Orange Bass/Treble Booster whistle

[gigimarga]

hello,

i had built an Orange Bass/Treble Booster using this schematic http://www.geofex.com/FX_images/orngbst.gif and i replaced the 10K resistor from the transistor collector with a 10K log pot for volume, as i saw in a Rangemaster schematic (i solder the 0.1mF output cap on the wiper of the pot).

when i turned the pot at minimum a strong whistle (oscilation) appear and i don't understand why.
i had built over 5 Rangemasters and i had no problems with them!!

thx a lot!

[gigimarga]

 

[R.G.]

It's likely that it oscillates because the pot is effectively using +9V as "ground" instead of the real circuit ground. I think it will quit if you put a 100K volume pot on the output to real ground and take the output off the wiper of this new pot.

[gigimarga]

Ok...i will try...thx a lot!

[Ronsonic]

The pot should come after the output cap. A larger value pot is probably in order, try 50 or100K, but the main thing is that the pot has to be isolated from DC. First off, it is inserted into the circuit where it doesn't belong and really screwing up the output, and even if a big enough pot prevents the oscillation, the pot will be noisy because of the voltage on it.

Ron

[soulsonic]

That circuit shouldn't even be built.
Anyone who's built one knows...

[gigimarga]

why not to build?!!!