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In Reply to: RE: Amplitrex 1000 testing MA1 6SN7, matching posted by Cousin Billy on November 09, 2008 at 09:49:57
Hi Bill,
The plate current gives you an idea about the gain and will also tell you how well the sections match up. However, this is only based on how the tubes reacts in your tester based on the plate voltage and bias voltage the tester is applying to the tube. The tube may or may not react the same way when you plug it in to your circuit unless the parameters are the same as the tube tester. However, the tester will give you some idea about the tube regarding gain and GM. It sounds like you have some very good tubes and if you have some where the GM matches but the plate current is off a little bit--don't worry about it. Like I said before--try and get within 5% but don't obsess over it. The tester is just something you can use to sort them out and get them matched up as close as possible. Sure it would be nice to have perfectly matched pairs but in reality you might not be able to. You probaly would not be able to hear any difference between them anyway. It's also nice to have a tester so that you can identify the dead or weak tubes. I agree with you---if you are a serious tube user, you must have a decent tube tester, but don't get some sort of nervosa over it.
Jack
Follow Ups:
Can you tell us more about the Amplitrex 1000? Is that one of those computer-based modules that gives actual plate curves?
Hello Lew
Yes, this is a computer compatible tube tester. I have not yet hooked it up that way, but when I have the time and the patience, it will be done.
I am still using it as a stand alone unit. I am very impressed so far. I have tested my 50 12SX7's. I have about 8 that are both very strong and perfectly matched. The best two pieces occupy V1a of each amp, the next best two V1b and so on. For V4 of each amp I have used the strongest, though not very well matched tubes. Is this OK?. iP Plate 8.8 and 9.6 on one (both have Gm of 3000), and 9.0 and 10.0 on the other (Gm 3000 and 3300). Would these tubes have been OK for V1a or b?. Should I use lesser strengthed but better matched tubes in the V4 position?. I figure these 'drive' the 6AS7's, so strength is more important then closeness.
Jack above seems to think it is the iP which is more important then the Gm. Do you concur?.
I have tested 40 6AS7's. The numbers are all over the board. When I reinstall them (After Ralph upgrades the MA1's with the MA2's power supply, I will be trying to run speakers which dip to 2.5ohms between 80 and 120Hz, please, no comments, I know I know I know, but there is no harm in trying), again, when I reinstall the6AS7's, should I add up all the numbers of each bank and try to balance them. Should I try to add up each row of each amp to the same numbers. Or am I just an excessive compulsive gone mad.
My MP1 mklll preamp has obviously 6SN7's. The front most tube on each side is the most important. These have RCA Grey glass. They are not very well matched 8.6 and 7.8, both with Gm of 3000 on one tube, and 7.2, 7.0, with Gm 2800 each. Is this OK, or should I put my best matched Chinese 6SN7's in this spot?.
All my 12AT7's test 50% with poorly matched sides. I will try to source better tubes.
Thanking everyone in advance.
Another aphorism: "a little knowledge is a dangerous thing"...
I will actually try to answer your many questions later this evening, when I am at home. Choosing between plate current and Gm as comparators may not be so different as you think.
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