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I have a MP3 preamp.
I have tried several Raytheon VT231 tubes, and all of them, after a few working minutes, light the preamp DC offset red lights.
The manual says that, when these red lights stand on, the preamp must be switched off inmediately, because of DC issues.
Is it really dangerous, when these DC offset lights keep on glowing?
Maybe it is a compatibility issue between the MP3 and the Raytheons?
The amps are Atma-sphere M60s.
I´d like to use these Raytheons because I like the sound. I also use Tungsols WGT, maybe somewhat lighter.
Thanks in advance
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
Follow Ups:
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.
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).
.
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
Thanks Legendre,I thought that VT231 was just a militar code for 6SN7.
About matching, I´ll 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 won´t 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 that´s 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 won´t 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.
If this is happening when you are listening to music, and if there are no nasty sounds coming from your speakers, then the DC offset problem is probably not "dangerous", but it is probably best to resolve the problem. Assuming that the circuitry in your MP3 is working properly, the DC offset light just indicates that the tubes you inserted are too different from one another electrically for the circuit to compensate, thereby leaving a little excess DC voltage. There is no inherent incompatibility problem between Raytheon tubes and the MP3. You just need tubes that are better matched to each other. For example, you may be using "used" tubes where one of the pair or one of the two sections in each tube is near the end of its lifespan. If you have several Raytheons (i.e., more than 2), then you can try experimenting with different combinations until you find a pair that don't light up the warning LED. (I am assuming that you are using the Raytheons in the circlotron output section of the preamp. The DC offset circuitry is monitoring those tubes.) I had a very similar experience with my MP1 and was able to resolve it with the help of a Hickok tube tester to select matched pairs.
Thanks Lew,
There are no nasty sounds coming out the speakers when the lights are on, but as the manual says that it may be dangerous, that´s why I am worried.
I only have that issue with several Raytheons I´ve tried. I have used several other tubes without problems, so I guess that the preamp works fine.
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
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.
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
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 :-)
I am beginning to think that Raytheons simply have compatibility issues in Atmasphere preamps.
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 ... :-)
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
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 ....
_____________________________________I am from Galicia, so if you find me in a staircase, you won´t know whether I go up or down ...
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