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Thread: B&K 650 tube matching?

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    B&K 650 tube matching?

    Is it possible to match the tubes with B&K 650 dynamic mutual conductance tester? Or is it only a more expensive emission tester?

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    It's a better test than an emission tester and should give a more representative comparison between tubes and triode halves for matching purposes. I just took a look at the schematic and the test voltages aren't as high as you'd see in many tube amps but should provide reasonable test conditions. You won't actually know if a tube tested would hold up under actual conditions in an amp, but I don't think your matching would be any worse than many of the tube vendors claims.

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    Thanks Mick. So what makes the 650 model so special and different than he regular emission testers? The "Life test" (which would be the key to the matching, right?)

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    Don't forget the joker g1's Avatar
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    I'm interested in anyone's answer about the 'life test' as I had come across people warning against using that function and it was not clear as to why.

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    Quote Originally Posted by boroman View Post
    Thanks Mick. So what makes the 650 model so special and different than he regular emission testers? The "Life test" (which would be the key to the matching, right?)
    If you want the best possible matching for your amp, the tester should have the same operation conditions as your amp, first & foremost B+. Followed by screen grid voltage and bias voltage. FWIW there's one tube dealer I know of in the USA, Jim McShane in Chicago, who will put together matched sets tested at hi voltage conditions specified by the customer. Very handy when you have to retube an SVT or something like that, and don't mind spending extra money for extra high quality service. Short of that, your amp makes the best tester of all. The drawback is, you'll be putting extra wear & tear on your output tube sockets as you swap tubes. After hours & hours of faffing, sooner or later you'll quit seeking ultimate perfection and decide you've reached the point of "good enough." As our resident expert Enzo says "it's just a guitar amp, not some NASA research project." Wiser words were never typed.

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    Quote Originally Posted by Leo_Gnardo View Post
    If you want the best possible matching for your amp, the tester should have the same operation conditions as your amp, first & foremost B+. Followed by screen grid voltage and bias voltage. FWIW there's one tube dealer I know of in the USA, Jim McShane in Chicago, who will put together matched sets tested at hi voltage conditions specified by the customer. Very handy when you have to retube an SVT or something like that, and don't mind spending extra money for extra high quality service. Short of that, your amp makes the best tester of all. The drawback is, you'll be putting extra wear & tear on your output tube sockets as you swap tubes. After hours & hours of faffing, sooner or later you'll quit seeking ultimate perfection and decide you've reached the point of "good enough." As our resident expert Enzo says "it's just a guitar amp, not some NASA research project." Wiser words were never typed.
    I agree with Leo, the amp is the best place to test, as all the variables that go into a pair of tubes operation are in the amp when it's running. If you can find a tester that can do that, then you are set.

    I've changed a lot of tubes over the last couple of years in attempts to get a balanced output, and for a guitar amp I now feel it's generally a big waste of time, as some of the best sounding amps I currently own have seriously out of balance output tubes.

    If it's a Hi-Fi amp we are talking about, then perhaps the rules change because you want to reproduce a recorded waveform accurately, or perhaps not, depending on what your ear says.
    Guitar amps are a mixed bag of inconsistencies, and everything that's already "off" seems to effect everything else, so balancing just the output tubes alone will probably not be all that effective in achieving a sonic improvement, IMHO, and as tubes age they often change differently, so balance is probably a temporary condition, under a controlled set of operational variables.

    It's an opinion I now have, and one that formed by shaking away my presumptions one by one through testing. Now, I just trust my ears !

    That's the end game for me, if it sounds good and nothing is going to blow up or cause a amp fire in the near future, I consider it a success and leave it be.

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    Last edited by HaroldBrooks; 02-10-2020 at 04:00 AM.
    " Things change, not always for the better. " - Leo_Gnardo

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    My Eico 666 has a "Merit" test, which I can imagine is likely similar to a "Life Test." The instructions explicitly state to use that feature as minimally as possible, and be extra sure that the various heater, plate, & grid voltages are set properly & the tube is fully warmed. They state openly that it puts the tube under some pretty extreme stresses as far as emmission, & that holding the lever down any linger than needed to simply get a yes/no reading can damage the tube. I'm imagining it's almost like forced cathodde stripping...

    On another bote, the previous owner took all of the common (& some not-so-common) amp tubes and made whole different instruction tables for plate & grid settings, generally trending towards higher plate volts & lower grid volts. (I think it's actually got a screen setting too; I haven't looked at it in a while...) So if I follow those it should be a little more "realistic" for amp operating conditions. Props to the guy who did the work & possibly blew up a few tubes in the process... I have all the original manuals & schematic for it. It's a fun toy.

    Jusrin

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    Basic emission testers often just strap the plate and grid together and measure AC current flow between these and the cathode. Sometimes at low voltages - 70v or 90v. Others just apply AC to the grid and measure current at the cathode, leaving the plate out of the equation altogether. A mutual conductance test goes a step further, but as stated, the real way to test a tube is in the actual circuit it's intended to run in.

    For a random box of tubes this is likely to risk putting a shorted tube into an amp, or creating premature wear on the socket. The main thing with preamp tubes is a tester doesn't tell you whether the tube is microphonic. There's a neat design of tester that helps with this that would be easy to build and I have one planned using a cheap voltage converter board to go through the hundreds of preamp tubes I have. Could really do with a B9A ZIF socket.

    For power tubes I just do a static match at 480V in a basic test 4-socket rig that operates the tube as it would do in an amp. This has never failed to give a decent usable match for a guitar amp. Since the price of factory-test-only tubes has increased I just buy matched sets, but I used to by lots of unmatched Shuguang 6L6 and El34 very cheaply and match them myself. I think the design was from Prowess Amps but its simple and much better than a tube tester for my purposes. I have an acquaintance with a Mullard punched card tester that does do a real test of tubes but is way beyond what I would need for guitar amps and takes up a lot of space.

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    On my stark 966 (hicock), the merit and life tests are 2 different things. You hold the merit down till the needle stabilizes like Justin mentioned. But there is also a 'life test' switch that you throw while observing merit, and see how much the reading drops. As far as I know, the life test drops the heater voltage.

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    On my Sencore the life test also reduces heater voltage to give some idea if there's enough life left in the tube to still give a decent conduction figure with 10% reduction in emission. In effect, it tells you if the tube is close to the end of the emission deterioration curve.

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