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In Reply to: RE: NOS 12AT7 at "first 2 position (Cascode?)" in MP3? posted by MP on February 13, 2017 at 10:28:47
Check me out on the wiring of the pins, but if your 12AU7s are running on 12V filaments, you can sub a 12FQ7, which sound better than any of the common 12AU7 swaps, but that's ONLY if your preamp provides a 12V filament supply. (12AU7s can be wired to run on either 12V or 6V; I don't know what Ralph does in the MP3.) If you have a 6V filament supply, then check the pinouts to see whether a 6FQ7 would work. (You can look this up on-line.) I really don't know for sure, but I suspect the answer is no. Among common 12AU7s, I found that real Amperex Bugleboys, that I had owned since around 1975, so I knew they were the real deal, sounded the best. And 7316s are nice too.
I find Ralph's quote to be a bit confusing: "I would remove the rear 4 12AT7s (you need some of them in place, even though you are not using them)." Did Ralph clarify this ambiguous statement by confirming that you can remove the four phono tubes? In your case, you might not know there was any problem, because you are probably not measuring voltages. But both plate and filament voltages might rise if you pull those tubes. Or, if the voltages are regulated, then some parts of the PS might overheat. (More voltage has to be dissipated in the regulator, which means heat goes up.) This could hypothetically shorten the lifespan of some components or the tubes, but nothing would "blow up" so you might not know about it until tubes wear out prematurely, etc.
My understanding of "I would remove the rear 4 12AT7s (you need some of them in place, even though you are not using them)." is " remove the rear four 12AT7, AND (leave the first 2 12AT7 and rear number 7 CCS 12AT7 in place, probably due to Phono/linestgae unit configuration).
I checked line stage MP3 specs online, there are only two 12AU7 and two 6SN7.
Re-read this quote: "you need some of them in place, even though you are not using them". there seems no need to use any NOS tube!
We've eliminated that tube in the updated version of the MP-3.
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