Vox 830 Distortion Cap mod??

[dumbassbob]

I have a vox 830 dist,booster,it is well made as far as parts,but it is bright.The C-1 cap just says 47 I guess that is 47 pf..The curcuit is sort of a Boss DS1 and a tube screamer.. Would changing some of these PF caps to a higher value give me a warmer fuller sound...I use it with a bright Plexi 100 watt ri.Are Vox AC30 on the dark side.Is that why there so brite

[Jazz P Bass]

It is all about the circuit, not the individual parts.
Post a scheme.

[dumbassbob]

It is all about the circuit, not the individual parts.
Post a scheme.

I have done this to aBoss SD1 and changed the SI Diodes to GE and most of the Coupling caps,and it made a huge differece in the sound,same thing with my Boss DS1 distortion,turned both into alot nicer pedals...I just wanted to knowif getting rid of the 47pf and puting in higher value cap would help to warm it up..I thought maybe these pedals where made to plug into the RI Vox AC30 when they first came out in 1989-92

[Jazz P Bass]

Here is the schematic.
Now, which cap are you aiming at?
You have to watch 'warming up' a distortion pedal.
The lows sound like crap distorted.
I see that they use 0.22 coupling caps.
That low value will certainly kill the bass.

[J M Fahey]

What an un-Voxy circuit !!!
Although it probably uses all the correct graphics, logo, knobs, etc.
Oh well.
15K input impedance ?
What were they thinking?
C1 is 47nF (.047uF), not 47 pF , and increasing it will not help, since the guitar is loaded after it with such a low impedance.
Maybe it looks fine if driven by a generator, but bad for a regular guitar pickup.
I'd replace R2 with a 150K to 470K resistor, and see what happens.
The bass cut is provided for elsewhere else anyway, as in: R4+C3 and R8+C6 , but this bass cut is necessary to avoid farty muddy bass, don't touch them.
The active filter/gyrator built around T1 provides some boost around 280 Hz, probably to "increase warmth".
The tone control is pure Nig Nuff type, and you know that can be set real buzzy.
What happens if you set tone control to "bassy" ?
I'd only raise R2 and check.
Post results.

[pdf64]

'15K input impedance ? What were they thinking?'
Perhaps they were hoping to emulate the way that a fuzz faces's low input impedance gives it good 'turn down ability' from the guitar?
Pete

[Mark Hammer]

There are two clipping stages. In the first stage (IC1a), the 47pf cap both limits how close the high frequencies get to the clipping threshold of the diodes, and also trims off some of the treble content resulting from that clipping. However, it doesn't remove very much of it. Indeed, even at max gain, it's effect really only kicks in around 16-17khz.

In the 2nd clipping stage (IC1b) the output of IC1a is reboosted and then fed to a 2nd pair of clipping diodes to ground. The overall design is a bit like a Tube Screamer fed to a DOD 250, and then to a Big Muff toe control. The 47pf feedback cap plays essentially the same role, except that it only limits what is fed to the clipping diodes, and does not trim the fizz from the product of that clipping. The real filtering of the treble, and "warming", is provided by the action of C10 and C11. C10 trims starting around 3400hz, and C11 at around 13,300hz.

Of course, what they have to trim is a function of what is being sent for clipping. So there is some rationale for changing the values of either, or both, C4 and C7. My own feeling is that replacing C4 and C7 with 150pf would limit the top end that is being clipped, and allow for the other filtering sections to do their job better.

Note as well that the little subcircuit built around T1 is actually an EQ-pedal-like gyrator circuit that is functioning like a fixed resonant boost. You might want to consider lifting just one end of C13 to see if you like the result without the resonant boost. If you don't mind popping a hole in the chassis, it might even be useful to insert a toggle to engage/disengage the resonant boost circuit.

[dumbassbob]

Here is the schematic.
Now, which cap are you aiming at?
You have to watch 'warming up' a distortion pedal.
The lows sound like crap distorted.
I see that they use 0.22 coupling caps.
That low value will certainly kill the bass.

Thanks for All the input and to Jazz P. for the schemactic.It doesn't sound too bad,I will post back....

[dumbassbob]

I tred changing R2 to 150K,it made it sound like a farting fuzztone I put the old R2 back in .The schematic doesn't mach up to my Vox830 like C-7 is not 47PF it is a 47UF electrolytic ,and R2 is not 15K it is 05.20K and C1 IS 47PF and not 47N.It is funny how a pedal won;t sound good in one amp,but great in another.One reason it was sounding bad the battery clips were covered in corosion,and the battery had a leak after I cleaned it,it sounded OK

[Jazz P Bass]

.One reason it was sounding bad the battery clips were covered in corrosion,and the battery had a leak after I cleaned it,it sounded OK

My kind of mod.

[Mark Hammer]

Sounds to me like the component IDs in the schematic I was using do not correspond to either the schematic you were using or to the legending on the PCB.

But yes, poor connections between the battery and power-supply lines on the board can result in very poor performance....much as you might expect from a battery that supplied barely a tickle on your tongue.