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Thread: Tube testing question

  1. #1
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    Tube testing question

    I've acquired 20 or so used 6aq5 power tubes. Judging by the look of them, I suspect some may be bad. I'm thinking of finding out which ones work by installing them in an amp. If I do that, and some of them are bad, do I risk damaging the amp?

    Thanks

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    If they are shorted ,possibly. try them one at a time and use a smaller fuse, maybe fast acting. If any are white toss em. And use a bulb limter or variac to power up slowly.

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    Quote Originally Posted by billyz View Post
    If they are shorted ,possibly. try them one at a time and use a smaller fuse, maybe fast acting. If any are white toss em. And use a bulb limter or variac to power up slowly.
    Indulge yourself and buy a tube tester.

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    Testers are good. I use a TV7 d/u . They are getting expensive though. Smoke em if you got em.

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    Quote Originally Posted by billyz View Post
    Testers are good. I use a TV7 d/u . They are getting expensive though. Smoke em if you got em.
    that's a good unit. I use a Hickok 532 that is older than me by two years, but it works like a champ, and I've got a backup in case this one ever dies even though it never had the mod for 9 pin miniatures. I also have a Mercury that I use only for gM testing 12 pin compactrons like the 6C10, 6K11, 6BK11 and 6U10.

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    Thanks everyone for your advice. I was hoping to do this cheaply, but maybe it is time to look into a tester.

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    Don't despair if you don't have a tube tester. Many of them are crap. If you don't get a good Hickock or other known good model, then don't bother.
    Most testers do not test at the voltage of the amplifier anyway. I have not measures my TV7, but I would not be surprised if it only got to 165VDC.

    They can find shorts and tell you the strength of the tube . They do not help you match for current draw though. I use the amplifier for that anyway. The Transformer shunt method, it is dangerous too. You can test a tube on a tester and it may look ok and then once you get it in the amp, POOF.

    the light bulb limiter is easy and cheap to make.

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    Can you test for shorts with a continuity tester (i.e. testing continuity between pins)?

    Can you tell me more about a light buld limiter?

    Thanks

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    I suppose you could test for shorts between the pins. The filaments would look almost like shorts though. And some only occur at higher voltages. I personally never check the pins for shorts.

    Go to New Page 1

    search under tube amps, soft on power test.
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    Now I understand what the theory is; and it looks simple enough to build.

    Thanks

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    Quote Originally Posted by billyz View Post
    Don't despair if you don't have a tube tester. Many of them are crap. If you don't get a good Hickock or other known good model, then don't bother.
    Most testers do not test at the voltage of the amplifier anyway. I have not measures my TV7, but I would not be surprised if it only got to 165VDC.

    They can find shorts and tell you the strength of the tube . They do not help you match for current draw though. I use the amplifier for that anyway. The Transformer shunt method, it is dangerous too. You can test a tube on a tester and it may look ok and then once you get it in the amp, POOF.

    the light bulb limiter is easy and cheap to make.
    Kinda depends on what you want to use them for, Doc. I use a couple emission testers (Knight and Eico) although the occasional Jackson comes my way) for sorting through vast piles of flea market sweepings. They will also tell you if a tube's shorted and that's valuable knowledge.

    Some of the audiophools seem to think the B&K 747 and 747B are great stuff. I never liked them much but I make money from every one I can find. The earlier B&Ks like the 500 and 600 used the Hickok circuit-as did everyone when the patent ran out in the early fifties but they're otherwise limited and need cumbersome adapters for all sorts of stuff.

    The circuit that Hickok used was invented in 1934 by a fellow named Job Barnhart who worked for them-talk about your unsung geniuses, he made a lot of money for a lot of people.

    As an exercise in something or other I took a known good 6L6GC and tested it on my Hickok after a thirty minute warmup. I then took and put it in my Eico 625 emission tester and adjusted the load control t oget the same needle deflection as the Hickok got. Then I proceeded to test a number of tubes on both testers and you would be surprised how close the results on my cheapie Eico were to the ritzy Hickok.

