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Delayed Hum on MC30

angelaudio

angelaudio

Super Member
One of my MC30’s has started to produce a high pitched hum that starts about 5 or so seconds after being plugged in. It starts off loud and then slowly decreases in volume until it levels off. I’ve ruled out the preamp as an issue and have switched the tubes from one MC30 to the other. This MC30 has the power supply rebuilt in 2001 so none of the old caps are inside. What could be the issues here? Thanks in advance for any help.
 
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Out of curiosity, what type of caps were used (if you know off the top of your head). If the can cap was done with a CE can cap, I've heard a lot about them failing early on in life.
 
nickdinatale said:
Out of curiosity, what type of caps were used (if you know off the top of your head). If the can cap was done with a CE can cap, I've heard a lot about them failing early on in life.
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And as far as the other caps go they’re Illinois Capacitors WRM series
 
The metal can is the common negitive. These caps are typically used to filter low frequency pulses from the mains AC feed to the rectifer and then out of it. So 50,60 or 100,120, low frequency noise or hum.

I would expect one of the smaller caps intended to supress high frequency oscilation to be failing.
 
c_dk said:
The metal can is the common negitive. These caps are typically used to filter low frequency pulses from the mains AC feed to the rectifer and then out of it. So 50,60 or 100,120, low frequency noise or hum.

I would expect one of the smaller caps intended to supress high frequency oscilation to be failing.
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Thanks c_dk!
 
I'd be real curious who made the external multi-section can capacitor on that amp. These amps usually have a cap with 80-40-20 sections rated at 500 Volts or at least 475 volts. Yours is rated at 450 volts for two sections and I see no manufacturer name or manufacture date. The manufacture date would be something like 7236 (36th week in 1972) and should be somewhere on the can. I am aware of no new manufacture can cap with 100-40-20 sections in the past 20 years so perhaps this is an NOS from many, many years ago? Do both amps have this same exact can cap? I just looked at a bunch of Antique Electronic Supply catalogs from 1998 through 2008 and I don't see that they ever offered a cap, Aero-M or CE Manufacturing, with the values shown on your cap. I suspect it is an old cap from the 60's or 70's so you should look for that manufacture date. It should be etched/inscribed on the metal side of the can, somewhere. Personally, I'd replace it unless it has a recent (last 10 - 15 years ago) manufacture date. Just MHO.
 
tuberadios69 said:
I'd be real curious who made the external multi-section can capacitor on that amp. These amps usually have a cap with 80-40-20 sections rated at 500 Volts or at least 475 volts. Yours is rated at 450 volts for two sections and I see no manufacturer name or manufacture date. The manufacture date would be something like 7236 (36th week in 1972) and should be somewhere on the can. I am aware of no new manufacture can cap with 100-40-20 sections in the past 20 years so perhaps this is an NOS from many, many years ago? Do both amps have this same exact can cap? I just looked at a bunch of Antique Electronic Supply catalogs from 1998 through 2008 and I don't see that they ever offered a cap, Aero-M or CE Manufacturing, with the values shown on your cap. I suspect it is an old cap from the 60's or 70's so you should look for that manufacture date. It should be etched/inscribed on the metal side of the can, somewhere. Personally, I'd replace it unless it has a recent (last 10 - 15 years ago) manufacture date. Just MHO.
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I couldn’t find any date etched into it so it must have been a NOS part when installed. The previous owner did a full recap back in 2001, not sure if the van cap was also replaced at the same time but either way it looks (sounds) like I’m going to need a new one. I couldn’t find any can caps with the 100-40-20 values either when I searched so I’m going to go with the 80-40-20 at 500V. Thanks for your input @tuberadios69!
 
I had hum issue for different reasons, but when I changed the tubes, the hum went away. By the way, my MC30s are all original, including the cap cans. Sounds fantastic.
 
cspyon said:
I had hum issue for different reasons, but when I changed the tubes, the hum went away. By the way, my MC30s are all original, including the cap cans. Sounds fantastic.
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I’m also getting some popping so I’m pretty certain it’s a capacitor. Probably the can cap seeing as how the values are indicative of a NOS part. Your MC30’s must have been gently and properly used over the years! I have a pair of the early A-116B versions, one that’s had the power supply recapped completely and the other that’s had only a few caps replaced. I’m not firing them up again until I fix the issue though.
 
angelaudio said:
I’m also getting some popping so I’m pretty certain it’s a capacitor. Probably the can cap seeing as how the values are indicative of a NOS part. Your MC30’s must have been gently and properly used over the years! I have a pair of the early A-116B versions, one that’s had the power supply recapped completely and the other that’s had only a few caps replaced. I’m not firing them up again until I fix the issue though.
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Have you ever touched the cap can and checked its temperature while the amp is running? If it is hot to the touch, I hear it's a sign of failure. Mac gurus here and my technician say it should stay just warm, not hot.
 
That can cap is not original, and I’m pretty sure it looks like a custom can from hayseed hamfest by the polished can.

It is possible that it is failing, but try swapping tubes from the other amp, one by one, to see if you can isolate the problem that way. It could also be another cap underneath.

If it has bumble bees still installed, I’d swap them out as they are most likely defective.
 
x3workshop said:
That can cap is not original, and I’m pretty sure it looks like a custom can from hayseed hamfest by the polished can.

It is possible that it is failing, but try swapping tubes from the other amp, one by one, to see if you can isolate the problem that way. It could also be another cap underneath.

If it has bumble bees still installed, I’d swap them out as they are most likely defective.
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Tried swapping all tubes from the other MC30 and nothing. The power supply was rebuilt in 2001 with all new iC WRM caps so I’m eyeing the can cap now
 
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