Mc-30 odd sounds...

Hi folks, needing advice here. I have a pair of Mc-30 amps, they were last checked about 10 years ago ( I had a tech go through them and check them for marginal or faulty parts, and get them sounding sweet). Lately the one amp will make ( on occasion) loud deep rumbling sounds for about 3-5 seconds and then return to normal. This happens usually after the amp has been on for 30 plus minutes, and occurs maybe 1 out of 6 times. The other oddity is that when I turn the amp off it makes a similar sound after I have "thrown the switch" , although it is not nearly as loud. This happens maybe 3 out of 5 times.
Any idea as to what is going on? Possible things to check out? When it does act up I see no redplating or arcing. Thanks, Ed

Sounds to me like a capacitor is going bad. If your tech didn't replace ALL the caps in your amps previously, now is a good time to do so. That includes any remaining Sprague Bumble Bee caps (black with colored stripes) and all the electrolytics including the two external multi-section twist locks. You can determine their age by the date code on the side of these aluminum caps. For example 677-9018 equals the 23rd week in 1990. If they are original, they'll probably date back to the 1960's. Those caps usually cause hum as they go bad, however. If the selenium rectifier hasn't been replaced, the power transformer may run very hot. It needs to be replaced by a diode and a resistor to compensate for the increased voltage passed by the new diode. (Am I right guys?)

OOps! Error on date code of twistloc caps

Yikes! I meant to say a date code of 677-9018 equals the 18th week in 1990. Sorry about that!

Oops! Error on date code of twist lock caps

Yikes! I meant to say a date code of 677-9018 equals the 18th week in 1990. Sorry about that!