    I also have a small Superior that I keep in the car so when I'm sitting in the hotel room with my hamfest scores I can amuse myself.

    But you're right. Plate voltage in your average service type tube tester is not nearly what you'll see in line service. And, as I am sure you're aware, tubes are not linear devices. So the designers at places like Hickok et al took and stuck a push pin in the curve chart and said "That's where we're gonna test 'em." Plus, a lot of our beloved audio tubes worked at fairly low voltages in service, because they'd last a long time. A 6V6 or 6K6 in audio service in a table radio might only be running at 225 volts on the plates-heck, my BR6 Gibson amp only runs about that after the field coil speaker gets done with it.

    Pick up a copy of Alan Douglas' "Classic Tube Testers" I think it's called. It has a world of good information in it.


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    Lifetime Member Rob Mercure's Avatar
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    Simply put the tubes in circuit and see how they perform - that's the only sane way to "test" a tube and no laboratory ever trusted a tube tested in critical applications - instead a dummy circuit was used that duplicated real world operation conditions (something that no tube tester that I've seen does). The 6AQ5 is a truly rugged little bottle and I've used many of them in my own builds. You can cobble up a little "champ" type circuit with only a single triode voltage amp stage if necessary - no need for tone controls - and use 1 ohm, 1% resistor in the plate, screen, and cathode legs of the output tube. Use a moderate B+ such as 250 V and get the amp/tester running with known good tubes. Then sub in the tubes to be "tested" and note currents and noise. Nothing could be simpler and one could cobble up the tester - assuming that you've got the components - in less than an hour.

    That's how I "test" tubes.

    Rob

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    Quote Originally Posted by Rob Mercure View Post
    Simply put the tubes in circuit and see how they perform - that's the only sane way to "test" a tube and no laboratory ever trusted a tube tested in critical applications - instead a dummy circuit was used that duplicated real world operation conditions (something that no tube tester that I've seen does). The 6AQ5 is a truly rugged little bottle and I've used many of them in my own builds. You can cobble up a little "champ" type circuit with only a single triode voltage amp stage if necessary - no need for tone controls - and use 1 ohm, 1% resistor in the plate, screen, and cathode legs of the output tube. Use a moderate B+ such as 250 V and get the amp/tester running with known good tubes. Then sub in the tubes to be "tested" and note currents and noise. Nothing could be simpler and one could cobble up the tester - assuming that you've got the components - in less than an hour.

    That's how I "test" tubes.

    Rob
    That's a good single purpose test rig. The Maxi Matcher seems to answer some of your objections in the "operating environment" sphere, only thing is it's kind of limited (not to mention pricey) for the kind of work I do and thus, sadly, I have to rely on what Hickok hath wrought, useless as it is and futile as the efforts of all their engineers seem to have been.

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    Lifetime Member Rob Mercure's Avatar
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    Well my test rig actually have several sockets of various types and wiring so that I can "test" whatever questionable tube I need to. But for repair I generally trust good stock tubes and pull one out to replace a suspect tube - which is how I've done is for over 30 years. When I did TV repair in the 1970s (yes I admit my sin) the tube tester was only used to convince a customer that the tech had not replaced a good tube just to make a buck - a common rumor that circulated in the press that wasn't true of any tech I knew.

    While you tester is a better one if you use it for a while you'll still find out that it will "lie" to you from time to time as the conditions it duplicates aren't in any way "real world." In ciruit at circuit parameters is still the only way to test for anything other than truly gross defects such as shorts and extreme gas.

    We get wedded to the "absolutism" of gadgets

    Rob

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    Quote Originally Posted by Rob Mercure View Post
    Well my test rig actually have several sockets of various types and wiring so that I can "test" whatever questionable tube I need to. But for repair I generally trust good stock tubes and pull one out to replace a suspect tube - which is how I've done is for over 30 years. When I did TV repair in the 1970s (yes I admit my sin) the tube tester was only used to convince a customer that the tech had not replaced a good tube just to make a buck - a common rumor that circulated in the press that wasn't true of any tech I knew.

