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

[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]

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.

[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]

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

[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.

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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

[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?

[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.

[Sea Chief]

Pedro Vecino I'm now interested in trying your idea. I tested continuity between my C1 legs, good, so that mod -in this Korg AC30- must not be compatible.

Can I ask, did your MV mod prove to be effective, I mean really effective/ decent tones etc? I must admit & agree: I do like the Cut knob (although what exactly it does is a bit of a mystery! Presence, in other amps-?)

Anyway I'm willing to drill a similar hole, I think, just. I might try removing the plastic facia plate, in case of affecting resale value.
So what value dual pot was your addition? I might buy this & then establish exactly what you did there.

Thanks, SC

[Helmholtz]

So you did something wrong. Need to see pictures.
A MV doesn't change clean tone, just lowers loudness.
You should be able to vary loudness between zero and full power.

[Sea Chief]

Just tried this Helmholtz.. but doesn't seem to work.

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.

And turning amp off.. a nasty Pop too.

Hateful things these guitar amps: my nerves are are shredded because of them!!


Capt

[Sea Chief]

Yes, just adding a wire shorting/jumping the legs of the cap.
I'm not familiar with the layout of your amp, so can't say what is the best way to do.
If you don't melt the cap the mod is easy to undo if you don't like it.
I didn't try other MV's with an AC30.

Great. I think I may just be able to do without taking the board out. Woohoo.

So you found this way decent enough, with your AC30 (an original 60's JMI, or one like mine?) to not feel the need to try another way, say the VVR method?

Thanks- very helpful as always HH.

SC

[Helmholtz]

Yes, just adding a wire shorting/jumping the legs of the cap.
I'm not familiar with the layout of your amp, so can't say what is the best way to do.
If you don't melt the cap the mod is easy to undo if you don't like it.
I didn't try other MV's with an AC30.

[Sea Chief]

Helmholtz Did you find this C1 shorting method effective? In comparison to other MV amps, just as good?

Here's my amp, & I've located C1 (blue 'disc' looking cap, towards the top RHS, actually it's below the last Cut potentiometer, as one might expect)..

Would the suggestion here, be to add a blank wire link joining the two legs of C1? If I can -just- get a soldering iron tip to it. Or to remove the whole board, remove C1 & add a wire link across it's pcb holes.

Thanks, SC



[edit: sorry, I'm totally unable to add the photo of my amp: frustrating].

[Sea Chief]

I consider the Cut control to be essential in this amp, and even more so if there is an intention to install a master volume. It is practically the only element that allows you to center the harmonic content since the equalization itself does not allow it and also loses effectiveness with certain levels of distortion.

The best method for me is to replace the bias feed resistors (220K) with a double potentiometer with the cut control connected before it. Normally I use the low sensitivity input of the vibrato channel for this (quite a bit of work since it is necessary to accommodate the block of the six input jacks) but in this case the amp (Korg AC30) had a very bad master volume installed on the back and I used its hole.

Hi Pedro, thanks for this idea. It sounds interesting.

It's good to know there's possibly another method then, after the easiest way using the cut knob (ideal for me to start off), & before the much more involved VVR circuit addition method. Terrific: so I might well come onto your idea, at a later date.

I'll first try the easiest way suggested by HH (I'm not too knowledgeable you see) even if it's not quite as effective as yours. I'll report back.

Much appreciated, SC

[Sea Chief]

Simple solution:
Shorting the 4n7 cut capacitor (C1) turns the cut control into a MV.
That's what I did with my AC30 in the 70s.

Hi HH.

As simple as this? Well that's great then, ideal. So I just solder a link 'over it'? Sorry I'm actually not knowing what shorting means, with a cap!

----

So can I just ask then, regarding MV's in general: is the idea basically to drive some preamp tubes (or maybe all preamp tubes, equally?) -over- their intended function-? Is that th3 basic idea?

I know only that MV disregards overdriving the power tubes, due to it being added/ introduced -before- the power amp section.

Thanks, SC