Proco RAT 2 - High frequency noise when pedal is turned off

[rockaffe]

So, when the pedal is off there is this high frequency noise. The noise disappear when the gain pot is at 0 and gradually increse going to 10.
When the pedal is on everything works perfectly.
Any ideas?

Thanks!

[Mark Hammer]

The Rat applies a LOT of gain.  If the volume of the HF noise corresponds to he Gain control setting, that strongly suggests the layout or wiring may need to be altered.  It sounds like it is picking up something it shouldn't, and amplifying it...a LOT.

[antonis]

And make feedback cap (100pF) 220pF or so, just in case...

[FiveseveN]

Also make sure the input is muted (grounded) in bypass.

[rockaffe]

Just made few tests before modify the circuit and, with a buffer in front (I use JHS Little Black Buffer clone) the noise completely disappeared.

[Mark Hammer]

And make feedback cap (100pF) 220pF or so, just in case...

Good recommendation.  We may think of such feedback caps as "filters", but what they really do is drastically reduce the gain for frequency content above some designated frequency.  Happily, that reduced-gain corner frequency also depends on the feedback resistance; the higher the resistance, the lower the frequency where the gain-reduction begins.  Since the feedback resistance is being used to increase the gain, that means  you get MORE attenuation of whatever the whine source is as the gain is increased.  Handy.