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In Reply to: Re: Help with Atma-sphere MP3 and DC offset posted by Manuel on May 23, 2005 at 08:10:33:
Just to reiterate, it is not the Raytheons that are causing the problem per se; it is the fact that the two sections in whatever ones you are inserting are too far apart electrically for the balance circuitry to operate properly. To my recollection, the MP3 uses two sections in a single 6SN7 per channel for the circlotron output, so the sections within each tube need to be relatively well matched. Are they used tubes?
Yes, this preamp uses just 2 6SN7 tubes, one for channel.
I have now 3 matched??? pairs, bought from 2 different and (I think) reliable sellers.
I bought the tubes as NOS, they look like new, and if I believe what appears to be the tests results, they test strong and the 6 tubes very close one to another.
When I swap the tubes, with some combinations the red DC light sometimes lights on the left or the right channel alone, or on both channels at the same time.
Well, if there is no danger maybe I must not worry.
I´ll try to "scientifically" swap the 6 available tubes to look for all the possible matched combinations.
I like how these Raytheons sound, both in the MP3 preamp and in the V1 possition in the M60 amps.
Hey Manuel :-)
So is the offset the difference between sections within a tube or between the two tubes? What I'm reading isn't very clear.
My tubes are tested on calibrated Hickok TV-10B/U and 580A for mutual conductance. The gm values I've provided are accurate and not manipulated. I'm not sure how to test for "DC." If anyone has an idea, I'm all ears :-)
Hi Len, the DC offset is within a single tube, as I understood.
I´ve found two that work OK. I´ll send you an email.
I don´t know how to test tubes for DC offset, unless you buy a MP3 ... :-)
In theory, it should not matter much what brand of tube you use in the circlotron, because little to no gain is realized from that stage, but I agree with you that I can hear differences.
I thougth that the 6SN7 was the gain tube .... and the most audible are the 12AU7s ... so I spent my money in the wrong side.
Well, I am also using Mullard CV4003 (12AU7). I´d like to try Amperex 7316, but I was waiting to check the 6SN7s.
The preamp is line only.
What is the use of that circlotron?
thanks again ....
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