Strange ts808 problem

[Chris Goodson]

I recently built an 808 following the info at GGG.  It seems to work okay except that when I turn the pot the sound cuts out at about 7/8 of the rotation.  I tried replacing the pot, but nothing changed.  Anyone know a possible cause for this?

[GuitarLord5000]

Is the pot value you are using the same as the pot value on the schematic?

[Jay Doyle]

Which pot?

[Chris Goodson]

I'ts the drive pot(I meant to say that :oops:).  Also, I used the right value.

[Mark Hammer]

I constantly find wipers that don't make perfect contact through the entire rotation, and conductive elements that don't make perfect contact with the rivets holding them in.  Plenty of times a shpritz of contact cleaner can fix things, but occasionally you need to pop the back of the pot and really clean things out, squeeze the rivets with needle-nose pliers a little for more positive grip on the conductive element and even clean and retension the wiper.

Here's the part I don't understand.  In a non-inverting op-amp configuration like this one any sort of open circuit in the pot should not result in no signal at all.  Since the gain is a function of the ratio of (Rfeedback+Rground)/Rground, then an open circuit and infinite resistance from the pot should result in very high gain if the pot is part of the feedback loop (in this case constrained by the diodes), and some gain level a little higher than 1 if the pot is in the ground path like the DOD/MXR distortion boxes.

Not unless what you mean to describe is some sudden surge in gain (and current requirements) when the pot goes open, resulting in a momentary "squashing" of the signal until it regains its composure.

[Johan]

Here's the part I don't understand.  In a non-inverting op-amp configuration like this one any sort of open circuit in the pot should not result in no signal at all.  Since the gain is a function of the ratio of (Rfeedback+Rground)/Rground, then an open circuit and infinite resistance from the pot should result in very high gain if the pot is part of the feedback loop (in this case constrained by the diodes), and some gain level a little higher than 1 if the pot is in the ground path like the DOD/MXR distortion boxes.
.

..just a thought...if you have, in theory, infinit gain, the slightest difference in the resistors making up the 1/2voltsupply biaspoint would/could result in an DC offset that would also be amplified to infinity resulting in just DC on the output...have to test that tomorow...just thinking out loud...makes sence?...tired now, must sleep..

Johan

[mindwave_21]

I recall that the GGG used a 25K linear drive pot instead of a 20K linear, which is what tubescreamers actually use.  Could this have anything to do with the sound cutting?  I'm not sure how that affects the gain, but just thought you should know.