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

[Sea Chief]

Hi chaps,

Amp: Uk-made (marshall factory, mid 90's) AC30/ 6 TBX

I've read these are able to have an MV added. Is this possible, if so what would be the best method? Possibly using the cut knob-?

I'd like to be able to dial in some overdrive without taking out 1/2 of Ceredigion: so I think adding an MV might be better to try, before later maybe adding the 'more intrusive' VVR mod. One step at a time. An MV might suit me better, to use at low volumes- as I get older I can't seem to cope with volume as I used to.

Thanks, SC

[Sea Chief]

I'll try & add the schematic, but here is the model (the closest reissue to the original circuit), showing the control panel/ cut knob.

http://www.voxshowroom.com/uk/amp/ac30tbx.html

Thanks, SC

[Sea Chief]

I -think- this is the correct schematic:

https://www.drtube.com/schematics/vo...60-02-iss5.pdf

Thanks, SC

[Helmholtz]

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.

[J M Fahey]

Yes, the Cut control is not a regular Tone control but more like a PPIMV, only affecting Highs.

Shortimg the series cap makes it a full range attenuator.

[Pedro Vecino]

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.

[Helmholtz]

I consider the Cut control to be essential in this amp,...

Maybe to some but certainly not for me.
I played my AC30 for around 10 years and never used or missed the cut control.
I was after the early Rory Gallagher strat sound. Also used a Hornby-Skewes treble booster.
The cut control can only take away treble - nothing I could use.
In fact the silver Alnicos in that 1970 AC30 were rather dark sounding (less bright than earlier Vox Alnicos) but incredibly efficient.
I still have the amp but changed the speakers to mid-sixties silver Alnicos which are brighter but less efficient.

[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

[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]

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].

[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]

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

[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

[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]

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