McIntosh MC 30 low humming& hissing sound help!!!

[Victor]

Yes that is better. I like the quote about 30 watts being more than the power used in a 5000 seat auditorium. You have to love those old efficient speakers they used.

Thank You,
Victor

 

[redhog]

Yes that is better. I like the quote about 30 watts being more than the power used in a 5000 seat auditorium. You have to love those old efficient speakers they used.

Thank You,
Victor

Victor
You have to respect the logic!!
Down Beat Magazine is still in business but I could not find this article in their archives

Try this one more time

 

[R-Y-R]

Victor
You have to respect the logic!!
Down Beat Magazine is still in business but I could not find this article in their archives

Try this one more time

Is it just me or are these ones out of focus ? Thnaks for the efforts.

 

[Victor]

Just in case you are curious. There is a pair of C4s on ebay right now, the auction ends in 3 days.

Victor

 

[redhog]

Just in case you are curious. There is a pair of C4s on ebay right now, the auction ends in 3 days.

Victor

Thanks for the heads up I'll check it out
RH

 

[R-Y-R]

RH, check your Pm's. THX

 

[redhog]

With the MC30 gain all of the way down try the hum balance on the 30, no effect ? next pull out the first 12AX7, if it get quiet you have found the problem. Each small tube may be checked this way, if you get to the last 12AX7 and still have hum you have a weak bias supply filter or possibly a bad main filter though if replaced a short time ago it should be good.

Hi dewickt

I pulled each of the small tubes and listen for increases or decreases in hum/hiss. I then put each tube back in the MC30. Then pulled the next set. The last set of 12AX7 quieted the hum/hiss to barely audible. Putting the tubes back in the MC30's the sound was more audible. Do you have any thoughts? :thmbsp: :thumbsdn: :scratch2:
RH

 

[dewickt]

Swap the first 12AX with the last 12AX, the problem with the last 12AX is that it will also pass any noise generated in an early stage and add it's own noise, if the final swap has no effect it may be time to check resistors, especially the 220K's on the last 12AX7.

 

[dshoaf]

My guess, for what its worth....

Terry, good job of eliminating what's *not* wrong.

I'd also bet that there are resistors out of spec in that amp. The last ones I recently re-did had well over 50% of the resistor compliment out of spec with more than one with hairline cracks in the casing.

Also, I'd not overlook the tube socket connectors as a source of noise. If they're loose, they can cause all sorts of gremlins, too. At the age of these amps, they really need to be looked at to remain stable and safe for operation, IMHO. Once done, though, they'll sing like they did as they left the factory back when Kennedy was in office.

Cheers,

David

 

[dewickt]

I agree on the MC30 resistors, just had 2 virgins on the bench and with minimum repairs they needed new bias supplies, new main filter caps, 2 of the last stage .22uf coupling caps, 12K's and 220K's. If I hd been given a free hand on them I would have replaced all the bumble bees and at least 4 more resistors. Even with the minimum repairs they did 35W before 1% THD and at 30W had less than .2% THD, not bad for 45+ year old amps.....