the inability to do that. Any argument that there is not enough definition in the allocation of function is valid. But there is no chance of it being reconciled unless one has the skills to define a circuit by looking at the schematic. So for the lay player any knob marked "volume", "gain" or even "master" has to be evaluated in use and not always consistent with what it means on their other amps. AND...

Asking any lay player to come on board with what "we" understand about the circuits is seldom gratified. Sea Chief may never understand the intricacies of gain structure. Even though it would help him there's nothing more we can do than we have to help him understand it.

[g1]

I don't understand you see, how a crappy 5w Randall RD5 (2 tubes) has a knob called 'Gain', which you just turn.. & disortion happens. It's not called an MV. So wtf is it exactly? And does it bear any similarities to an MV?

The RD5 has a 'gain' AND a 'volume'. The volume control is what is acting like a MV. You can only crank up the gain if the volume is set low. If volume was set full and you tried to crank the gain, you would be fleeing for the river.

[Pedro Vecino]

The AC30s from the 90s (made by Marshall and popularly known as Korg) are all the same. I attach their schematic.
The capacitor C1 you mention is usually a blue ceramic one (4n7) located next to the Cut control. It is very easy to identify it.
It may happen that the pins barely protrude from the plane of the board. If they protrude 1mm or more, it is possible to scrape that area with a small, sharp cutter, apply fresh tin and solder the bridge with a very fine-tipped soldering iron. If not, you will have to look for other points (the correct pin on the potentiometer and the side of the correct 1K5 resistor) but do not make a mistake with this because it is a delicate area to make a bridge.
Regarding the noises you mention, these are amps from 30 years ago and the quality of their potentiometers is not the best. Make sure they work well or clean them. In certain cases there is no other solution than to change the defective ones.

That formula for the cross master volume should work exactly the same as in the Helmholtz amp. The difference is that in an older amp it is much easier to do.

I think (that's my idea) that one of the differences between the cross master volume and the one I proposed (replacing the feed bias resistors with a double potentiometer) is that in the latter, when lowering the master, the resistance in series with the grids of the EL84 progressively increases. This rounds off the highs and in a certain way has a "braking" effect on the feel of the guitar, musically centering the response of the overdrive (it's very difficult for me to explain it in English).

Below is the diagram. I remind you that this operation is more complex. I would first try to make the cross master work properly. It would also be useful for comparison if necessary.

AC30Korg90s.pdf

[Sea Chief]

From the schematic in post 3, the cut control is a 220k audio taper pot wired as a 'backwards' variable resistor, ie such that full CW is a short, full CCW is max resistance.
It seems a rather user unfriendly way to arrange things. The range of adjustment will tend to be all squashed up into the last 1/4 turn.
A reverse audio or even a linear taper would have been a better choice.

Hi pdf, so now I'm wondering why C1 was suggested as the Cut control.

What is known, is HH shorted C1 in his "70's AC30" . But that doesn't neccessarily correspond, with my "90's AC30" which although a firm remake of the original Dick Denny circuit, was made in the marshall factory, & the Vox name owned by Korg.

So all I can do, is put up, what I diligently research as being my AC30 circuit in #3. My research, with my amp knowledge though, isn't -anything remotely- on a par with yours, HH's, or Chuck's. But it's all I can do.

What I need, ideally, is for someone to say definitively "Yes- that is the correct schematic for your amp" or "No.. that's not" & then I can trawl & try to obtain the correct one (or ideally they might say "THIS ONE.. is correct for your AC30/6 TB" [Top Boost, Korg owned, Marshall-made, 1995 amp]).

Thanks, SC

[Sea Chief]

What happened to your attenuator? I know you made one. Considering that you like the tone/effect of the cut control with the amp at normal volumes then why not use the attenuator to reduce volume? Admittedly this does nothing to preserve the power tubes life but if this is about tone instead then why not use the attenuator? Otherwise the best option might be VVR.

JM2C

Hi Chuck, good question (sorry to not reply, I only just spotted this post tucked away among yesterday's posts).

Well I lost confidence in it, whether I wasn't sure I made it 100% correctly (I cut one major wire, & it made no difference to it- why I cut it/ I cant recall, had I spotted a mistake maybe), because I remember it didn't seem to be too effective. Kinda like it was only partially working. Or if it was, then it was dissapointing & not useable. That's what it seemed.

The MV preferable, because A) it seemed simpler, & B) it's less of an anxiety-inducing method (so I recall using the MV knob on one amp I had, my MusicMan 4-10 for eg). No extra box to make, then to plug into/ out of too. Just use a knob on the amp too/ always there, within the amp.

One thing that concerned me with the attenuator, was if the knob I'd made somehow suddenly failed.. the village would implode, I'd have a heart attack with the sudden volume (seriously- it might well scare me into having one), & police would be round too.

