ROG 18

[ildar]

Just finished the 18-my first ROG project!    It sounds pretty nice, except I'm getting some oscillation at high gain settings. It's there if I run it through my preamp, less so if it goes straight through my power amp.
Could the FETs be misbiased?

[B Tremblay]

Have you checked the FET pin voltages with a DMM?

What type of enclosure are you using?

[ildar]

Well, the oscillation problem is solved, now something else: when I turn the gain pot all the way down, the signal cuts out completely, as if the volume was turned down. Of course, the volume control works normally. Is this normal for this circuit?

BTW, I answered my own question about driving my Boogie power amp with one of these pedals-a resounding OH YEAH!

[RDV]

...when I turn the gain pot all the way down, the signal cuts out completely, as if the volume was turned down. Of course, the volume control works normally. Is this normal for this circuit?

You could put a 10k or so to ground on pin 1 of the volume control to stop that if you want.

RDV

[B Tremblay]

...when I turn the gain pot all the way down, the signal cuts out completely, as if the volume was turned down... Is this normal for this circuit?

Yes.  The reason?  Because it actually is a volume control.  We just called it Gain, since that's a more accurate description of its function in that circuit.

In most of the amp circuits of which we've developed a FET emulator, there is no Master Volume present.  If we didn't add the Master Volume, then the distortion would only be available when the Gain (the original amp's Volume) was cranked, which would also result in a very loud output.  With the Master Volume in place, the user can dial in the amount of distortion with the Gain pot, then set the output with the Master Volume.

We've considered adding a low resistor to prevent the Gain functioning as a "second volume control" at the lower end of its rotation, but opted to remain true to the original amp circuit.  However, RDV's suggestion is the same as I would offer - simply add some small series resistance between lug 1 of the Gain pot and ground.

[ildar]

Thank you, gentlemen. I will follow that suggestion.