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Re: The Role of the 12AT7 ??

Posted by hopper chu on February 24, 2000 at 08:41:58:
Hi Marc,
You sez:
>Which incarnation of the Magus do you have? I had an A2 version for 10 >years. In my experience, the 12AT7, while not as amenable to tube rolling >as the two 12AX7's are (as you found out), tends to be the tube to check >1st when you start getting tube hiss-it goes more frequently than the >other ones. Don't know why that is, as it's "only" a cathode follower.
Do you know how this cathode follower is designed?? What is the plate voltage at the top of the tube and the voltage between cathode and ground (and what value of resistor)? And also, how do they wire the filament (e.g. 6.3VAC no grounding, or biased up with some DC; 6.3VDC maybe?)
What I am thinking is if the cathode-filament voltage is too high for the 12AT7 in this case (max. of 90V!!). This is a classic mistake in the 7199 phase splitter stage of the Dynaco ST-70. This over-voltage will shorten the life of the tube.
regards,
hopper