LBP2 - Distorting/Overdriving

[ubersam]

I recently built an LBP2 based on the GGG schematic. I did not have all the correct resistor values so I substituted them with the closest values that I had on hand, also, I substituted the 390ohm resistor with a smaller value (220ohm) for more gain. All the substitutions are shown in the schematic below (in red):

I tried the finished booster on the clean channel of my Road King II. It sounds fine except is seems to be distorting, or going into overdrive. Could this be because of the substituted values? Or, is the output big enough that it is just slamming the front end of the amp, sending it into overdrive? I was hoping for a 'clean' boost, more of a volume boost without the additional overdrive. Another observation I had was that the booster seemed a bit on the trebly side.

I have some ideas and I wanted to inquire with you all to see if they would work. Would an MPSA18 have more overhead before distorting than a 2N5089? If I changed the collector resistor to a 100k pot and the emitter resistor to 10k, will the circuit then have variable gain ranging from 1 to 110? Will I there be a clean boost in the lower gain settings? If I changed the caps from 0.1uf to 0.2uf, will it be less trebly? The schematic below shows the changes I was thinking about:

(The schematics above are based on GGG's LBP2 schematic.)

[Angle Loss]

Until the those in the know step in, I can give this advice.  First change the 220 ohm (normally 390) to a higher value.  I had a 390 and went to a 780 (from what I recall) and it decreased the overdriving.  I would start there first.  The rest of the guys can tell you what else you might want to do.

[newbie builder]

Yes .22uf will add lots of bass but I think it almost makes it tooo muddy and bassy-- fine though if it's a lead boost, not for chords though. 780ohm to 1K is pretty clean, I have a switch between the 1k for a clean boost and 390ohm as a dirty boost as well as stock input/output caps vs. .22 in both places. Makes for a very versatile and tweakable booster.

[amz-fx]

Increase the gain and you will increase the distortion as the circuit runs out of headroom. This article explains why:

http://www.muzique.com/lab/boost.htm

regards, Jack

[ubersam]

Thanks all. I tried the 2nd schem last night. I got a little gain happy and substituted 1Ks in place of the 10Ks. Up to about a third of the 100ks pots sweep sounded o.k. but distorted in the higher settings. Past that, things get ugly. I don't know how else to decribe it but you all probably know what I mean.

The 0.22uf cap did give more bass but as newbie builder mentioned, it made it more muddy. I'll try a 0.15uf next. If I were to make the LBP a variable gain booster, where would be the best spot for a pot? At the collector or at the emitter?