MC30 60hz hum

[mikemc99]

I recently acquired a pair of MC30's. One was original which I replaced caps and rebuilt the PS w/ silicone diode and bias resistor. This MC30 works great. The other MC30 was built by another person with all new caps and resistors. That MC30 has a steady 60hz hum on the speaker output regardless of where the gain is set. I can hear the hum from a couple feet away. I've tried swapping the tubes with my good amp without any change. The hum seems to go away when I remove either 12AX7 which seems to imply to me that it is something at the "front" of the amp. I've spent hours going through this amp and verifying all components are correct values and landed correctly. Everything seems correct (and measures correctly). I've compared measurements to my good amp and still haven't found a smoking gun. It seems that it may have something to do with the feedback circuit. When I put a 3k resistor in parallel with R13, the 60hz hum virtually disappears and is on par with my other amp. I verified that R13 is 2.2K (correct value according to schematic I have). I also swapped C7 with another 470pf cap w/o a change. I've also tried swapping C12 and the other electrolytic caps w/o change. Does anyone have any suggestions what next steps I should take to help eliminate this hum?
Thanks,
Mike

P.S. the 60hz is read from my scope on the output of amp. It isn't 120hz.

 

[mikemc99]

Followup...
I finally sat down and spent a bit of time to figure out what would be causing the hum. I had the two MC30's side by side comparing the wiring and components. All the components were the correct value and tested good. However, I did notice some differences in the routing of the wiring. Specifically, the ground wiring. The previous owner of the amp that had the hum completely rebuild the amp but decided to have the ground path routed a little differently. I noticed on the MC30 that I build (which was 100% original before I replaced caps), the ground for the signal input would go directly to the chassis ground on C12. The amp that the other person worked on had the ground go through the turret board components instead of being directly routed to the chassis ground. I routed the ground identically to my other MC30 and...no more hum! This was a little bit of a bugger to locate because electrically everything measured identically. Thankfully, I had an unmolested MC30 to use as a reference. I can now enjoy these without that constant humming.

 

[macyjrm]

Well done! Welcome to posting on AK. We look forward to your future contributions.

 

[cspyon]

Followup...
I finally sat down and spent a bit of time to figure out what would be causing the hum. I had the two MC30's side by side comparing the wiring and components. All the components were the correct value and tested good. However, I did notice some differences in the routing of the wiring. Specifically, the ground wiring. The previous owner of the amp that had the hum completely rebuild the amp but decided to have the ground path routed a little differently. I noticed on the MC30 that I build (which was 100% original before I replaced caps), the ground for the signal input would go directly to the chassis ground on C12. The amp that the other person worked on had the ground go through the turret board components instead of being directly routed to the chassis ground. I routed the ground identically to my other MC30 and...no more hum! This was a little bit of a bugger to locate because electrically everything measured identically. Thankfully, I had an unmolested MC30 to use as a reference. I can now enjoy these without that constant humming.

Nice work! That's good to know. In my case, vintage AU7 Telefunken was to blame. When I changed them with RCA clear top AU7, the audible hum disappeared.