    While you tester is a better one if you use it for a while you'll still find out that it will "lie" to you from time to time as the conditions it duplicates aren't in any way "real world." In ciruit at circuit parameters is still the only way to test for anything other than truly gross defects such as shorts and extreme gas.

    We get wedded to the "absolutism" of gadgets

    Rob
    True about the absolutism of gadgets. There's a human tendency to demand gnat's ass readings, which I suspect is why everyone's so gaga over digital stuff when analog works just fine. A tube tester's a tool and it's best to know it's limitations and what it's capable of. As you point out there's a lot going on in an audio circuit that a tube tester may not be capable of telling you.

    My background's in aviation, and there we use test equipment and we bet the ranch on the results. I've a bit of experience using strain gauge torque testing equipment and various electronic test sets-in particular when adjusting engine control analog computers. I may not be quite as suspicious as you are of the genre, but that's only because most of it is good stuff and not mere cooking grade devices like the tube tester down at the drugstore.

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    I have a friend. He found out that tube testers, especially a lot of them, get wives really upset. He talked to his ham swapfest buddy, and they decided to do a tube tester calendar to upset wives. His buddy brought him something like 30 tube testers over the years, but he never made the calendar. He did give me a Hickok for safe keeping.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by Rob Mercure View Post
    ... the tube tester was only used to convince a customer that the tech had not replaced a good tube just to make a buck - a common rumor that circulated in the press that wasn't true of any tech I knew.
    Hi Rob. Its been a long time.

    You bring up an interesting point about tube testers and honest techs. I've encountered a few less than honest ones. In working on gear for people, I've found capacitors that were installed that were deliberately under voltage for the required application. As you know, if a repairman uses a cap beyond its rated voltage specs, the cap will ultimately fail. Less than honest repair guys would deliberately use lower voltage caps so that the units being repaired could be counted on to fail outside of the 30-day warranty period. I know one fellow who had his gateway CRT monitor repaired 2 times by the same guy for the same problem. i got it when it failed the 3rd time. when i popped the hood, it was easy to see which part had been serviced, and it was a cap that was being used beyond its rated voltage. i repaired the monitor using the proper part, and put an end to the dishonest repairman's revenue stream. i also told the customer exactly what happened and gave them the old part.

    much to my surprise, i was reading through an old thordarson catalog and i found an article that specifically addressed ethics in performing repairs on radio gear. the article specifically mentioned the practice of using "condensers" that weren't rated for a high voltage application, just so the customer could be counted upon for return business. the article specifically advised against this sort of thing, suggesting that repair techs should act ethically at all times, so that dishonest behavior would not end up destroying their business.

    this sounds a lot like the kind of practice that was referred to by the news media in your TV repair days -- techs deliberately performing improper repairs for customers who don't know any better. the problem is real, and i guess its been around for a long time. thinking about this kinda makes me nervous every time i take my car in for repairs...

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    Can you match power tubes (with a cathode current / plate voltage bias probe) in your amp, but taking out the preamp tubes so as to not have to have them subject to the on-off cycling when you install / remove the power tubes you are testing? I wondered if this would damage the amp (to start it and check idle current but with preamp tubes removed.

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    It should raise heater and plate voltage a bit. Which will affect your readings. How much? Try it both ways and record your results, then you will know. Otherwise no damage to the amplifier will result. I doubt the the few extra on off cycles will be noticeable to your preamp tubes.

    You could also do a hot swap with the standby on . Use a glove or rag to not get burned. Sometimes this is not practical though. I do it all the time when matching and balancing output tubes for an amp.

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    Quote Originally Posted by billyz View Post
    It should raise heater and plate voltage a bit. Which will affect your readings. How much? Try it both ways and record your results, then you will know. Otherwise no damage to the amplifier will result. I doubt the the few extra on off cycles will be noticeable to your preamp tubes.

    You could also do a hot swap with the standby on . Use a glove or rag to not get burned. Sometimes this is not practical though. I do it all the time when matching and balancing output tubes for an amp.
    It might amount to a hill of beans but not much more.

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    billyz - Thanks for the advice!

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