Vox Treble Boost Problems After Modding

[MikeHop]

I've just made a treble boost pedal utilizing the Vox circuit. After finding the sound was too toppy I changed the 500p input cap to 4n7 which sounds much better to my ears. I also included a 100K pot on the output as per schematic mods. On the in-between sounds of the neck and middle pickup, it sounds great, but when I use the bridge pickup it distorts - as mentioned on the schematic when the cap value is changed. I've been told that the problem is probably due to the bias being changed due to the change of cap, ie., lets thru more bass which overloads the booster. The biasing resistors are 100k, +v to tx base and 22k, tx base to earth. Can someone please advise if biasing is the problem, and what I should change the value of these resistors to, so I get a clean sound when using any pickup?



Thanks.

[Mick Bailey]

Does the booster distort at all frequencies with the bridge pickup, or just the lower positions on the lower strings?

[eschertron]

How far down do you have to turn the volume on the guitar before it cleans up? That will give an idea of the headroom needed for the signal.

[MikeHop]

Thanks for replies.
I'm using an Ibanez Roadstar II RS440. The booster distorts at all frequencies. When using the bridge pickup in humbucking mode, the sound cleans up when the vol pot (500K Lin) is turned back by approx 50%. When in single coil mode, the vol needs to be turned back by approx 1/3rd.

[eschertron]

Thanks for replies.
I'm using an Ibanez Roadstar II RS440. The booster distorts at all frequencies. When using the bridge pickup in humbucking mode, the sound cleans up when the vol pot (500K Lin) is turned back by approx 50%. When in single coil mode, the vol needs to be turned back by approx 1/3rd.

If you're at 50% and you can clean it up, try running the pedal off two 9v batteries in series (18v) to get the same headroom at full volume on the guitar. From the schem it appears nothing would be adversely affected...

[MikeHop]

I'll give that a try on the weekend. If it does work at double the voltage, am I right in thinking that if I replace the biasing resistors with ones half their value, I could get a similar result from 9v? Reason I'm asking is that I have a place ready for it on my pedalboard, which only has 9v psu.

[eschertron]

I'll give that a try on the weekend. If it does work at double the voltage, am I right in thinking that if I replace the biasing resistors with ones half their value, I could get a similar result from 9v? Reason I'm asking is that I have a place ready for it on my pedalboard, which only has 9v psu.

The reason I suggested doubling the voltage source is to get 2X the headroom for the gain stage, to allow the louder signals through without clipping. If you are running out of head room and want to use just the single 9V, then you'll need to adjust the gain of the circuit down. Not sure if that will kill the whole boost 'feel' of the stage. My transistor design chops are rusty enough that I'd have to go look to see what can be changed to center the bias or tame the gain a little.

[Mick Bailey]

Check that the collector voltage (measured from collector to ground) is 1/2 the supply voltage; this gives the maximum +/- output voltage swing. This can be tweaked by adjusting the 100k resistor.

If your collector voltage is centred then the input signal may just be driving the circuit too hard and you need to reduce the inherent gain of the circuit or attenuate the input. You could move the volume control to the input side and see how you like it - it also means you retain the flexibility of the original circuit (if you reduce the gain of the circuit by changing the resistors you may find there isn't enough gain for weaker pickups) The control needs to go before the input cap. To retain the sparkle you may need a 470pf bypass cap on the pot.