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In Reply to: Help with Atma-sphere MP3 and DC offset posted by Manuel on May 23, 2005 at 03:48:13:
Manuel,If the DC lights are coming on, it's due to a section mis-match within the tube. The MP-3 likes to see closely matched sections, but it doesn't care if the two output tubes are not well matched.
The VT-231 is not a direct 6SN7 replacement, so it might be that the DC servo on the MP-3 doesn't have enough range to manage this tube. It might also be that the sections are just very badly matched.
We ship the MP-3 with the common Chinese 6SN7. Out of every hundred or so of these, I come across one or two that won't zero out in an MP-3, so it's pretty rare.
I'd test those VT-231 on a good tester, and see if they are truly section-matched as well as you were told they are.
Follow Ups:
VT-231 is 100% compatible with 6SN7GT. It's merely a military designation for 6SN7GT, nothing more, nothing less. 6SN7GTA and 6SN7GTB have slightly different operational specs, but 99.9% of the time they will work just fine in 6SN7GT applications (you may encounter more problems vice versa, but it's still very rare).
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_____________________________________I am from Galicia, so if you find me in a staircase, you wonīt know whether I go up or down ...
Thanks Legendre,I thought that VT231 was just a militar code for 6SN7.
About matching, Iīll try to swap my 6 available tubes and look for the best pair, if possible.My main concern is: Is it dangerous when the DC lights keep always on?
_____________________________________I am from Galicia, so if you find me in a staircase, you wonīt know whether I go up or down ...
Manuel,Like I said - it's not an issue of pair matching, but matching of the internal sections. If you find a tube that works right in one channel, then just leave it there and try different ones in the other channel.
The DC thing isn't really 'dangerous', and mainly depends on whether or not your power amp can tolerate DC on its input. But you really don't want to keep running with the DC light(s) on - find a tube that works, and use it.
Also, the MP-3 will make more noise if the DC lights are on, in the same way our power amps will, if they have DC at their output.
Thanks again, Legendre, as I said, I am an ignorant about electronics.
Now I understand this issue. The thing that fools me is that sometimes a single tube lights the DC light and then, when I swap the other channel, sometimes not ..... ?????The amps are the Atmas M60, which can regulate the DC , I guess thatīs what the DC swith is used to do.
I am using in the amps V1 position the tubes that swith on the DC lights on the preamp, and then correct the DC and bias in the amps. Am I doing right?By the way, I like them very much
Thanks again
_____________________________________I am from Galicia, so if you find me in a staircase, you wonīt know whether I go up or down ...
(1) As far as I know, you are correct, there is no difference between a "VT231" and an early type "6SN7GT". VT231 is merely the US military name for the latter tube. There are later versions of the 6SN7, called GTA and GTB that differ from GT types in their electrical maximum ratings, i.e., the latter two types can tolerate more plate voltage and a higher plate dissipation (Watts). This is not at all an issue in the MP3 or in the M60, except where you might want to use GTA or GTB types in the driver stage of the M60. (V1 is NOT the driver stage. V1 is an excellent place to try the Raytheons where they will make the most difference in sonics.)(2) When you move the Raytheon(s) from the circlotron circuit of your MP3 to the input stage of the M60, the bias-ing of the M60 has nothing to do with that tube any longer. The bias adjustment is merely for the output stage in both the preamp and the amplifier. Your m60 uses 6AS7s in its output stage.
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