Tube testing question

[Johnrcurry]

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

[billyz]

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.

[Prairie Dawg]

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.

[billyz]

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

[Prairie Dawg]

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.

[Johnrcurry]

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

[billyz]

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.

[Johnrcurry]

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

[billyz]

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.

[Johnrcurry]

Now I understand what the theory is; and it looks simple enough to build.

Thanks

[Prairie Dawg]

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.

[Rob Mercure]

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

[Prairie Dawg]

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.

[Rob Mercure]

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

[Prairie Dawg]

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.