110 RD 50 - Distortion on Low Notes

Aaron_S

Hi - I've got an '82 110RD50 that sounds great but I get a distorted sound under most low notes and note combinations. This happens on the clean and limiter channels and all volume ranges. I've ruled out the speaker and power tubes. I've also performed a complete electrolytic cap job...that didn't have any affect on the distortion I'm experiencing.

Any recommendations for troubleshooting or parts replacement would be appreciated!
I love the tones and responsiveness of the amp...just want to get the distortion issue sorted out so I can use for gigs and jams.

Thanks - Aaron

Music Man Equipment
MM 2-10-65

Member for

11 years 2 months
Profile picture for user mm210

mm210 Sat, 10/31/2015 - 16:27

Are the IC's socketed in the rd50? If so, cleaning the sockets. I'm assuming that you have cleaned and re-tensioned the tube sockets. Also, the schem for the 50 shows a 12ax7 in the front end. Have you changed that? Mike.

Full Name
Aaron Strain
Music Man Equipment
110 RD-50
65 Reverb Head

Member for

9 years 7 months

Aaron_S Sat, 10/31/2015 - 17:27

Thanks for the suggestions! Yes, the IC's are socketed. I've just cleaned them as well as cleaning/retensioning the tube sockets. I tried swapping the 12AX7 too. The distortion is still happening. It's most noticeable on two note combinations.

Full Name
Lars Verholt
Music Man Equipment
HD-130 head, 210X cab, 210RH cab, RD-50 110 combo, Sixty-Five 112 combo, RP65-112

Member for

11 years 2 months

lmv Sat, 10/31/2015 - 20:44

I'll bet you a tuna sammich that you have crossover distortion going on. One thing to try is to swap the power tubes around hoping that any mismatch in the tubes could be compensated for by a complementary mismatch (if one can say so) in the drive transistors. If this does not help, the next step is to check that your cathode resistors are within spec along with the resistors in the bias circuit. In the end, you may have to get a new set of driver transistors or power tubes - or both - to get rid of the crossover distortion. Since there is no bias adjustment in these amps, it can be a pain in the neck to get rid of the distortion.

Cheers,
Lars Verholt

Full Name
Aaron Strain
Music Man Equipment
110 RD-50
65 Reverb Head

Member for

9 years 7 months

Aaron_S Mon, 11/02/2015 - 04:58

Good news! I replaced the Driver Transistors with 2N6488's from Mojotone. That cleaned up the crossover distortion and amp is sounding normal now. Thanks to all who contribute to this site...fantastic reference! - Aaron

Full Name
Ian Azrikan

Member for

11 years 2 months

inertian Sat, 11/07/2015 - 05:31

If you have ruled out the speaker and power tubes and replaced all 12 electrolytic capacitors correctly (double check polarity), test the zener diode (D9 on the schematic). If it is failing it could cause the distortion you are experiencing. Does the distortion get noticeably worse when you hit the strings harder?