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

[g1]

I tested continuity between my C1 legs, good, so that mod -in this Korg AC30- must not be compatible.

Do you mean it measured 0 ohms across C1 with your jumper installed?
Have you verified that C1 in your amp is connected to where the schematic shows? It is more likely that the schematic is not compatible, rather than the mod.
If the schematic is not a match, you won't be able to do either mod.

Like Helmholtz said above, pictures will be a great help.

[Helmholtz]

[USER="534"] I do like the Cut knob (although what exactly it does is a bit of a mystery! Presence, in other amps-?)

The cut control does essentially the opposite of a presence control as it attenuates treble while presence boosts treble.

What is the value of the cap (label) you jumpered?

[Sea Chief]

Do you mean it measured 0 ohms across C1 with your jumper installed?
Have you verified that C1 in your amp is connected to where the schematic shows? It is more likely that the schematic is not compatible, rather than the mod.
If the schematic is not a match, you won't be able to do either mod.

Like Helmholtz said above, pictures will be a great help.

Hi g1.

The site is glitchy for adding pics/ tried for ages last night, all manner of ways. Will try again tmrw.

I do know that my amp is the same as Pedro Vocinio's (Korg AC30) with his photo on pg 1 with the dual pot added. Which means if he did this successful MV mode to his, there shouldn't be any logical reason why I can't copy that, into my amp with success (regardless of whether the schematic is or isn't a match). That's just logic, to me. But again I'm not in the know.

---

I meant I measured continuity from one leg to the other, 0.2 ohms, meaning I successfully soldered a link across C1 cap legs. Surely if HH's mod was done on his 70's AC30, different to my 90's Korg reissue, the likelihood is our amps are not compatible.. rather than my schematic not compatible to my amp:

The schematic shows that it's version 5, 1994. So I believe this means it's correct for my amp, if the run of these Korg reissues was 10 years from 1993-2003 (as says in the Vox showroom glossy web info page I linked to).. so I don't see how else I cannot accredit this schematic, to my amp. But I'm not the expert as you know.

Thanks, Capt

[Chuck H]

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

[g1]

Firstly I get no discernable change to the clean tone, whatever rotation the Cut knob is.
Secondly I get no volume at all, with the Cut knob full (& some nasty noises too).

Thirdly, I'm getting unwanted intrusive noises (some making me jump in fear.. similar to my Twin Reverb, occasional loud noises).
Lastly there's a very big lessening of the overall volume, it seems. I only tried the Vib Trem channel.

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).

[Chuck H]

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).

This is a matter of understanding the signal chain. IMHO this is the single most important aspect of tone for guitar. I'd guess that most failed resolutions here on the forum regarding modifications are the result of the poster being unable to understand what's happening in the actual signal chain. Since the guitars and amps came up together in time and guitar amps are actually signal processors rather than just utilitarian. This is an important thing to understand. Not a problem if you just want to plug a guitar into a digital signal chain and take what's given. But if you're an artist that want's to know his canvas this shortcomming can be a problem.

[Justin Thomas]

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. Which, admittedly, I have had terrible results with IF I am using it to ditch a LOT of volume (actual loudness). It definitely does not play well with all circuits, especially if the PI isn't nicely balanced. Honestly, after using the dual-pot post-phase-inverter MV that I was suggested on my last build, I don't plan to ever go back to a Crossline!

I can see where SeaChief might be getting bad clean tone & nasty artifacts. He's using a quick&dirty solution that may just really not like his goals. Might need to use a different MV design...

Jusrin

[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..

[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

[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.

[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.

[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.

[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

[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

[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