Modding a Vox AC4TV

[Gary_S]

I have one of these little amps and it's cool but i always want more high end out of it plus i feel the tone control doesn't do that much.

I was thinking of reducing the size of the coupling caps to brighten the tone up a bit and introduce a bit more high end sparkle.

The attenuator in particular sounds too thick to me with not enough high end zing, although from what i read that seems to be a problem with a lot of these type attenuator circuits, they do cut the level for lower volume practice but it just doesn't sound as good.

I'll include the schematic at the bottom here and welcome any advice on what's best to change.

One other question; on many modern amps i see the pots mounted directly onto the PCB, are they just soldered directly to the traces there? My amps have never had that being the wiring go's directly to the pots lugs. What is the goal in doing it this modern way? Is there a benefit apart from saving money? or is it easier for them to assemble like that...?

Thanks.

 Gary

[spacelabstudio]

I have one of these little amps and it's cool but i always want more high end out of it plus i feel the tone control doesn't do that much.

I was thinking of reducing the size of the coupling caps to brighten the tone up a bit and introduce a bit more high end sparkle.

The attenuator in particular sounds too thick to me with not enough high end zing, although from what i read that seems to be a problem with a lot of these type attenuator circuits, they do cut the level for lower volume practice but it just doesn't sound as good.

I'll include the schematic at the bottom here and welcome any advice on what's best to change.

Making the coupling caps smaller will filter out low end.  Which may be what you want but isn't what you say you want.  C15 and C20 both roll off high end.  You can try removing them altogether or replacing them with smaller values so the roll off happens at a higher frequency.

One other question; on many modern amps i see the pots mounted directly onto the PCB, are they just soldered directly to the traces there? My amps have never had that being the wiring go's directly to the pots lugs. What is the goal in doing it this modern way? Is there a benefit apart from saving money? or is it easier for them to assemble like that...?

Ease of assembly, which saves money.

Chris

[Gary_S]

Making the coupling caps smaller will filter out low end.  Which may be what you want but isn't what you say you want.  C15 and C20 both roll off high end.  You can try removing them altogether or replacing them with smaller values so the roll off happens at a higher frequency.

Thanks Chris, yeah i want more high end or a bit of a bass or midrange cut so that the high end becomes more defined and with some bite which i feel the amp lacks a little bit.

Ease of assembly, which saves money.

Chris

Knew it! had to be that. Corner cutting and penny pinching really. What i found was that my other amp; a Marshall 4010 from 1982, has the old way of doing the pots. I saw the inside of a chassis of a later one and they had switched the pots to this PCB mount idea which i thought was a bit off.

[tubeswell]

That schematic throws aways lots of gain the pre-amp, and the attenuation between the first two stages will roll off the high end somewhat. You could try bypassing R1 (with something between 500pF and 5nF - need to experiment a bit) and see how it goes.

[Gary_S]

Thanks guys for the advice. I'll make a couple of small changes and then fire it back up and review it, see how it sounds. It doesn't need much, the sound is already pretty cool but it needs a bit more definition  

EDIT: Removed C15 and C20 this afternoon. Will have to wait to see what the sound is like. Very small and fiddly chassis to access. Took me awhile to get those 2 little caps out. Quite intricate and very small PCB. Sucked the solder with some wick to get rid of it then pulled them out. It'll be interesting to see if there's much difference. Took my time removing these caps as i didn't want to destroy them in case i want to re-install them at some time.

[Gary_S]

Did that mod and it didn't do anything really. Very little difference in the sound. Maybe feels like a bit more high end, but that might be my imagination.

Really what i want is for the tone control to add more treble as i turn it up. I know it's a subtractive tone control but you know what i mean i'm sure? I looked at altering the cap that go's to the tone control C4, i thought about making that smaller, at the moment it's 220pF. Maybe that would make the control give me more high end?

[Willabe]

That schematic throws aways lots of gain the pre-amp, and the attenuation between the first two stages will roll off the high end somewhat. You could try bypassing R1 (with something between 500pF and 5nF - need to experiment a bit) and see how it goes.

Did you try this?



                Brad     

[sluckey]

That schematic throws aways lots of gain the pre-amp, and the attenuation between the first two stages will roll off the high end somewhat. You could try bypassing R1 (with something between 500pF and 5nF - need to experiment a bit) and see how it goes.

And also put a 120pf to 500pf on the volume control, between wiper and top side.

[Gary_S]

That schematic throws aways lots of gain the pre-amp, and the attenuation between the first two stages will roll off the high end somewhat. You could try bypassing R1 (with something between 500pF and 5nF - need to experiment a bit) and see how it goes.

Did you try this?



                Brad    

No not yet as i'll need to work out how to do it as it's an extremely small PCB and there's very little room to position anything. I'll have to take it apart to see if it's feasible to do that.

I was looking at the easiest things first like removing those 2 caps to see if that would work but it hasn't.

If i do this i'm gonna have to have the bypass cap nearly right literally on top of the 220k R1 resistor. That resistor might dissipate some heat will that be ok for that cap? There's no other way to fit it as the PCB is tiny and everything is jammed in.

[sluckey]

Just use gator clips to temporarily clip it to the resistor to see if you even want to do this. If you like it, put a hook at the end of each cap lead and hook to the resistor. Add solder. You don't even need to remove the board. There will not be any heat dissipated by that resistor.

[Willabe]

I don't think heat will be a problem with that R. Very small signal there.

I would just leave the caps leads a little long so the cap sits up a 1/4" or so above the R.

And try what Sluckey said too.


               Brad      

[Gary_S]

I don't think heat will be a problem with that R. Very small signal there.

I would just leave the caps leads a little long so the cap sits up a 1/4" or so above the R.

And try what Sluckey said too.


               Brad      

Thanks, i'll try this and see how it goes with it.  

EDIT: Cheers Sluckey just read your post above. Yeah that's a great idea! i'll try it out. Thanks.