Clipping input on GK MB150?

tornadobass · #1 03-23-2008, 08:10 AM

I just picked up an older used GK MB150 combo for small gigs, practices, jams, etc. It just got back from the factory where it had the power supply upgraded to the current version, plus a bench test, so it has a clean bill of health.

Anyhow, I've noticed that the input clips on an open E with a RSII pickup, producing audible distortion. This happens even at low volumes, so it's not the power amp. And my G&L 5 string absolutely needs the -10db pad turned on to avoid crunchy low notes. If I back off the volume a bit on my BugBass (w/BassMax), I'm fine and don't need the pad.

Anybody else find that this little rig clips easier than expected?

BluesMan845 · #2 03-23-2008, 08:20 AM

I used to use this amp ( it's now fried and sitting in a closet ) and I never noticed any input clipping issues. I ran a Carvin LB70 ( 9v active electronics ) into it and was able to get the amp quite loud before clipping distortion would occur.

If your bass has a really hot signal going into the front of the GK, maybe try turning the bass' output down like you have and use the BOOST knob to get back some of the volume you are losing at the input level.

As a side note. I was a huge GK fan for years, having owned 4 different amps/combo's but all of them but one (GK700RBII) went down and needed repair. My MB150 was repaired twice before the most recent breakdown which would cost me nearly $400 to fix. Not sure if I just had some really bad luck or if GK just had a bad run for a while as far as Q.C. .

Anyway, good luck.

fdeck · #3 03-23-2008, 06:01 PM

A piezo pickup, if necessary, can be padded by a capacitor in parallel with the pickup. But it might take some experimentation, starting with roughly 0.01 microfarads. Prefer polymer film type, voltage rating irrelevant. One can get these at Radio Shack. If you buy two, you can try them individually, in series, or in parallel, for 3 gain options.

Also, if doing this on a trial basis, no soldering is needed. Just sneak the cap off of the plug on one end of a regular cable, and twist the wires on.

speedster · #4 03-24-2008, 10:00 AM

I have a GK MB150 and had a MB 200, on occassion it starts to cut out sometimes. What it ends up being is the 1/4" jacks getting dirty.

Usually the send and returns get dirty from lack of use, if you run a plug in and out of the jacks they usually clean up.

I take it in once a year now and have it pulled apart and the jacks cleaned at a shop for $25 it's good insurance.

Other than that never had an issue with the GK, this problem is not just isolated to GK amps. Lots of amps have the same issues....

Beautiful electronics and crappy jacks......

tornadobass · #5 03-24-2008, 10:38 AM

Thanks for the ideas. After reading the MB150 manual, I noticed that if the amp distorts at low volumes, try the input pad. I did that with my G&L and the distortion went away. As suggested, the boost knob can make up a little gain, if needed.

With my BugBass, it fortunately has a volume control built in. I've never quite understood how running a BassMax piezo directly into a 250k pot would work, but somehow it does. In that case, keeping the volume pot to about half prevents distortion and avoids using the pad.

My URB with RSII pickup poses a dilemma. Run straight into the amp, the amp has some distortion, but only on the low E string. With the HPF-pre...helpful for its phase reverse switch, there's that bit of gain boost and it distorts more easily...a good candidate for the pad.

Interesting idea about the cap inside the jack. Long ago, I had a Polytone pickup, the brass one that went between the bridge legs. It had a cap in the jack, but I thought that was to filter some highs rather than to lower the output level.

I don't think what I'm hearing is the effects loop jacks since it goes away by using the input pad. But I do remember having that problem on a 400RB head a few years ago.

bolo · #6 03-24-2008, 10:45 AM

Quote:

Originally Posted by speedster

I have a GK MB150 and had a MB 200, on occassion it starts to cut out sometimes. What it ends up being is the 1/4" jacks getting dirty.

Usually the send and returns get dirty from lack of use, if you run a plug in and out of the jacks they usually clean up.

I take it in once a year now and have it pulled apart and the jacks cleaned at a shop for $25 it's good insurance.

I'm sure the periodic maintenance by a qualified tech is a good thing. But you can do your own PM of sorts on the jacks w/ a shot of one of the appropriate Caig DeoxIT products. You don't need a lot. Spray the end of 1/4" cable, insert it into the jack, rotate it a bit and you're done.