Higher Coupling Caps; Vox Bass boost? Or Vox tone hell?

[nedhogan]

The coupling caps for vox ac30s are so small they arent even an option on the AmpBooks calculator website. They basically cut a heck of a lot of bass from the signal starting at 1KHz!
I can't decide whether to put some larger value Cg coupling caps in, because after playing other amps whenever returning to the ac30 it sounds good, but very thin.
They range, for some reason, from 47nf to 470pf (excluding the tone caps which appear to also be Cg's).
Im thinking an order of magnitude higher (470pf to 0.0047uf etc) will flatten out the reasponce across all frequencies.
I might just piggy back on the existing caps with some clips, just to see what it sounds like.

Before I go buying £15 of caps does anyone have an opinion on this or some advice?

Will 15dB of extra gain on the lowest bass freq's be rediculous? Or will it totally change the characteristics of the amp, in a bad way?

Or maybe just up size the ones on V1?

[Steve Conner]

Yes. The lack of bass (or treble boost, depending which way you view it) is part of the AC30 signature sound, IMO.

[nedhogan]

Yes. The lack of bass (or treble boost, depending which way you view it) is part of the AC30 signature sound, IMO.

Now, I don't think of them like that, I always think of them as being quite muddy when they are cranked (still with plenty of treble bite like), and mine is. but not muddy enough! I mostly use it with the neck pickup, because the bridge one is always far too thin sounding (on most if not all guitars).

I agree that changing the way it sounds is sacrilege but Im after my own sound not that of any particular amp. I have never been closer to the sound I like with any other amp but its still not quite there yet and I think a touch more low end might just be what it needs.

The question is how much bass and low mids will it add. I suppose theres only one way to find out.

[Steve Conner]

Something doesn't add up about that. The smallest value you gave is 47nF, which is 0.047uF. That's a fairly standard coupling cap value for tube amps, and ends up giving you a cutoff frequency around 16Hz with the usual resistor and pot values.

That doesn't square with "not even an option on the calculator".

500pF would be a really undersized capacitor.

[pdf64]

I don't understand the issue raised, there's only one 'undersized' coupling cap in a 60s AC30, 500pF feeding on the bright channel vol control.
As that's the only channel linked to the tone controls (assuming that it's a top boost version), it's valid to increase it; 1n, 2n2, 4n7, 10nF should allow a progressively low roll off. Try them all and see which you prefer; as there's a bass control there should be some flexibility.
But as for the rest of them, you may have got muddled up with the calculator.
Could you explain how you came to this conclusion?
Pete.

[diagrammatiks]

your definitely converting some numbers wrong or something.

a 470n coupling cap in front of a distorting stage is going to go into blocking. possibly forever.

[nedhogan]

Oops I alway think that nano is smaller than pico. Derrr
oh well.
Its strange that there isnt that much difference bass wise between the two channels seen as the normal channel has a normal value coupling cap.

I guess I'll have to think of some other way to get more bass out of it. Maybe up size the bass cap in the tone circuit to 47nf. Or just try up sizing the 500pf one.

[diagrammatiks]

well which ac30 circuit are you looking at?

the top boost channel is supposed to be very bright.

[nedhogan]

your definitely converting some numbers wrong or something.

a 470n coupling cap in front of a distorting stage is going to go into blocking. possibly forever.

Yeah that was a typo I meant 470pf to change to 0.0047uf.

[nedhogan]

well which ac30 circuit are you looking at?

the top boost channel is supposed to be very bright.

AC30cc2x.

I lowered the bright cap which made a small difference. My other 30watt amp thats similar, vaguely, to a ac30 schematic has heck loads of bass. more than it seems possible for the ac30 to ever have, and Im a little puzzled as to why.

I'll upload both schematics possibley and someone may be able to tell me why the other has so much bass.

I appologize for my getting the cap sizes mixed up, I might have noticed on looking again but probably not, Im terrible at remembering which cap sizes are equivalent to each other!

[nedhogan]

AC30cc2x.

I lowered the bright cap which made a small difference. My other 30watt amp thats similar, vaguely, to a ac30 schematic has heck loads of bass. more than it seems possible for the ac30 to ever have, and Im a little puzzled as to why.

I'll upload both schematics possibley and someone may be able to tell me why the other has so much bass.

I appologize for my getting the cap sizes mixed up, I might have noticed on looking again but probably not, Im terrible at remembering which cap sizes are equivalent to each other!

[Tage]

This is not an AC30cc2x. Is this the other amp you have?

[nedhogan]

Theres two schematics.

[Tage]

gotcha, one thing you might concider is the slope resister for the tone stack, it's a 100k, you could lower it to say a 56 or 47 and see if that gets you in the ball park, you can even just piggy back another 100k for 50k and see if you like it. it will give a slight boost to the mids and maybe thicken things up for you. even the cap that feeds the tone pot could be brought up to 100p or 220p, allowing more high mids through with the treble.

Also I think a 2.2nf cap in that position may help out your low end issues

[J M Fahey]

The AC30 is "as is".
Modifying it will add little and lose a lot.
If you need more bass (fine with me) leave electronics as-is and plug the head into a closed back 4x12" ... and stand back.
KILLER sound.