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Noise & Hum Rating

I am having a hum issue - steady state regardless of whether the pre-amp is connected, and no change with an increase in volume - and the amp has been back to Cary where it checked out fine. At this point I've assumed it's a ground loop.

The manual for my amp, a Cary V12R, states "Noise/Hum: > 84db below rated output." Output on the amp is 50 watts triode and 100 watts UL. Does the rating suggest that there will always be some hum and that this is normal? Does anyone know how this rating works at a practical level? Cary didn't have much of an explanation except to say that yes, some hum is normal.

Thanks!
grimace
jea48
2,944 posts
 
Al,

Thanks for the kind words.

I Like your idea about trying the 4 ohm tap for the speakers.

Jim
grimace OP
624 posts
 
I did switch to the 4 ohm taps, and they are quieter. Not silent, but definitely quieter. There also doesn't appear to be any difference in sound, which is good.

I was a bit disappointed in Cary's service for a number of reasons. I don't think they looked very hard at the amp, although they were quick to sell an overpriced capacitor upgrade, and to try to charge me $40 each for EH EL 34s that cost $13 from any number of sources. If I ever send it out in for service again, it won't be to them.
jea48
2,944 posts
 
and to try to charge me $40 each for EH EL 34s that cost $13 from any number of sources.

Like ARC tested tubes,..... there are tested good tubes and then there are very good closely matched tested tubes.

Out of all the tubes Cary buys from the distributor I would bet they probably throw at least half of them away or sell them to another tube vendor because the tubes did not meet their testing standards.

You pay for what you get when buying new production tubes....
Quality control in tube factories ain't what it used to be in the 1950s and 1960s. Even the 1970s and 1980s....
grimace OP
624 posts
 
That's nonsense. My amp takes a dozen EL 34s. Cary tried to tell me that the tubes that were in it were bad. When I had them tested locally not only weren't they bad, they were actually very strong, and except for a little degradation in two of them, pretty well matched. Cary said they were all bad and needed to be replaced, and it simply wasn't true. They also said that the tubes were causing the hum, which also wasn't true.

The EH tubes they wanted to sell me are nothing special. Electro-Harmonix EL34s are an inexpensive tube. Tube Depot, Doug's Tubes, Amplified Parts - all of which were highly recommended by A-gon members - and a few others sell that same tube for $13 (or $27 a pair) matched and tested, and graded for early or late distortion - a pretty thorough QC process.

IMHO Cary was less than upfront about the state of my existing tubes, and trying to gouge me for replacements. Even if Cary does go through such a rigorous QC process themselves, that's not an excuse to triple the price of the tube, especially when the same service is available through some other reputable sources.
jea48
2,944 posts
 
That's nonsense. My amp takes a dozen EL 34s. Cary tried to tell me that the tubes that were in it were bad. When I had them tested locally not only weren't they bad, they were actually very strong, and except for a little degradation in two of them, pretty well matched.
02-25-12: Grimace
What kind of tube tester was it? Service or laboratory tester?

When was the tester last calibrated?

http://alltubetesters.com/articles/howto_test_tubes.htm
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