Vintage Vox AC4 - Dark sounding?

[mbratch]

Hey all...

I got me-self a Vox AC4 custom build which is nearly stock. The differences are:

1.5M in place of 5.5M for a little more gain
0.015uF in place of 0.05uF cap for a brighter sound
68k input resistors in place of 100k input resistors.

Everything else is stock to the schematic (http://www.blueguitar.org/new/schem/vox/ac_4.gif) as far as I can tell.

I was unable to find any kind of good review information about this amp doing a Google search other than "this amp is awesome". But I'm finding the following to be the case:

• The tone control doesn't have much change across the entire sweep. Noticeable but not dramatic.
• Tremolo same thing: it goes from lowest setting moderately fast, to highest setting just a bit faster
• The tone is overall quite dark. Les Paul low notes are really not that good on neck pickup. Certainly a lot of great rock "girth" on bridge. Fender strat is OK. Need the tone all the way up to get much Vox sparkle/jangle.

My first question is: are these conditions normal for a Vox AC4? My second question is: are there some tried and true mods to improve it in some of these areas? What would you recommend for a speaker (I'm looking for an 8 or 10 inch speaker, not 12-inch).

Thanks
Mark

[HTH]

The 0.05uF coupling cap from the plate to the volume pot is much bigger than you'd normally get on a Vox amp (hence the raunchy rock tone these amps can get). For more of that chime, I'd be looking at changing that cap to anywhere between 500pF and 4700pF.

For a more responsive tone control, look at the one Vox use on the AC4TV, which is in turn basically the 18w Marshall type of arrangement. The stock tone control is just rolling off treble, but that may work out fine if you scale the 0.05uF coupling cap down.

[mbratch]

The 0.05uF coupling cap from the plate to the volume pot is much bigger than you'd normally get on a Vox amp (hence the raunchy rock tone these amps can get). For more of that chime, I'd be looking at changing that cap to anywhere between 500pF and 4700pF.

For a more responsive tone control, look at the one Vox use on the AC4TV, which is in turn basically the 18w Marshall type of arrangement. The stock tone control is just rolling off treble, but that may work out fine if you scale the 0.05uF coupling cap down.

Thanks HTH I will give that a try. The AC4 has some interesting tones in there, but it's somewhat muddy in the low/low-mid range that spoils it for me. We'll see what happens with the cap change. I'll also have a look at the AC4TV tone circuit.

[HTH]

no problems dude, post back and let us know how you get on.

[mbratch]

no problems dude, post back and let us know how you get on.

Replacing the 0.015uF EF86 plate coupling cap (formerly 0.05uF in the original circuit) with a 1,000pF was a noted improvement! I tried 500pF and it was a bit too bright for the particular speaker I was trialing. Once I get cracking on a cab and speaker, I might need to adjust it a bit. We'll see. I'm stoked, though, as this really brought some definition on the low end as well as added more jangle on the top. With that change, the somewhat shallow sweep of the tone control is not an issue. I also bumped one of the caps in the trem circuit from .01 to .02 to get a slower trem frequency on the low end of the adjustment. It's almost perfect.

[HTH]

nice one dude, glad its working out for you.