Lava Rim 2 Build with fuzz drive pot issue

[NevynShyne]

Hello all.
It's my first time posting after lurking for several months. I've poured over the archives and used the search function exhaustively and have learned a LOT! Thanks!

I've completed my first build on a proto-board. It's the Lava Rim 2 (minus the diodes as of now). A rather basic fuzz that I figure was a good starting point. I'm using 2N9304 (hfe at 160ish) in Q1 and NTE123 (hfe 280 ish) in Q2. I've followed the schematic as well as I can figure and the circuit works and sounds as expected except for one thing... dot dot dot

When I peg the A5k pot for the fuzz drive, the circuit gets extremely noisy and harsh. It works fine up till you hit the max sweep of the pot. Even at what I figure to be 99% full on, it's fine...But once I hit 100% - NOISE and extreme treble.

I replaced the pot with other identical pots (all A5k Alpha pc mount) and it's the same thing every time. I thought that maybe the bias was wrong and I was simply over-powering something, but double-quadruple checked everything and bias seems to be right. I even changed the 100k feedback resistor to a 47k resistor thinking it would lower the gain a little. It did quite a bit to the SOUND of the circuit (a good thing, I believe), but did not alleviate the weird A5k pot issue. Interestingly enough, doubling the 100k feedback resistor to 200k made it sound like the gain was on the drive pot was at 100k regardless of it's setting, making me believe that gain was the issue with the pot. Make sense?

The circuit sounds darn good except for this one little issue. Would changing the A5k pot to a higher ohm pot be a fix? Is this something that I'll just have to accept as 'normal' in this circuit?

Any advice is welcome and appreciated. I've enjoyed the *crap* outta my foray into pedal building so far, and I think I'm addicted!

Carl

[NevynShyne]

Ooops. The pot is a B5k, not the A5k. Still got the 'issue', but figure it's probably what the pot does at it's max setting. I'll be subbing different value pots and trying different caps in the drive portion to see if it helps. It's not a big deal, but darnit! I want it perfect.
Carl

[aron]

Carl,

One option is simply to stop the problem. I believe you can put a 100-470 ohm resistor in series from the pot to ground. I think what is happening is that the pot doesn't like to be grounded. (At least that's what I think)

[NevynShyne]

Hi Aron. Thanks for the response and the very fun circuit! I will try the resistor. It makes sense.

Carl

[NevynShyne]

Hi. I tried your suggestion and guess what? It works! Thanks for the coolio circuit.  Now I gotta take it off the proto board and put it on something that can go into a box. woot!