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In Reply to: RE: Rebuild of MC30s posted by Lee of Omaha on April 29, 2016 at 17:53:58
In addition to the somewhat abstract opinion I posted below, it's worth noting that the most significant difference between the 'AU7 and other varieties is linearity. However, I'm pretty sure that the more feedback the circuit applies, the less significant an individual tube's linearity will be. This is yet another reason I wouldn't change out the 'AU7 in this amplifier. Not only will it devalue the amp, it's extremely unlikely you'll hear a difference. I do agree with others who are suggesting power supply modifications, and in particular the use of higher grade parts for filtering. Many improvements have been made to components like capacitors over the last 30-40 years.
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Buy Chinese. Bury freedom.
Follow Ups:
Using a 12BH7 in place of the 12AU7 requires zero wiring changes, while improving linearity. IMO, installing a 12BH7 is a "no brainer". The doubled heater draw is, as previously indicated, a non-issue.
Eli D.
Edits: 04/30/16
I don't think so, Eli. These amplifiers are already extremely linear. The high degree of feedback that makes such performance possible completely swamps the 12AU7's native characteristics.
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Buy Chinese. Bury freedom.
Jack
When you put a 6CG7 in place of the 12AU7,the difference in noticeable as the 6cg7 is more linear but not only that,distortion drops by .25% coming out of the voltage amp on measurement.What the 6cg7 is doing is keeping the distortion down and the circuit more linear the longer it is on.The 12AU7 runs awful hot in the Mac amps but it looks nice in there..The 12BH7 can give you can idea but it doesn't give the stand up difference that we get with the 6cg7.
A friend you get for nothing,an enemy has to be bought
"distortion drops by .25% coming out of the voltage amp"
But does it improve performance of the amplifier at the speaker terminals? It's difficult to imagine how Mac could have missed something so simple if it reduces distortion by so much. Personally, I'm skeptical.
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Buy Chinese. Bury freedom.
You have got to be kidding me...the 12AU7 is much cheaper.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
"You have got to be kidding me...the 12AU7 is much cheaper. "
Do I understand your comment correctly? The 12AU7 was selected because McIntosh was too cheap to do better? I think it's just as likely that the 12AU7s they used were better than we know, or maybe even hand-picked at the factory. I'll never believe that an improvement of this magnitude would have been ignored by Mac. It doesn't make sense.
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Buy Chinese. Bury freedom.
this coming from the same folks that brought us the 12AX7 cathode so they could be common with the input tube. You are free to believe what ever you wish about their engineers ability to dodge the bean counters. Do show me some proof of this idea of selecting 12AU7's.
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
But does it improve performance of the amplifier at the speaker terminals? It's difficult to imagine how Mac could have missed something so simple if it reduces distortion by so much. Personally, I'm skeptical.
It definitely improves it. BTW the measuremnt was .15% and not .25%.I looked at my paperwork so that was my mistake. The big improvement you get from the 6CG7 is after warmup. In the Mac amps there is a noticeable improvement. I was skeptical as you were but then I tried it after Pak Protector said do it Mikey,and I never looked back.Try it for yourself and tell us what you think but everyone that made the switch,loves it.
A friend you get for nothing,an enemy has to be bought
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