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In Reply to: Re: M-60 dc offset/ bias run amok posted by cousinbillyl on November 16, 2004 at 21:26:33:
I do not agree with this. The amps are not 6sn7 tubetesters, and I don't believe one can state that if they work in the amp then they are good. I recently had some 12sx7s in my amps that 'worked' fine in the amps, but I knew they sounded horrible and sure enough I sent them off to be tested and they tested horribly. You can test the 6as7 powewr tubes in the amps though. The original posters post is another thing all together and I hope someone with more knowledge can reply.
Follow Ups:
You are correct. It is not a tube tester. I did not give enough information. I simply wanted to state that a 6SN7 is not tested under as harsh a load in a tube tester. It is under a greater load in the amps.Now a question. I loved my 12SX7's in my M60's, and am in the process of getting my MA1 Silver's upgraded. How long do you suspect the 12SX7's will last?. I only have a dozen. How many spares should I keep around?.
Which are the better 12SX7's you have tried?.
> > How long do you suspect the 12SX7's will last?.I really have no idea how long they will last. I'm as far from an expert in tubes as there is.
> > I only have a dozen. How many spares should I keep around?.
Beat's me. All I know is it's important to have a full compliment of well matched, good driver tubes in your amps.
> > Which are the better 12SX7's you have tried?.
I have only tried the RCA's that Mark Gilmore picked up for me. I don't know if there are better out there, or if there are any other kinds for that matter.
> > Any and all information would be appreciated.
Well, its nice to know what little I could provide in the way of information is appreciated :-) To be honest, I'd just get some RCA's that are good, and a few spares that are good, and keep an eye on the ones that are in your amps. If ASOG was up I know there is a post or two about where you can get NOS 12SX7s. I'd like to get some more since I discovered that all of my spares are useless. Maybe whoever has the ASOG archive can do a quick search and let us know what tube supplier(s) was recommened.
Brian, I remember your post on ASOG about having a bunch of 12SX7s that had non-matching sections, which you then concluded were not useable. Here is a case where you may want to try some of those "bad" tubes in your amps. As mentioned above, most tube testers test at very low plate voltages. A tube may behave very differently depending on plate voltage. At the actual working voltages in your Atma-spheres, some or all of your so-called bad tubes may function quite well. As I wrote before, in my experience NOS 12SX7s have very well-matched sections, as per the read-out of my Hickok 533A tube tester. (Hence the information I get from testing is no better than yours, provided you used a Hickok or other tester that measures transconductance.) I selected some for use in my amps, and they have been going strong for about a year now. I expect them to last for several more years.
I am confused, Lew.> > At the actual working voltages in your Atma-spheres, some or all of your so-called bad tubes may function quite well. As I wrote before, in my experience NOS 12SX7s have very well-matched sections, as per the read-out of my Hickok 533A tube tester. (Hence the information I get from testing is no better than yours, provided you used a Hickok or other tester that measures transconductance.)
Isn't that a contradiction? You say they have well matched sections in the tester, yet I think you are impling that even if they dont test well they may behave differently in the amps. I am not sure what the case is for trying these tubes that test poorly in my amps.
I had this experience. I recieved about 20 12SX7s from Mark and he said that they came from a tube swap in California. These would be used NOS. I also purchased another 10 new NOS from another source.
I don't have a tester myself so I didn't know what each tube tested out as and I put a set of 8 fresh ones in my amps one day and the sound was horrible. I sent the 8 to a friend who has M60s and a Hickock tester. He put the tubes in his amps, and the sound was horrible. He then tested them, and most of them had poorly matched sections. I then sent him the rest of my tubes and he tested all of them and came up with 8 excellant ones (probably from the 10 I bought new). He listened to them, they sounded great, and he sent them to me and they sounded great. He said the remaining tubes didn't test well. I haven't recieved them back yet, but I could, and I could give them a listen but I don't expect that it would be worthwhile. I think I would be better off just ordering some new spares.
First of all, the "N" in the acronym "NOS" stands for "new", meaning never used. So when you first said that your tubes were NOS I assumed they were never used before. (The term "used NOS" is an oxymoron.) In fact, I think I wrote back on ASOG that your test results might indicate your tubes were not truly NOS but were in fact used ones. If you found bad tubes among a bunch of used ones, that is no surprise. Second, you apparently found that a majority of known truly never used NOS tubes tested OK in your friend's tester AND work well in your amps, so there is no discrepancy between your experience and mine.BUT what I wrote yesterday was just to confirm that one cannot necessarily rely on a tester like mine and most other Hickoks to select tubes for matched sections because of the big difference between the plate voltage developed in a tube tester and the typical plate voltages developed in actual use (probably much higher in the latter case). Tubes are nonlinear devices; you cannot necessarily extrapolate performance at one set of conditions from measured performance at another set of conditions. It just so happened that testing in my Hickok seemed to correlate with function in my amplifiers, but results could have been otherwise. There are a rare few testers that do permit the application of selected plate voltage and bias voltage and with which one can truly emulate the conditions of actual use in an amplifier.
I got mine, and am getting more, from www.halfin.comHe says he has about 1000 RCA's. He will match and low noise them for you.
Bill
PS I can't believe how much better the Silvers are with the 12's over the 6SN7's. Happy listening.
Ya, thats the link that was posted at asog. I didn't realize there were so many different brands of 12sx7. What ones have you purchased? The RCAs? And how much were they?
My original purchase was for 12 12SX7GT's. Matched low noise pairs. $25.00 each plus shipping. They are 1954 Grey Glass. I was told the grey glass was tinted for RF rejection.Even though they have many types listed on their website, they have mostly RCA's.
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