Maybe I need to rebuild it. I don't know. I just thought the MV was the -simplest- first port of call, to try again, at this decade+ journey I was on (& kinda gave up as the attenuator didn't glean good results) just to get some playably-decent overdrive, with an amp. That's all I'm after, it's not much on paper to strive for. But it's been nigh on impossible to achieve for me, one amp to another.

I don't understand you see, how a crappy 5w Randall RD5 (2 tubes) has a knob called 'Gain', which you just turn.. & disortion happens. It's not called an MV. So wtf is it exactly? And does it bear any similarities to an MV? It's all such an infernal mystery. Why dies a marshall have more than one of these 'disortion' knobs (called, well not distortion, or overdrive, but something else entirely.. Gain? MV? Drives me fkn mad trying to understand it.. let alone what to do to my amp!).

SC

[Helmholtz]

Well admittedly I haven't had or used an MV in decades, so I was asking earlier on, what exactly an MV does (does it overdrive the small preamp tubes- I asked this Q) to establish this.

A MV has no effect on the preamp whatsoever. Preamp meaning everything before the phase inverter tube.
The MV lowers the signal delivered to the power tubes.
This means that except for high MV settings you won't get power amp distortion anymore.
As most tube amps sound best with at least some power amp distortion, a good power attenuator would be a better solution than a MV (as Chuck mentioned).

I also forgot that a 'trick' of using one, is to turn it to max.. then dial up the Volume. Or is the 'trick' idea to turn the volume to max & dial up the MV from it having no output? (Seemingly this was -confusingly- at the end of it's rotation.. so I dial it 'backwards'?? You see I'm right away, confused at this juncture).

This has already been answered by g1.

The added link wasn't liked by the amp, I know that much. It wasn't happy. I'd like to establish why before I try it again (I removed the link panicking wondering if I'd ruined the amp- thankfully not). IE is this schematic.. correct? I think it is, but I'm the least knowledgeable.

Still wondering if you jumpered the wrong component judging by the noises. The cut cap in your amp might not be C1 if the schematic doesn't match.
Only you can find out if the schematic is correct for your amp.

[Helmholtz]

I'm thinking that shorting the cap across the Cut control doesn't just make the Cut into an MV, but turns it into a Crossline MV.

Yes. Also the cut control uses the crossline principle, but only for the treble.
Crossline MV works ok with an AC30 (not using global NFB) at not too low settings.
The idea was to propose something simple and reversible to allow the OP to test if he likes a MV at all.

[pdf64]

From the schematic in post 3, the cut control is a 220k audio taper pot wired as a 'backwards' variable resistor, ie such that full CW is a short, full CCW is max resistance.
It seems a rather user unfriendly way to arrange things. The range of adjustment will tend to be all squashed up into the last 1/4 turn.
A reverse audio or even a linear taper would have been a better choice.

[Sea Chief]

It's a master volume control, what are you expecting it to do?
It should not change the clean tone.
At one end of the rotation it should be zero volume. At the other end of the rotation it should be as if it was not there (full output possible depending on channel volume setting).
It may operate in reverse because it was a cut control, so zero volume at full rotation may be the way it is.
Don't worry about the noises for now, it may be coincidence. Does it give a full range of output power capability?

edit: as for the clean tone, a master volume allows you to turn down the loudness, so you can turn up the channel volume. So you need to turn the master to a low setting, then crank the channel volume up. That will change the clean tone (make it dirtier).

Good point g1. What am I expecting, good question.

Well admittedly I haven't had or used an MV in decades, so I was asking earlier on, what exactly an MV does (does it overdrive the small preamp tubes- I asked this Q) to establish this. From here I can possibly (unlikely tho) understand how to use it. MV has always been a mystery to me.

I also forgot that a 'trick' of using one, is to turn it to max.. then dial up the Volume. Or is the 'trick' idea to turn the volume to max & dial up the MV from it having no output? (Seemingly this was -confusingly- at the end of it's rotation.. so I dial it 'backwards'?? You see I'm right away, confused at this juncture).

The added link wasn't liked by the amp, I know that much. It wasn't happy. I'd like to establish why before I try it again (I removed the link panicking wondering if I'd ruined the amp- thankfully not). IE is this schematic.. correct? I think it is, but I'm the least knowledgeable.

Thanks, Capt

[Chuck H]

The AC30 is a very specific beast WRT tone across the ENTIRE circuit. So... I haven't experimented with VVR but maybe? Attenuator, I think so. YMMV.


EDIT: My own experience with the "crossline" MV would be the same as Justin. It's ok for taking off a few watts. Past that you may as well plug into anything else because the tonal qualities of the amp in use is decimated anyway..